US 5,886,368 AGrant

Transistor With Silicon Oxycarbide Gate and Methods of Fabrication and Use

Issue Date:1999-03-23

•40 Claims

•11 Drawing Sheets

Abstract

A CMOS-compatible FET has a reduced electron affinity polycrystalline or microcrystalline silicon oxycarbide (SiOC) gate that is electrically isolated (floating) or interconnected. The SiOC material composition is selected to establish a desired barrier energy between the SiOC gate and a gate insulator. In a memory application, such as a flash EEPROM, the SiOC composition is selected to establish a lower barrier energy to reduce write and erase voltages and times or accommodate the particular data charge retention time needed for the particular application. In a light detector or imaging application, the SiOC composition is selected to provide sensitivity to the desired wavelength of light. Unlike conventional photodetectors, light is absorbed in the floating gate, thereby ejecting previously stored electrons therefrom. Also unlike conventional photodetectors, the light detector according to the present invention is actually more sensitive to lower energy photons as the semiconductor bandgap is increased.

Metadata

Assignee

Micron Technology, Inc.

Inventors

Leonard Forbes
Joseph E. Geusic
Kie Y. Ahn

Application Information

Application Number:US 9021329

Filing Date:1997-07-29

Priority Date:1997-07-29

Art Unit:281

Classifications

IPC:

H01L 310312

Field of Search:

25776;77;315;316;323;364;411;412

Patent Drawings (11 sheets)

Description

FIELD OF THE INVENTION

The present invention relates generally to integrated circuit technology, and particularly to a silicon oxycarbide gate transistor, such as a floating gate transistor, and complementary metal-oxide-semiconductor (CMOS) compatible methods of fabrication, and methods of use in memory and imaging devices.

BACKGROUND OF THE INVENTION

Field-effect transistors (FETs) are typically produced using a standard complementary metal-oxide-semiconductor (CMOS) integrated circuit fabrication process. Such a process allows a high degree of integration for obtaining a high circuit density with relatively few processing steps. Resulting FETs typically have gate electrodes composed of n-type conductively doped polycrystalline silicon (polysilicon).

The intrinsic properties of the polysilicon gate material affects operating characteristics of the FET. Silicon (monocrystalline and polycrystalline) has intrinsic properties that include a relatively small energy bandgap (E.sub.g), e.g. approximately 1.2 eV, and a corresponding electron affinity (.chi.) that is relatively large, e.g. .chi.=4.2 eV. For example, for p-channel PETs fabricated by a typical CMOS process, these and other material properties result in a large turn-on threshold voltage (V.sub.T) magnitude. As a result, the V.sub.T magnitude must be downwardly adjusted by doping the channel region that underlies the gate electrode of the FET.

Another drawback with polysilicon gate FETs arises during use as a nonvolatile memory device, such as in electrically erasable and programmable read only memories (EEPROMs). EEPROM memory cells typically use FETs having an electrically isolated (floating) gate that affects conduction between source and drain regions of the FET. A gate dielectric is interposed between the floating gate and an underlying channel region between source and drain regions. A control gate is provided adjacent to the floating gate, separated therefrom by an intergate dielectric.

In such memory cells, data is represented by charge stored on the polysilicon floating gates. Fowler-Nordheim tunneling is one method that is used to store charge on the polysilicon floating gates during a write operation and to remove charge from the polysilicon floating gate during an erase operation. However, the relatively large electron affinity of the polysilicon floating gate presents a relatively large tunneling barrier energy at its interface with the underlying gate dielectric. The large tunneling barrier energy provides longer data retention times than realistically needed. For example, a data charge retention time at 85.degree. C. is estimated to be in millions of years for some floating gate memory devices. The large tunneling barrier energy also increases the time needed to store charge on the polysilicon floating gates during the write operation and the time needed to remove charge from the polysilicon floating gate during the erase operation. This is particularly problematic for "flash" EEPROMs, which have an architecture that allows the simultaneous erasure of many floating gate transistor memory cells. Since more charge must be removed from the many floating gates in a flash EEPROM, even longer erasure times are needed to accomplish this simultaneous erasure. There is a need in the art to obtain floating gate transistors allowing faster storage and erasure, such as millisecond erasure periods for flash EEPROMs.

Other problems result from the large erasure voltages that are typically applied to a control gate of the floating gate transistor in order to remove charge from the floating gate. These large erasure voltages are a consequence of the large tunneling barrier energy between the polysilicon floating gate and the underlying gate dielectric. The large erasure voltages can result in hole injection into the gate dielectric. This can cause erratic overerasure, damage to the gate dielectric, and introduction of trapping states in the gate dielectric. The high electric fields that result from the large erasure voltages can also result in reliability problems, leading to device failure. There is a need in the art to obtain floating gate transistors that allow the use of lower erasure voltages.

Halvis et al. (U.S. Pat. No. 5,369,040) discloses a charge-coupled device (CCD) photodetector which has transparent gate MOS imaging transistors fabricated from polysilicon with the addition of up to 50% carbon, and preferably about 10% carbon, which makes the gate material more transparent to the visible portion of the energy spectrum. The Halvis et al. patent is one example of a class of conventional CCD photodetectors that are directed to improving gate transmissivity to allow a greater portion of incident light in the visible spectrum to penetrate through the gate for absorption in the semiconductor substrate. However, the absorption of photons in the semiconductor substrate is limited to high energy photons exceeding a bandgap energy of the semiconductor substrate. There is a need in the art to detect lower energy photons independently of the semiconductor bandgap energy limitation. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, the above described needs are unresolved in the art of fabrication of imaging devices, FETs, and EEPROMs using CMOS processes.

SUMMARY OF THE INVENTION

The present invention includes a transistor having a silicon oxycarbide (SiOC) gate. The transistor includes a source region, a drain region, and a channel region between the source and drain regions. A gate is separated from the channel region by an insulator. The gate is formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w. The SiOC composition w is selected approximately between 0 and 1.0, such as to establish a desired value of a barrier energy between the gate and the insulator. In one embodiment, the gate is an electrically isolated floating gate and the transistor includes a control gate, separated from the floating gate by an intergate dielectric.

In another embodiment, the present invention includes an imaging device that is capable of detecting low energy photons independent of a semiconductor bandgap energy. The imaging device includes a source region, a drain region, and a channel region between the source and drain regions. A floating gate is separated from the channel region by an insulator. The floating gate is formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w. The SiOC composition w is selected approximately between 0 and 1.0, such as to establish a desired value of a barrier energy between the gate and the insulator. The floating gate is adapted for emission of charge from the floating gate in response to absorbed incident photons.

In another embodiment, the present invention includes a memory device that is capable of providing short programming and erase times, low programming and erase voltages, and lower electric fields in the memory cell for improved reliability. The memory device includes a plurality of memory cells, each providing a transistor. Each transistor includes a source region, a drain region, and a channel region between the source and drain regions. A floating gate is separated from the channel region by an insulator. The floating gate is formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w. The SiOC composition w is selected at a value approximately between 0 and 1.0, such as to establish a desired value of a barrier energy between the gate and the insulator. A control gate is located adjacent to the floating gate and separated therefrom by an interlayer dielectric.

In another embodiment, the present invention includes a method of producing a SiOC gate transistor on a semiconductor substrate. Source and drain regions are formed, thereby defining a channel region between the source and drain regions. An insulating layer is formed on the channel region. A gate is formed on the insulating layer. The gate comprises a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w. The SiOC composition w is selected at a value approximately between 0 and 1.0. In one embodiment, the method also includes forming a second insulating layer on the floating gate and forming a control gate on the second insulating layer.

In another embodiment, the present invention includes a method of using a transistor having a floating gate formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w, such as for data storage. The SiOC composition w is selected at a value approximately between 0 and 1.0, such as to establish a desired value of a barrier energy between the floating gate and an insulator. Data is stored as charge on the floating gate. Data is read by detecting a conductance between source and drain regions of the transistor, wherein the conductance varies based on the stored data charge on the floating gate. Data is erased by removing charge from the floating gate.

In another embodiment, the present invention includes an method using a transistor having a floating gate formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w, such as for light detection of low energy photons. The SiOC composition w is selected at a value approximately between 0 and 1.0, such as to establish a desired wavelength of incident light absorption to which the floating gate is sensitive. Charge is stored on the floating gate. Incident light is received at the floating gate, thereby removing at least a portion of the stored charge from the floating gate by the photoelectric effect. A change in conductance between the transistor source and drain is detected. According to one aspect of the invention, low energy photons are detected independently of a semiconductor substrate bandgap energy.

Thus, the present invention includes a CMOS-compatible FET having a low electron affinity SiOC gate that is either electrically isolated (floating) or interconnected. According to one aspect of the present invention, the SiOC composition w can be selected to provide the desired barrier energy at the SiOC-SiO.sub.2 interface, such as 0<w<0.4, or 0.5<w<1.0.

In one embodiment of the present invention that is particularly useful in a flash EEPROM application, the SiOC composition w is selected to provide the desired programming and erase voltage and time or data charge retention time. The lower barrier energy and increased tunneling probability of the SiOC gate advantageously provides faster programming and erasure times for floating SiOC gate transistors in flash EEPROM memories. This is particularly advantageous for "flash" EEPROMs in which many floating gate transistor memory cells must be erased simultaneously. Writing and erasure voltages are also advantageously reduced, minimizing the need for complicated and noisy on-chip charge pump circuits to generate the large erasure voltage. Lower erasure voltages also reduce hole injection into the gate dielectric that can cause erratic overerasure, damage to the gate dielectric, and introduction of trapping states in the gate dielectric. Reducing the erasure voltage also lowers electric fields in the memory cell, minimizing reliability problems that can lead to device failure, and better accommodating downward scaling of device dimensions.

In another embodiment of the present invention, the SiOC composition w is selected to decrease the data charge retention time to a desired value. Since conventional data charge retention times are longer than what is realistically needed, a shorter data charge retention time can be tolerated in order to obtain the benefits of a smaller barrier energy. The data charge retention time can be selected between seconds and millions of years by selecting the value of the SiOC composition w, such as to obtain different memory functionality.

In another embodiment of the present invention that is particularly useful for an imaging application, the SiOC composition w is selected to provide sensitivity to the desired wavelength of light. Unlike conventional photodetectors, light is absorbed in the floating gate, thereby ejecting previously stored electrons therefrom. Also unlike conventional photodetectors, the light detector according to the present invention is actually more sensitive to lower energy photons as the semiconductor bandgap is increased.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like numerals describe substantially similar components throughout the several views.

FIG. 1 is a cross-sectional schematic diagram illustrating generally one embodiment of a FET provided by the invention, which includes an electrically isolated (floating) or interconnected gate including a silicon oxycarbide (SiOC) compound.

FIGS. 2A and 2B are energy band diagrams that illustrate generally the barrier energy at the interface between gate and an adjacent thin oxide layer (or other gate insulator).

FIG. 3 is a graph that illustrates generally electron affinity, .chi., as a function of bandgap energy.

FIG. 4 is a graph that illustrates generally barrier energy height versus tunneling distance for SiOC and conventional polysilicon gates.

FIG. 5 is a graph that illustrates generally the relationship between retention time and barrier energy, and also the relationship between erase time and barrier energy.

FIG. 6 is a schematic/block diagram that illustrates generally a flash EEPROM memory having memory cells that include an SiOC gate transistor according to the present invention.

FIG. 7 is a cross-sectional schematic diagram of the floating gate transistor that illustrates generally its application according to the present invention as a light detector or imaging device.

FIG. 8 is a cross-sectional schematic diagram that illustrates generally how incident light is detected by the absorption of photons by the floating gate.

FIG. 9 is a graph illustrating generally barrier energy versus tunneling distance, and further illustrating the absorption of light energy by the floating gate.

FIG. 10 is a graph illustrating generally barrier energy versus tunneling distance, and distinguishing photoelectric absorption of incident light in the SiOC floating gate from valence-to-conduction band electron transitions.

FIG. 11 is a graph that illustrates generally, by way of example, absorption coefficients as a finction of wavelength and photon energy.

FIGS. 12A, 12B, 12C, 12D, 12E, 12F and 12G are cross-sectional schematic diagrams that illustrate generally examples of process steps for fabricating n-channel and p-channel SiOC gate FETs according to the present invention, including the fabrication of SiOC floating gate transistors.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, a specific embodiment in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. This embodiment is described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural and electrical changes may be made without departing from the scope of the present invention. The terms wafer and substrate used in the following description include any semiconductor-based structure having an exposed surface with which to form the integrated circuit structure of the invention. Wafer and substrate are used interchangeably to refer to semiconductor structures during processing, and may include other layers that have been fabricated thereupon. Both wafer and substrate include doped and undoped semiconductors, epitaxial semiconductor layers supported by a base semiconductor or insulator, as well as other semiconductor structures well known to one skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.

The present invention discloses a field-effect transistor (FET) having a gate that is formed of a silicon oxycarbide (SiOC) material, which includes any material that incorporates silicon, oxygen, and carbon into the gate region of a FET. The invention is described with respect to microcrystalline (.mu.c) SiOC materials, but is understood to include both .mu.c and polycrystalline SiOC materials. The SiOC gate FET includes characteristics such as, for example, a lower electron affinity than a conventional polycrystalline silicon (polysilicon) gate FET. The present invention also discloses a tailored SiOC material composition for obtaining particular device characteristics for use in conjunction with a particular gate dielectric, or for particular applications, such as data storage (memory) and imaging.

FIG. 1 is a cross-sectional view illustrating generally, by way of example, one embodiment of a n-channel FET provided by the invention. The invention is understood to also include a p-channel FET embodiment. The n-channel FET includes a source 102, a drain 104, and a gate 106. A gate dielectric, such as thin silicon dioxide (oxide) layer 118 or other suitable insulator, is interposed between gate 106 and substrate 108. In one embodiment, source 102 and drain 104 are fabricated by forming highly doped (n+) regions in a lightly doped (p-) silicon semiconductor substrate 108. In another embodiment, substrate 108 includes a thin semiconductor surface layer formed on an underlying insulating portion, such as in a semiconductor-on-insulator (SOI) or other thin film transistor technology. Source 102 and drain 104 are separated by a predetermined length in which a channel region 110 is formed.

According to one aspect of the invention, gate 106 is formed of silicon oxycarbide (SiOC) material, which includes any material that incorporates silicon, oxygen, and carbon into gate 106. The silicon oxycarbide material forming gate 106 is described most generally as Si.sub.x O.sub.y C.sub.z, where x, y, and z are composition variables that define the SiOC material composition. In one embodiment of the present invention, the silicon oxycarbide material forming gate 106 is described more particularly as SiO.sub.(2-2w) C.sub.w, where the SiOC composition variable w defines the empiric relationship between the number of oxygen and carbon atoms in the SiOC compound and the SiOC composition w is between 0<w<1. For example, but not by way of limitation, SiOC materials having a composition between that of stoichiometric silicon carbide (SiC) and stoichiometric silicon dioxide (SiO.sub.2) are included within the present invention.

According to another aspect of the invention, the SiOC composition w is selected at a predetermined value that establishes the value of a barrier energy (also referred to as a barrier potential, potential barrier, tunneling barrier, interface barrier, or barrier) between gate 106 and thin oxide layer 118 (or other gate dielectric). For example, in one embodiment, the SiOC composition is approximated by w=0.5. However, other embodiments of the invention include less oxygen and more carbon (i.e., w>0.5) or less carbon and more oxygen (i.e., w<0.5). For example, but not by way of limitation, one embodiment of the SiOC gate material is illustrated by 0.25<w<0.5. Another example embodiment is illustrated by way of example, but not by way of limitation, by 0<w<0.4, such as in a light detector having sensitivity including visible light. Still another embodiment is illustrated by way of example, but not by way of limitation, by 0.5<w<1.0, such as in a SiOC floating gate memory application having a shorter data charge retention time than a polysilicon gate. As described below, the SiOC composition w is selected as a predetermined value in order to tailor the barrier energy for particular applications. In one embodiment, the SiOC composition w is uniform over a particular integrated circuit die. In another embodiment, the SiOC composition w is differently selected at different locations on the integrated circuit die, such as by additional masking or processing steps, to obtain different device characteristics on the same integrated circuit die.

In one embodiment, an insulating layer, such as silicon dioxide (oxide) 114 or other insulating layer, is formed by chemical vapor deposition (CVD). Oxide 114 isolates gate 106 from other layers, such as layer 112. In another embodiment, gate 106 is oxidized to form at least a portion of oxide 114 isolating gate 106 from other layers such as layer 112. In one embodiment, for example, layer 112 is a polysilicon or other control gate in a floating gate transistor. According to techniques of the present invention, the floating gate transistor is used in an electrically erasable and programmable read-only memory (EEPROM) memory cell, such as a flash EEPROM, or in a floating gate transistor photodetector or imaging device, as described below. In these embodiments, gate 106 is floating (electrically isolated) for charge storage thereupon. The present invention offers considerable advantages to the known EEPROM techniques used for charge storage on floating gate 106. In another embodiment, for example, layer 112 is a metal or other conductive interconnection line that is located above gate 106.

The upper layers, such as layer 112 are covered with a layer 116 of a suitable insulating material in the conventional manner, such as for isolating and protecting the physical integrity of the underlying features. Gate 106 is isolated from channel 110 by an insulating layer such as thin oxide layer 118, or any other suitable dielectric material. In one embodiment, thin oxide layer 118 is a gate oxide layer that can be approximately 100 angstroms (.ANG.) thick, such as for conventional FET operation. In another embodiment, such as in a floating gate transistor, thin oxide layer 118 is a tunnel oxide material that can be approximately 50-100 .ANG. thick.

FIG. 1 illustrates generally, by way of example, a complementary metal-oxide-semiconductor (CMOS) compatible n-channel FET that includes an SiOC gate 106, which may be floating or electrically interconnected. In one embodiment, for example, the FET can be formed on substrate 108 using an n-well CMOS process for monolithic CMOS fabrication of n-channel and p-channel FETs on a common substrate. The invention includes both n-channel and p-channel FET embodiments. Thus, with appropriate doping, the FET of FIG. 1 can be a p-channel FET. The SiOC gate FETs are useful for any application in which conventionally formed polysilicon gate FETs are used, including both electrically driven and floating gate applications.

FIG. 2A illustrates generally the barrier energy at the interface between a polysilicon gate and an adjacent thin oxide layer (or other gate insulator). Silicon (monocrystalline or polycrystalline) has a bandgap energy of about 1.2 eV, and an electron affinity, .chi., of about 4.2 eV. Silicon dioxide (oxide) has an electron affinity, .chi., of about 0.9 eV. The electron affinities, .chi., of each of the polysilicon gate and adjacent thin oxide layer are measured with respect to the vacuum level 200, as illustrated in FIG. 2A. In the polysilicon gate, the electron affinity, .chi., is defined by the difference between the semiconductor conduction band edge 205 and the vacuum level 200. In the adjacent thin oxide layer, the electron affinity, .chi., is defined by the difference between the oxide conduction band 202 and the vacuum level 200. The difference between the polysilicon electron affinity of .chi..apprxeq.4.2 eV and the oxide electron affinity of .chi..apprxeq.0.9 eV results in a barrier energy at the Si-SiO.sub.2 interface between the thin oxide layer and the polysilicon gate of about 3.3 eV. In a memory application using a polysilicon floating gate, the 3.3 eV barrier energy results in long data charge retention times (estimated in millions of years at a temperature of 85 degrees C.). The 3.3 eV barrier energy results in large erasure voltages and long write and erase times, with the accompanying problems described above. In an imaging application using a floating gate, the 3.3 eV barrier energy requires relatively high energy photons (i.e., high frequency and short wavelength) for photoelectric emission of stored electrons from the floating gate.

In FIG. 2B, the gate material is approximately stoichiometric silicon carbide (SiC) having an electron affinity of .chi..apprxeq.3.7 eV. The electron affinity in the adjacent oxide gate insulator is .chi..apprxeq.0.9 eV. The resulting barrier energy is approximately 2.8 eV, which is lower than the 3.3 eV barrier energy obtained using a polysilicon gate. In a memory application using a floating gate, the 2.8 eV barrier energy results in shorter charge retention times than are obtained than in the case described with respect to FIG. 2A, together with smaller erasure voltages and shorter write and erase times. In an imaging application using a floating gate, the 2.8 eV barrier energy requires less photon energy (i.e., lower frequency and longer wavelength) to eject electrons from the floating gate than in the case described with respect to FIG. 2A.

The present invention includes SiOC gate 106, which has particular advantages over polysilicon or SiC gates used in floating and driven gate FETs. For example, the SiOC-SiO.sub.2 interface between gate 106 and thin oxide layer 118 provides a lower barrier energy than a Si-SiO.sub.2 interface or a SiC-SiO.sub.2 interface. Moreover, the SiOC composition w can be selected to tailor the SiOC-SiO.sub.2 barrier energy, such as between 2.8 eV and 0 eV, to suit the requirements of a particular application, as explained below.

FIG. 3 is a graph that illustrates generally electron affinity, .chi., as a function of bandgap energy. Silicon, Diamond (C), SiC, and SiO.sub.2 are illustrated, together with region 300 that illustrates the expected range of SiOC electron affinity 0.9 eV<.chi.<3.7 eV as the SiOC composition w varies between w.apprxeq.1 (i.e., approximately SiC) and w.apprxeq.0 (i.e., approximately SiO.sub.2). The SiOC-SiO.sub.2 interface barrier energy is given by the difference between the SiOC electron affinity 0.9 eV<.chi.<3.7 eV and the SiO.sub.2 electron affinity of 0.9 eV. At the SiOC-SiO.sub.2 interface, the resulting barrier energy can therefore be established at a value approximately between 2.8 eV and 0 eV by selecting a predetermined value of the SiOC composition w.

FIG. 4 is a graph illustrating generally barrier energy versus tunneling distance for SiOC and conventional polysilicon gates. The SiOC electron affinity provides a smaller barrier energy than a polysilicon or even a SiC gate. The smaller SiOC barrier energy reduces the energy to which the electrons must be excited to be stored on the SiOC gate 106 by thermionic emission. The smaller barrier energy also reduces the distance that electrons stored on the gate have to traverse, such as by Fowler-Nordheim tunneling, to be stored upon or removed from the SiOC gate 106. The reduced tunneling distance allows easier charge transfer, such as while writing or erasing data in a floating gate transistor in a flash EEPROM memory cell. In FIG. 4, "do" represents the tunneling distance of a typical polysilicon floating gate transistor due to the barrier energy represented by the dashed line "OLD". The tunneling distance "dn" corresponds to a SiOC gate 106 and its smaller barrier energy represented by the dashed line "NEW". Even a small reduction in the tunneling distance results in a large increase in the tunneling probability, because the tunneling probability is an exponential function of the reciprocal of the tunneling distance.

Floating Gate Memory Device

In one embodiment, the invention provides a memory device including an SiOC floating gate transistor. Floating gate 106 can be programmed, by way of example, but not by way of limitation, by providing about 12 volts to control gate 112, and providing about 6 volts to drain 104, and providing about 0 volts to source 102. This creates an inversion layer in channel region 110, in which electrons are accelerated from source 102 toward drain 104, thereby acquiring substantial kinetic energy. High energy "hot electrons' are injected through thin oxide layer 118 onto the SiOC floating gate 106. Floating gate 106 accumulates the hot electrons as stored data charges.

The change in the charge stored on floating gate 106 changes the threshold voltage of the n-channel floating gate FET of FIG. 1. When control gate 112 is driven to a read voltage during a read operation, the change in charge stored on floating gate 106 results in a change in current between drain 104 and source 102. Thus, detection of the change in charge stored on floating gate 106 by sensing drain-source current conductance advantageously uses the appreciable transconductance gain of the floating gate FET of FIG. 1. Either analog or digital data can be stored as charge on floating gate 106 and read back as a conductance between drain region 104 and source region 102.

In one embodiment, the SiOC composition w of the floating gate 106 is at a predetermined value 0<w<1 to establish the exact barrier energy at the SiOC-SiO.sub.2 interface between the SiOC floating gate and SiO.sub.2 or other adjacent gate insulator. The lower barrier energy and increased tunneling probability of the SiOC gate 106 advantageously provides faster programming and erasure times for floating SiOC gate transistors in flash EEPROM memories. This is particularly advantageous for "flash" EEPROMs in which many floating gate transistor memory cells must be erased simultaneously. The large charge that must be transported by Fowler-Nordheim tunneling during the erasure of a flash EEPROM typically results in relatively long erasure times. By reducing the tunneling distance and increasing the tunneling probability, the SiOC gate 106 reduces erasure times in flash EEPROMs.

The increased tunneling probability of the SiOC gate 106 also advantageously reduces the voltage required for writing and erasure of the SiOC floating gate transistor. Polysilicon floating gate transistors typically require complicated and noisy on-chip charge pump circuits to generate the large erasure voltage, which typically far exceeds other voltages required on the integrated circuit. The lower erasure voltages of the SiOC floating gate transistor are more easily provided by simpler on-chip circuits. Lower erasure voltages also reduce hole injection into the gate dielectric that can cause erratic overerasure, damage to the gate dielectric, and introduction of trapping states in the gate dielectric. Reducing the erasure voltage also lowers the electric fields, minimizing reliability problems that can lead to device failure, and better accommodating downward scaling of device dimensions.

Lowering the barrier energy also decreases the data charge retention time of the charge stored on the SiOC floating gate 106. Conventional polysilicon floating gates have a data charge retention time estimated in the millions of years at a temperature of 85 degrees C. Since such long data charge retention times are longer than what is realistically needed, a shorter data charge retention time can be accommodated in order to obtain the benefits of a smaller barrier energy. According to one aspect of the present invention, the SiOC composition w is selected to establish the particular data charge retention time. In one embodiment, the exact value of the SiOC composition w is selected to establish a barrier energy that is large enough to prevent electrons from being thermally excited over the barrier at high operating temperatures, such as at a temperature of 85.degree. C., as this could allow the stored data charges to leak from the floating gate over a long period of time. For example, the data charge retention time can be selected between seconds and millions of years by selecting the value of the SiOC composition w. For example, by selecting the SiOC composition w to provide data charge retention times on the order of seconds, then the memory device can be made to emulate a dynamic random access memory (DRAM). In another example, by selecting the SiOC composition w to provide data charge retention times on the order of years, then the memory device can be made to emulate a hard disk drive. According to one aspect of the present invention, one memory device provides different memory functions by selecting the SiOC composition w such as, for example, approximately between 0.5<w<1.0. In one embodiment, floating gate transistors having different SiOC compositions are provided on the same integrated circuit, thereby providing differently functioning memory cells (e.g., having different data charge retention times) on the same integrated circuit.

FIG. 5 is a conceptual diagram, using rough order of magnitude estimates, that illustrates generally how erase and retention times vary with the barrier energy for a particular value of erasure voltage at a particular temperature of 85.degree. C. The probability of thermal excitation and emission over or tunneling through the barrier is an exponential function of the barrier energy. A lower barrier energy provides exponentially shorter erase and retention times. The particular memory application requirements determine the needed memory retention time, whether seconds or years. From this memory retention time, the barrier energy required and the erase time for a particular voltage can be determined using an engineering graph similar to that of FIG. 5. In one embodiment, the SiOC composition w is selected to provide a retention time on the order of seconds or years, depending upon the function required for the memory device. According to one aspect of the present invention, for example, the memory device can emulate or replace DRAMs or hard disk drives by selecting the SiOC composition w to establish the appropriate data charge retention time.

FIG. 6 is a simplified block diagram illustrating generally one embodiment of a memory 600 system, according to one aspect of the present invention, in which SiOC gate FETs are incorporated. In one embodiment, memory 600 is a flash EEPROM, and the SiOC gate FETs are floating gate transistors that are used for nonvolatile storage of data as charge on the SiOC floating gates 106. However, the SiOC gate FETs can have electrically interconnected gates 106, and can be used in other types of memory systems, including SDRAM, SLDRAM and RDRAM devices, or in programmable logic arrays (PLAs), or in any other application in which transistors are used.

FIG. 6 illustrates, by way of example, but not by way of limitation, a flash EEPROM memory 600 comprising a memory array 602 of multiple memory cells. Row decoder 604 and column decoder 606 decode addresses provided on address lines 608 to access addressed SiOC gate floating gate transistors in the memory cells in memory array 602. Command and control circuitry 610 controls the operation of memory 600 in response to control signals received on control lines 616 from a processor 601 or other memory controller during read, write, and erase operations. Voltage control 614 is provided to apply appropriate voltages to the memory cells during programming and erasing operations. It will be appreciated by those skilled in the art that the memory of FIG. 6 has been simplified for the purpose of illustrating the present invention and is not intended to be a complete description of a flash EEPROM memory.

Floating Gate Imaging Device

According to another aspect of the present invention, the SiOC floating gate transistor of FIG. 1 is used in light detection applications, such as a photodetector or imaging device. In this embodiment of the invention, light is detected by the absorption of photons by the SiOC floating gate 106. This is distinguishable from other types of imaging devices, such as sensors using a charge-coupled device (CCD) or a photodiode detector, in which light is absorbed in the semiconductor substrate, thereby producing charge carriers that are detected.

According to one embodiment of the present invention, charge is stored on the SiOC floating gate 106, such as by known EEPROM charge storage techniques. The imaging device is exposed to incident light. Incident photons having enough energy to eject an electron by photoelectric emission from floating gate 106 are detected by a resulting change in drain-source conductance of the imaging device. Thus, the light detector of the present invention advantageously utilizes the appreciable transconductance gain of the floating gate transistor. In one embodiment of the invention, the wavelength to which the light detector is sensitive is established by selecting the SiOC composition w of floating gate 106.

FIG. 7 is a cross-sectional schematic diagram of the floating gate transistor that illustrates generally its application according to the present invention as a light detector or imaging device. In FIG. 7, floating gate 106 is charged by the injection of hot electrons 700 through thin oxide layer 118 under the SiOC floating gate 106. This change in charge on floating gate 106 changes the threshold voltage of the n-channel floating gate FET. As a result, when control gate 112 is driven to a read voltage during a read operation, a large change in drain-source current is obtained through the transconductance gain of the floating gate transistor.

FIG. 8 is a cross-sectional schematic diagram that illustrates generally how incident light 800 is detected by the absorption of photons by floating gate 106. The photons must have enough energy to cause electrons 700 stored on floating gate 106 to overcome the barrier at the interface between floating gate 106 and thin oxide layer 118 and be ejected from floating gate 106 back into the semiconductor or SOI substrate by the photoelectric effect. A small electric field in thin oxide layer 118, such as results from the presence of electrons 700 stored on floating gate 106, assists in emission of electrons 700 toward substrate 108. Detection or imaging of visible wavelengths of incident light 800 requires a low electron affinity floating gate 106. The present invention allows the electron affinity of floating gate 106 to be tailored by selecting the particular value of the SiOC composition of floating gate 106.

FIG. 9 is a graph illustrating generally barrier energy versus tunneling distance, and illustrating the absorption of light energy by the floating gate. In FIG. 9, incident photons impart sufficient energy to electrons 700 stored on floating gate 106. The electrons 700 can therefore overcome the "new" barrier 900, which represents the barrier energy at an SiOC-SiO.sub.2 interface. As a result, electrons 700 are ejected from floating gate 106 back toward the semiconductor or SOI substrate 108, thereby discharging floating gate 106. "Old" barrier 905, which represents the barrier energy at a Si-SiO.sub.2 interface, is higher than "new" barrier 900 of the SiOC-SiO.sub.2 interface. As a result, a light detector having an SiOC floating gate 106 is sensitive to lower energy photons than a light detector having an polysilicon (Si) floating gate. Similarly, a light detector having an SiOC floating gate 106 is sensitive to lower energy photons than a light detector having a SiC floating gate.

In one embodiment, SiOC floating gate 106 is doped n-type to maximize the number of conduction band electrons 700 in floating gate 106, thereby increasing the absorption of incident light by photoelectric emission. Visible light has a photon energy of about 2 eV. For detection of visible light, the barrier energy at the interface between floating gate 106 and thin oxide layer 118 should be less than or equal to about 2 eV. However, most common gate materials have larger barrier energies at an interface with an adjacent silicon dioxide insulator. For example, a conventional polysilicon floating gate 106 results in a barrier energy of about 3.3 eV.

According to one aspect of the present invention, SiOC is used as the material for floating gate 106. The SiOC composition w is selected for sensitivity to particular wavelengths of light, and the barrier energy is established accordingly. For example, in one embodiment, the SiOC composition w is selected in the range approximated by 0<w<0.4 such that barrier energy is less than or equal to about 2 eV. As a result, the floating gate transistor light detector's sensitivity includes visible light. According to another aspect of the invention, the floating gate transistor light detector is made sensitive to different portions of the light spectrum by adjusting the barrier energy through the selection of the SiOC composition w. The SiOC composition w can also be different for different floating gate transistors, such as on the same integrated circuit, in order to yield different sensitivities to different wavelengths of light.

FIG. 10 illustrates generally how the above-described absorption of incident photons in the SiOC floating gate 106 by photoelectric emission is distinguishable from, and independent of, valence-to-conduction band electron transitions, which is the common photon absorption mechanism of most diode or CCD photodetectors or imaging devices. Light detection by conventional photon absorption is illustrated by the band-to-band electron energy transition 1000. Light detection by photon absorption according to the present invention is illustrated by the photoelectric emission 1005 of a conduction band electron 700 from floating gate 106 over the barrier 900 between the floating gate conduction band 1010 and oxide conduction band 1015.

The semiconductor bandgap is defined by the energy difference between the semiconductor conduction band 1010 and semiconductor valence band 1020. Exciting an electron from a valence band 1020 low energy state to a conduction band 1010 high energy state requires absorption of an incident photon of energy exceeding the bandgap of the semiconductor material. For the large bandgap of SiOC, these band-to-band transitions occur only at very high photon energies, such as for ultraviolet light. By contrast, photoelectric emission 1005 of electrons from the SiOC floating gate 106 only requires that the incident photon energy exceed the barrier energy 900 between floating gate 106 and thin oxide layer 118. Since the present invention allows the barrier energy 900 to be less than the 2 eV energy of a visible photon by an appropriate selection of the SiOC composition w, a wide spectrum of light detection is obtained.

In conventional photodetectors, only high energy photons are detected as the bandgap is increased (i.e., as the bandgap becomes larger, first red, then blue, and finally ultraviolet light is required for band-to-band photon absorption). According to the present invention, a larger bandgap typically results in a smaller barrier energy 900, thereby allowing detection of even lower energy photons as the bandgap is increased (i.e., as the bandgap becomes larger, the detector becomes sensitive not only to ultraviolet, but to blue, then red, and finally to infrared wavelengths). As a result, the present invention can be used for visible and infrared light detection and imaging, including camera-like operations, and can employ lenses, shutters, or other such known imaging techniques.

FIG. 11 is a graph that illustrates generally, by way of example, the band-to-band absorption coefficient as a function of wavelength and photon energy for several different materials. Band-to-band light absorption by Si, SiC, and SiOC materials are illustrated generally by lines 1100, 1105, and 1110, respectively, which require progressively higher photon energies. By contrast, photoelectric emission of electrons from floating gate 106, according to the present invention, requires progressively lower photon energies as the material composition is changed from Si to SiC to SiOC.

For example, red light has a photon energy of approximately 2 eV. Since the bandgap of SiOC is much higher than 2 eV, band-to-band photon absorption of red light is minimal, as seen in FIG. 11. However, the SiOC composition w is selected such that the barrier energy between SiOC floating gate 106 and adjacent thin oxide layer 118 is much less than 2 eV. In this embodiment, incident photons generate negligible electron-hole pairs or valence-to-conduction band transitions in floating gate 106. Instead, absorption of photons is substantially entirely the result of photoelectric emission of previously stored electrons from floating gate 106. While the quantum efficiency associated with the photoelectric effect can be low (e.g., less than one electron emitted per one hundred photons) the floating gate transistor offers appreciable transconductance gain. Emitting a single electron from the floating gate 106 changes the number of electrons flowing out of the drain 104 by thousands. By adjusting the SiOC composition w of floating gate 106, the floating gate light detector device can be adjusted for optimum response over almost the entire optical spectrum, from infrared through visible light to ultraviolet.

The SiOC gate 106 is doped to enhance its conductivity. In floating gate applications, the SiOC film need not be very conductive since it is not used for interconnection wiring. An SiOC floating gate 106 need only be sufficiently conductive to allow for redistribution of carriers in the floating gate 106. In one embodiment, the SiOC floating gate 106 is doped n-type, to increase the number of conduction band electrons stored on floating gate 106, thereby increasing the sensitivity of the photodetector.

Process

FIGS. 12A, 12B, 12C, 12D, 12E, 12F, and 12G illustrate generally examples of CMOS-compatible process steps for fabricating n-channel and p-channel SiOC gate FETs according to the present invention, including the fabrication of SiOC floating gate transistors. The transistors can be produced on a silicon or other semiconductor substrate, an SOI substrate, or any other suitable substrate 108. Only the process steps that differ from conventional CMOS process technology are described in detail.

In FIG. 12A, substrate 108 undergoes conventional CMOS processing up to the formation of the gate structure. For example, field oxide 1200 is formed for defining active regions 1202. In a bulk semiconductor embodiment, well regions are formed, such as for carrying p-channel transistors.

In FIG. 12B, an insulating layer, such as thin oxide layer 118 or other suitable insulator, is formed on substrate 108, such as by dry thermal oxidation, including over the portions of the active regions 1202 in which transistors will be fabricated. In one embodiment, thin oxide layer 118 is a gate oxide layer that can be approximately 100 angstroms (.ANG.) thick. In another embodiment, such as in a floating gate transistor, thin oxide layer 118 is a tunnel oxide material that can be approximately 50-100 .ANG. thick.

In FIG. 12C, a thin film 1206 of conductively doped polycrystalline or microcrystalline SiOC is then deposited, such as by chemical vapor deposition (CVD) over the entire wafer, including over thin oxide layer 118. The SiOC composition w of film 1206 is differently selected according to the particular barrier energy desired at the interface between the gate 106 and adjacent thin oxide layer 118, as described above.

The SiOC film 1206 can be in situ doped during deposition, or doped during a subsequent ion-implantation step. The conductive doping can be n-type or p-type. In one light detecting embodiment, the SiOC film 1206 is conductively doped n-type for enhanced photoelectric emission of electrons from floating gate 106 in response to incident light, as described above. In another embodiment, the SiOC film 1206 is conductively doped p-type using a boron dopant, which advantageously diffuses from the SiOC gate 106 less easily than from a polysilicon gate during subsequent thermal processing steps.

In one embodiment, for example, SiOC film 1206 is deposited using a Two Consecutive Decomposition and Deposition Chamber (TCDDC) system, providing the structure illustrated in FIG. 12C. One such example of depositing SiOC, in the unrelated technological application of solar cells, is disclosed in an article by R. Martins et al., entitled "Transport Properties of Doped Silicon Oxycarbide Microcrystalline Films Produced By Spatial Separation Techniques," Solar Energy Materials and Solar Cells, Vol. 41-42, pp. 493-517, June 1996. See also an article by R. Martins et al., entitled "Wide band-gap microcrystalline silicon thin films," Diffusion and Defect Data Part B (Solid State Phenomena), Vol. 44-46, Pt. 2, pp. 299-346, 1995.

SiOC film 1206 can also be deposited using other techniques such as, for example, low pressure chemical vapor deposition (LPCVD), or enhanced CVD techniques known to those skilled in the art including low pressure rapid thermal chemical vapor deposition (LP-RTCVD). The conductivity of the SiOC film 1206 can be changed by ion implantation during subsequent process steps, such as during the self-aligned formation of source/drain regions for the n-channel and p-channel FETs.

In FIG. 12D, SiOC film 1206 is patterned and etched, together with the adjacent thin oxide layer 118, to form SiOC gate 106. SiOC film 1206 is patterned using standard techniques and is etched using plasma etching, reactive ion etching (RIE) or a combination of these or other suitable methods. The etch rate of SiOC film 1206 can be significantly increased by using magnetron enhanced RIE.

FIG. 12E illustrates one embodiment in which SiOC gate 106 is oxidized after formation, providing a thin layer 1210 represented by the dashed line in FIG. 12E. SiOC gate 106 can be oxidized, for example, by plasma oxidation similar to reoxidation of polycrystalline silicon. During the oxidation process, the carbon is oxidized as carbon monoxide or carbon dioxide and vaporizes, leaving the thin layer 1210 of silicon oxide over SiOC gate 106. In one embodiment, thin layer 1210 is used as, or as a portion of, an intergate dielectric between floating and control gates in a floating gate transistor embodiment of the present invention.

FIG. 12F illustrates generally a self-aligned embodiment of the formation of n-channel FET n+ source region 102 and drain region 104. For a p-channel FET, p+ source drain regions can be similarly formed. The doping of SiOC gate 106 can be changed by ion implantation, such as during the formation of n-channel FET or p-channel FET source/drain regions, or subsequently thereto. For example, a p-type SiOC film 1206 can be deposited, and its doping then changed to n+ by leaving SiOC gate 106 unmasked during the formation of the n+ source region 102 and drain region 104 for the n-channel FET.

FIG. 12G illustrates generally the formation of an insulating layer, such as oxide 114 or other suitable insulator, after formation of n-channel FET source region 102 and drain region 104. In one embodiment, oxide 114 is deposited over the upper surface of the integrated circuit structure using a standard CVD process. Oxide 114 isolates SiOC gate 106 from other gates such as, for example, an overlying or adjacent control gate layer 112 where SiOC gate 106 is a floating gate in a floating gate transistor. Oxide 114 also isolates SiOC gate 106 from any other conductive layer 112, such as polysilicon layers, gates, metal lines, etc., that are fabricated above or over SiOC gate 106 during subsequent process steps.

Conclusion

The invention includes a CMOS-compatible FET having a low electron affinity SiOC gate that is either electrically isolated (floating) or interconnected. The SiOC composition w is selected to provide the desired barrier energy at the SiOC-SiO.sub.2 interface, such as 0<w<0.4, or 0.5<w<1.0. In a flash EEPROM application, the SiOC composition w is selected to provide the desired programming and erase voltage and time or data charge retention time. In an imaging application, the SiOC composition w is selected to provide sensitivity to the desired wavelength of light. Unlike conventional photodetectors, light is absorbed in the floating gate, thereby ejecting previously stored electrons therefrom. Also unlike conventional photodetectors, the light detector according to the present invention is actually more sensitive to lower energy photons as the semiconductor bandgap is increased.

Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art will appreciate that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

Claims

What is claimed is:

1. A transistor comprising: a source region; a drain region; a channel region between the source and drain regions; and a gate separated from the channel region by an insulator, the gate formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w, wherein w is a value approximately between 0 and 1.0.

2. The transistor of claim 1, wherein w is a value that is selected to establish a desired barrier energy between the gate and the insulator.

3. The transistor of claim 1, wherein the value of w is approximately between 0 and 0.4.

4. The transistor of claim 1, wherein the value of w is approximately between 0.5 and 1.0.

5. The transistor of claim 1, wherein the value of the barrier energy is approximately between 0 eV and 2.8 eV.

6. The transistor of claim 5, wherein the value of the barrier energy is approximately between 0 eV and 2.0 eV.

7. The transistor of claim 1, wherein the insulator is formed of silicon dioxide.

8. The transistor of claim 1, wherein the transistor is formed in a substrate selected from the group consisting essentially of a bulk semiconductor substrate and a semiconductor-on-insulator substrate.

9. The transistor of claim 1, wherein the gate is formed of a material selected from the group consisting essentially of a polycrystalline silicon oxycarbide compound and a microcrystalline silicon oxycarbide compound.

10. A transistor comprising: a source region; a drain region; a channel region between the source and drain regions; an electrically isolated floating gate separated from the channel region by an insulator, the floating gate formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w, wherein w is a value approximately between 0 and 1.0; and a control gate, separated from the floating gate by an intergate dielectric.

11. The transistor of claim 10, wherein w is a value that is selected to establish a desired barrier energy between the floating gate and the insulator.

12. The transistor of claim 10, wherein the intergate dielectric is formed of silicon dioxide.

13. The transistor of claim 10, wherein w is selected to provide a desired charge retention time of the floating gate.

14. The transistor of claim 10, wherein the transistor operates as an electrically erasable and programmable read-only memory (EEPROM) cell.

15. The transistor of claim 10, wherein the transistor operates as an imaging device.

16. The transistor of claim 10, wherein w is selected such that the floating gate absorbs incident photons and emits stored electrons in response to the absorbed incident photons.

17. The transistor of claim 16, wherein the floating gate is sensitive to incident photons of a wavelength that is established by the predetermined value w.

18. The transistor of claim 16, wherein the emission of electrons from the floating gate in response to incident photons changes a current conductance between the source and drain regions.

19. An imaging device comprising: a source region; a drain region; a channel region between the source and drain regions; and a floating gate separated from the channel region by an insulator, the floating gate formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w, wherein w is a value approximately between 0 and 1.0 that is selected to establish a desired barrier energy between the floating gate and the insulator for emission of charge from the floating gate in response to absorbed incident photons.

20. The device of claim 19, further comprising a control gate located adjacent to the floating gate and separated therefrom by an interlayer dielectric.

21. The device of claim 19, wherein the value of w is approximately between 0 and 0.4.

22. The device of claim 19, wherein the value of the barrier energy is approximately between 0 eV and 2.8 eV.

23. The device of claim 19, wherein the w is selected to establish an incident photon wavelength to which the floating gate is sensitive.

24. The device of claim 19, wherein the emission of electrons from the floating gate in response to incident photons changes a current conductance between the source and drain regions.

25. A memory device comprising: a plurality of memory cells, wherein each memory cell includes a transistor comprising: a source region; a drain region; a channel region between the source and drain regions; a floating gate separated from the channel region by an insulator, the floating gate formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w, wherein w is a value approximately between 0 and 1.0 that is selected to establish a desired barrier energy between the floating gate and the insulator; and a control gate located adjacent to the floating gate and separated therefrom by an interlayer dielectric.

26. The memory device of claim 25, further comprising: a row decoder; a column decoder; a voltage control circuit; and a command and control circuit.

27. The memory device of claim 26, further comprising a processor for providing address and control signals for selectively accessing particular ones of the memory cells.

28. The device of claim 27, wherein the value of w is approximately between 0.5 and 1.0.

29. The device of claim 27, wherein the value of the barrier energy is approximately between 0 eV and 2.8 eV.

30. The device of claim 27, wherein a charge retention time of the floating gate is established by the predetermined value w.

31. The device according to claim 27, wherein the memory device is a flash electrically erasable and programmable read only memory (EEPROM) device.

32. A method using a transistor having a floating gate formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w, the method comprising the steps of: selecting w at a predetermined value approximately between 0 and 1.0 to establish a value of a barrier energy between the floating gate and an insulator; storing data as charge on the floating gate; reading data by detecting a conductance between source and drain regions of the transistor, wherein the conductance varies based on the stored data charge on the floating gate; and erasing data by removing charge from the floating gate.

33. The method of claim 32, wherein w is selected to provide a value of the barrier energy that is approximately between 0 eV and 2.8 eV.

34. The method of claim 32, wherein w is selected at a value that is approximately between 0.5 and 1.0.

35. The method of claim 32, wherein the gate is a floating gate and w is selected to provide a desired charge retention time of the floating gate.

36. The method of claim 35, wherein the charge retention time is approximately between 1 second and 10.sup.6 years, as determined by the selected value of w.

37. A method using a transistor having a floating gate formed of a silicon oxycarbide compound SiO.sub.(2-2w) C.sub.w3 the method comprising the steps of: selecting w at a value approximately between 0 and 1.0 to establish a wavelength of incident light absorption to which the floating gate is sensitive; storing charge on the floating gate; receiving incident light at the floating gate, thereby removing at least a portion of the stored charge from the floating gate by the photoelectric effect; and detecting a charge in conductance between the transistor source and drain.

38. The method of claim 37, wherein w is selected to provide sensitivity to infrared light.

39. The method of claim 37, wherein w is selected to provide sensitivity to visible light.

40. The method of claim 37, wherein w is selected to provide sensitivity to ultraviolet light.

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