Ultra-linear multi-channel field effect transistor

[0001] (1) Field of the Invention

[0002] This is a continuation-in-part application of U.S. patent application Ser. No. 09/618,884, filed Jul. 18, 2000, and amended in Jan. 11, 2002. This invention relates to field effect transistors, in particular to a multiple channel, ultra-linear field effect transistors.

[0003] (2) Description of the Related Art

[0004] A field effect transistor (FET) normally is a square-law device. In the current saturation of the drain current (I_D) vs gate-to-source voltage (V_GS) of the characteristic of a FET, the basic relationship is given as:
I_D=K(V_GS−Vt)²  (1)
where K is a transconductance parameter, and Vt is the threshold voltage. When an input voltage is applied as a change in V_GS, the output current, which appears as the change in ID, does not vary linearly with V_GS. This square-law relationship causes non-linearity (harmonic distortion) and limits the dynamic range of amplifiers.

[0005] In equation (1), the threshold voltage Vt is assumed to be constant, based on a uniform impurity concentration N(x) of the semiconductor substrate. This threshold voltage is the voltage required to creating a maximum depletion layer in the substrate after strong inversion.

[0006] When the substrate concentration is not uniform, equation (1) must be modified, it was revealed by R. A. Pucel in a paper “Profile design for distortion reduction in microwave field-effect transistors” Electronic Letters, vol. 14, P. 204, 1978, that the I_D can be characterized in terms of the non-linear distortion components as:
ID=Ido+Gm⁽⁰V_GS+½gm⁽¹⁾V_GS²+⅙gm⁽²⁾V_GS³+ . . .   (2)
where gm⁽ⁿ⁾ V_GSⁿ is the nth order distortion and gm(n) is the transconductance and its derivatives with respect to the gate voltage. Linear device operation occurs at signal levels (V_GS˜0) or when the higher order terms have been eliminated. Signal distortion in amplifiers is due to the higher order terms, which become important at high signal levels.

[0007] To improve the linearity of a FET, the transconductance must be constant with varying gate voltage V_GS. The transconductance is strongly dependent on the electron distribution in the channel of the FET. Thus the design of a linear transistor demands paying attention to carrier distribution.

[0008] A measure of the linearity of a FET is the third order intermodulation ratio (IMR), which is the ratio of the power generated in a spurious third order intermodulation signal relative to the power in the dc desired signal. IMR has been shown to be proportional to the ratio of the third order component relative the first order (fundamental) component in Equation (2). Further more, Pucel has shown that this ratio can be expressed as:
IMR∝|gm⁽²⁾/gm⁽⁰⁾|=(κε_o/q)²|d/dx(1/x³N(x)|/N(x)  (3)
where N(x) is the charge distribution in the channel, κ is the relative dielectric constant, εo is the permittivity of free space, q is electronic charge and equation (3) is evaluated at the depletion edge boundary. The requirement for a highly linear device is for IMR to be small (close to 0). Therefore, from equation (3), either x³ N(x) is constant or x³ tends to infinity. The former occurs when N(x) varies as 1/x³. In the U.S. patent application Ser. No.09/618,884, we proposed to implement such a doping profile using multi-channel to achieve linearity in Hetrojunction Field Effect Transistors (HFETs). However, the invention was limited to specific doping profile in selected semiconductor materials such as InP and GaAs. Further, the lower mobilities of the HFET limit its applications to lower frequencies.

[0009] In a standard single channel High Electron Mobility Transistors (HEMTs), higher mobilities and sheet concentrations can be achieved as compared to HFETs. Therefore HEMT devices offer higher breakdown voltages and cut-off frequencies, low noise and higher power. There are numerous prior arts on single conducting channel HEMTs and its application to low noise and power applications. In U.S. Pat. No. 6,121,641 Ohno addressed the shielding of traps and suppression of short channel effect by insertion of p-type layer in a single conducting channel HEMT, and U.S. Pat. No. 5,767,539, Onda disclosed a field effect transistor having various designs for donor supply layer but with only one channel. Further, in the U.S. patents to Mishima (No. 5,633,516 ), to Nakayama (No. 5,856,685), to Hida (No. 6,049,097), and to Matloubian (No. 5,663,583) variations to the single channel HEMT such as; two charge supply layer on either side of the channel layer, different material for spacer, various compositions of buffer and or channel have been proposed. However, there is no provision that the device can improve the linearity of the drain characteristic. U.S. Pat. No. 5,739,559 (the '559) to Isheda et al discloses a HEMT device having improved linearity. The patent '559 relates to a HEMT having asymmetrical carrier supply layers sandwiching a channel layer. More specifically, the heterojunction barrier height between the lower carrier supply layer and the channel layer is greater than the heterojunction barrier height between the upper carrier supply layer and the channel layer. Further, the patent '599 discusses only a single channel HEMT and silent regarding a HFETs in general, or having multiple channels in HFET and HEMT, or any specific carrier concentration and type of doping, or different materials for the multiple channels.

[0010] U.S. Pat. No. 6,015,981 to Gluck (“the patent '981”) relates to a HFET having high modulation efficiency. FIG. 1, shows the cross-sectional view of the patent '981. As this figure illustrates, the HFET includes supply layer 3 having a doping layer 8, a layer packet 4 having multiple channels 41 and, and a supply layer 5 having a doping layer 9. The packet 4 has at least two repetitions of undoped channel 41 and undoped layer 42. The doping layers 8 and 9 serves to provide carrier concentration in the channels and doping layer 9 is characterized as having only a small number or no charge carriers. More specifically the patent '981 discloses a single doping layer 8 below the packet 4. Further the uniform doping is proposed which increases the gate to channel distance degrading the FET performance. As regards the electron concentration profile for the multiple channels HFET, the patent '981 has limited disclosure and teaches only concentration increases with distance from the gate. Further the patent is silent regarding different materials among the various multiple channels and teaches only Si/SiGe materials with GaAs/AlGaAs as an alternate material system.

[0011] Thallium (Tl) compounds of varying composition can be lattice matched to GaAs, InP and InAs. High electron mobilities have been predicted for TlP, TlAs, and TlSb (Schilfgaard et al, Applied Physics Letter, Vol. 65, pp 2714, 1994). Therefore FET with thallium compounds channels has the advantage of achieving highest gm, and cut-off frequencies exceeding the current state-of-the-art. Recently Tl compounds (U.S. Pat. No. 5,841,156 and references therein) have been proposed as detector, FET and HBT material. However, multi-channel HEMT or HFET for linearity has not been proposed.

[0012] An object of this invention is to obtain a linear dc I_D variation as a function of V_Gs and a constant transconductance variation in the characteristics of a FET over a wide range of V_GS. Another object of this invention is to obtain a sharp impurity gradient in the channel of a FET. Still another object of this invention is to apply the multi-channel concept to other semiconductor material systems such as TlInP, TlGaInP, InAs, InAsSb etc selected from the Table 1. A further object of this invention is to optimize the impurity gradient in the channels of a HFET to obtain constant cut-off frequency variation over a wide range of V_GS.

[0013] These objects are achieved by using multiple channels for a FET. Alternate layers of doped and undoped different kinds of semiconductors form heterojunctions in the multiple channels. The heterojunctions confine the electrons in separate thin spikes. A number of spikes of different electron concentrations can result in a sharp overall electron concentration gradient such as 1/x³ electron concentration profile. Such an electron concentration gradient can result in a linear variation of drain current with gate voltage to obtain a wide dynamic range.

[0014] In the second embodiment of the invention is to use multi-channel HEMT structure to increase the dynamic range of a FET. Specifically, one of the objects of this invention is to optimize the impurity gradient in the channels of a FET to obtain constant cut-off frequency variation over a wide range of V_GS.

[0015] These objects are achieved by heterojunction FET having multiple two dimensional electron (or hole) gas channels formed by using alternate layers of narrow band gap and wide band gap different kinds of semiconductors (form heterojunctions) deposited on a substrate with a buffer layer. In the invention multiple two dimensional electron (or hole) channels are formed in narrow band gap semiconductor by inserting a thin planar or spikes doping in the multiple wide band gap semiconductor (charge supply) layers. The multiple number of spikes of different accept or/donor concentrations in the wide band gap semiconductor can result in any arbitrary overall hole/electron concentration gradient or specifically 1/x³ electron concentration profile. Such an electron concentration gradient (1/x³) can result in a linear variation of drain current with gate voltage to obtain a wide dynamic range. In the third embodiment of the invention is to implement a multi-channel Insulated Gate HFET and HEMT to further increase the breakdown voltage and dynamic range of a FET.

[0026] Table 1 shows the material structures of the multi-channel FET (HFET/HEMT) of the present invention for the first and second embodiment.

[0027] FIG. 2 shows the basic structure of multiple channel heterojunction field effect transistor of the present invention. The structure has a substrate 10 and an undoped buffer layer 12 deposited on the substrate. The multiple channel HFET is formed with alternate undoped barrier layers 12, 14, 16, 18 of a first kind of semiconductor and doped channel layers 13, 15, 17 of a second kind of semiconductor. The two kinds of different semiconductor material form heterojunctions in the multiple channels. Another layer 20 is used to cap the multiple channels. An optional oxidation barrier is inserted (e.g. to protect layers such as AlSb, AlAsSb) between the layer 19 and cap 20. A thin layer of undoped GaAs cap can be used to improve the uniformity of the threshold voltage of the HFET. Alternately doped GaInAs (or InAs) can be used as the cap, but requires recess etching before forming metal gate. Over the layer 20, the source 21, gate 22 and drain 23 are formed laterally. Ohmic contacts are sintered to form the source and drain, which make contacts to the multiple channels. Various combinations of the barriers, channels, buffers and substrates can be selected from the Table 1. The number of channels are more than two and the conductivity of the channels can be either n or p-type. Uniform or delta or spike or planar doping can be used for the channel. The FET structure can be designed to give optimum noise figure parameters (peak gm at 10% I_dsat), uniform gain, F_t, and F_max with gate voltage. By proper selection of charge density, peak in gm close to pinch-off and or zero gate voltage can be realized in multi-channel FET. In the narrow band gap layer quantum dots, (e.g. InAs in GaAs, TlP or TlAs in GaInAs or GaAs) and sub-channel (e.g. InAs in GaInAs) can be inserted to increase the carrier density and mobility. The substrate can be InP, GaAs, GaN, Si, SiC or other substrates on which the FET heterostructure is transferred by bonding or lift-off or heteroepitaxial growth.

[0028] An example of the multiple channels GaN/AlN HFET having n-type conducting is shown in FIG. 3. The doped GaN layers form quantum wells and the undoped AlN layers form barrier layers and confine the dopants (electrons) within their own doped layers without diffusing into a neighboring doped layers. Thus, spikes of high electron concentration can be formed to achieve any desirable impurity profile. Any combination of N based wide band gap, narrow band gap, buffer, and of the channel (as shown in Table 1) can be used to implement multiple channels HFET. The use of thinner doped channels allows reducing the gate to channel distance to improve the charge control.

[0029] While FIG. 3 shows four doped layers, the number of doped layers and their distribution (variation) with depth are not limited to these numbers of layers. While the preferred embodiments of the invention have been described, it will be apparent to those skilled in the art that various modifications may be made in the embodiments without departing from the spirit of the present invention. Such modifications are all within the scope of this invention.

[0030] In the second embodiment of the invention is to use multi-channel HEMT structure to achieve ultra-linear device. A conventional and generic single channel HEMT structure uses a buffer layer, channel layer, spacer layer, charge supply layer, Schottky barrier, cap, source, drain, and gate formed on a substrate. In the single channel HEMTs higher mobilities and sheet concentrations can be achieved as compared to HFETs. HEMT devices offer higher breakdown voltages and cut-off frequencies, low noise and higher power. In the prior art variations to the single channel HEMT such as; two charge supply layer on either side of the channel layer, different material for spacer, various compositions of buffer and or channel have been proposed. Our device design consists of multiple channels (2-DEG or 2-DHG) and the location and number of the conducting channel distinguish from the prior art. In the multi-channel HEMTs higher mobilities and sheet concentrations can be achieved as compared to HFETs, the first embodiment of the invention. Therefore the proposed devices can be used where higher breakdown voltages and higher operating frequencies, low noise and higher power are required.

[0031] A generic structure of the said device is shown in the FIG. 4. In the figure the buffer layer 31 comprises of any semiconductor material to reduce dislocations of the channel and substrates or also arising from the lattice mismatched layers. For example InP, AlInAs, AlAsSb, AlGaAsSb and GaAs, AlGaAs, GaInP, AlInP for the InP and GaAs substrates respectively. The buffer layers can be metamorphic or graded for lattice mismatched semiconductor layers. The conducting channels 34, 38, 42 are formed by creating a two dimensional electron gas (2 DEG) or hole gas (2 DHG) in the narrow band gap semiconductor. The channel can be comprised of a binary, ternary, or quaternary pseudomorphic or lattice matched compound semiconductor layers. The channels can also contain embedded quantum dots, sub-wells. For example GaInAs, InAsP, InP, GaInAsP, GaAsSb and GaAs, GaInAs, GaAsSb lattice matched or pseudomorphic materials as the channel layers for InP and GaAs based FETs. The wide band gap layers 33, 35, 37, 39, 41, 43, and 45 can be AlInAs, AlSbAs, AlGaAsSb, InP and AlGaAs, AlAs, AlInP, GaInP material system or any combinations for InP and GaAs based FETs respectively. Typical thicknesses of wide band gap barrier and the channel are 2-10 nm.

[0032] The spike dopings 32, 35, 40, and 43 can be Si, Sn, Te and Be, C for n and p-type FETs. Spike provides the donor or acceptor supply layer for the FET. Using the spike doping or delta or planar doping reduces the distance between the gate and the channels that need to be optimize for sub-micron devices. Further, the breakdown voltage of the multi-channel HEMT will be higher due to quantum confinement. In the FIG. 4, the spike doping and the position of the spike can be adjusted to give the required doping profile for the linearity. Layer 45 is used as a Schottky layer or Schottky enhancement layer.

[0033] The multi-channel HEMT is capped with doped or undoped layer 46, or in combination of oxidation barrier. The cap can be N+GaAs, N+GaInAs and the oxidation barrier can be AlInAs for AlAsSb. For example the optional oxidation barrier can be used to protect layers such as AlSb and AlAsSb. The doped GaInAs (or InAs) used as the cap requires recess etching before forming metal gate. A thin layer of undoped GaAs cap can be used to improve the uniformity of the threshold voltage of the HEMT. Over the layer 46, the source, gate and drain are formed laterally. Ohmic contacts are sintered to form the source and drain, which make contacts to the multiple channels. Specific layers that can be used to implement the linear device are shown in the Table 1. In the table multi-channel HEMTs can be implemented on InP, GaAs, and sapphire substrates using lattice matched structures or mismatched structures using metamorphic buffer to accommodate the lattice mismatch. The number of channels can be selected depending on the linearity requirement, or other applications. The conductivity of the channels can be either n or p-type. The HEMT structure can be designed to give optimum noise figure parameters (peak gm at 10% I_dsat), and uniform gain, F_t, and F_max with gate voltage. For example by proper selection of charge density, peak in gm close to pinch-off and or zero gate voltage can be realized in multi-channel HEMT (FIG. 5). The substrate can be InP, GaAs, GaN, Si, SiC or other substrates on which the FET heterostructure is transferred by bonding or lift-off or heteroepitaxial growth. The combination of the wide/narrow band gaps are selected from Table 1: GaAs/(AlGaAs, GaInP, AlAs,AlInP), (GaInAs, GaAsSb, TlGaP, TlGaInP)/(AlGaAs, AlAs, GaInP, A;InP), (GaInAs, GaAsSb, GaInAsP, AlGaInAs, InP) /(AlInAs, AlInAsSb, AlGaPSb, InP, AlGaAsSb, AlAsSb), (TlGaInP, TlInP, TlGaInAs)/(AlInAs, AlAsSb, AlInAsSb, AlGaAsSb), InAsP/(AlInAs, AlGaAsSb, InP, AlInAsSb), (InAs, InAsSb, GaInAsSb, TlInAs)/(AlGaAsSb, AlSb, AlAsSb, AlGaSb, AlInAsSb), InSb/AlInSb, GaN/AlGaN, GaInN/AlGaN, etc. and any other combination of binary, ternary, quaternary III-V semiconductor. Ultra-linear HEMT can be extended to IV—IV semiconductor (Si, Ge, Sn, C), II-VI semiconductors (ZnSe, ZnS, CdTe, CdS, etc) or combinations of III-V and IV-/IV (e.g. GaP/Si) or IV—IV and II-VI (e.g. ZnS/Si). The wide band gap semiconductor layer under the metal gate provides the necessary Schottky barrier height.

[0034] An example of multiple channel AlInAsSb/GaInAs HEMT structure is shown in FIG. 6, in which layers 31, 33, 35, 37, 39, 41, 43, and 45 is AlInAsSb and layers 33, 38, and 42 is GaInAs over a InP substrate 30. Spike doping of Si 32, 36, 40 and 44 is used as the donor layer in the wide band gap semiconductor. A second example of the multiple channels HEMT uses AlInAsSb/InAs pair formed on metamorphic buffer. The metamorphic buffer is used to accommodate the lattice mismatch between InP and InAs. The structure can be also formed on GaAs substrate using the metamorphic buffer. The structure can be capped with an undoped layer of GaAs to produce a high quality Schottky contact.

[0035] Thallium (Tl) compounds of varying compositions can be lattice matched to InP, GaAs, and InAs substrates. For example TlGaInP, TlInP and TlGaInAs can be used lattice matched to InP and TlGAP, TlGaInP to GaAs and TlInAs to InAs substrate. In this invention we also propose Tl compounds for realizing multi-channel HENG and HFET for linearity. FIGS. 7 and 8 shows the typical layered structures of the HFET and HEMT. The material structure is not limited to these layers, but any combination of the material TlAs, TlP, TlSb with GaInAs, GaAs, InP, InAs and GaSb on InP and GaAs substrate can be used for the doped or 2 DEG or 2-DHG channel. Various combinations of the barriers, spacers, channels and buffers can be selected from the Table 1.

[0036] In the third embodiment of the invention is to use multi-channel HEMT and HFET structures to create an insulated gate FET (IGFET) (shown in FIG. 10) to achieve ultra-linearity in the device. An insulated gate FET structure uses a buffer layer 41, multiple channels of HFET or HEMT 42, barrier layer 43, sub channel layer 44, cap 45, source 46, drain 48, and gate 47 formed on a substrate. The layer 45 can be thermally grown or deposited insulator such as SiO₂, Si₃N₄, Al₂O₃, AlN oxides of GaAs, AlAs (Al₂O₃), AlGaAs (Al₂O₃) and InP (ex: InPO₄). The sub channel layer 44 (e.g. Si, Ge, InP, GaInAs, InAs etc.) provides lower surface state density between semiconductor and dielectric and also gives an additional channel. The IGFET offers higher breakdown voltages compared to HFET or HEMT, therefore these devices can be used for high power applications.

[0037] A variation of IGFET is to use all undoped wide band gap/narrow band gap layers and implants 49 (n or p) self aligned with gate to accumulate electron or holes in the channels. For example Si/SiGe, InP/GaInAs, GaAs/AlGaAs, GaN/AlN etc pairs can be used. Further, by selective implantation of n and p, a complimentary FET can be implemented. A further variation of the IGFET is to use the Type II band line up pair such a GaSb/InAs, GaAsSb/GaInAs to form complimentary FETs by accumulating holes in GaSb (GaAsSb) and electrons in InAs (GaInAs). In these material system, a single implant (Si) can be used for n and p doping (for Sb>0.6). The combination of wide band gap/narrow band gap, barrier, and of the cap can be extended to other semiconductor.

[0038] While FIGS. 6, 7 and 8 shows three 2-DEG layers and four charge supply layers, the number of 2-DEG and spike doped layer and their distribution (variation) with depth are not limited to these numbers of layers. While the preferred embodiments of the invention have been described, it will be apparent to those skilled in the art that various modifications may be made in the embodiments without departing from the spirit of the present invention. Such modifications are all within the scope of this invention.

[0039]

			Channel Layer
		Barrier Layers	Lattice Matched or	Schottty Barrier
Substrate	Buffer	(Spacer)	Pseudomorphic	Layer
GaAs	Lattice	AlGaAs/GaInP/	GaAs,TlGaInP	AlGaAs/GaInP/
	Matched	AlInP/AlGaPSb	GaInAs, GaAsSb,	AlInP
			GaAsN, GaInAsN
InP	Lattice	AlInAs/AlGaAsSb/	GaInAs, GaAsSb,	AlInAs/AlGaAsSb/
	Matched	AlASb/AlInAsSb/	GaInAsP, TlGaInP,	AlInAsSb/
		InP/AlGaPSb	TlInP, TlGaInAs, InP,
			AlGaInAs, InAsP,
GaAs	Metamorphic	AlInAs/AlGaAsSb/	GaInAs, GaAsSb	AlInAs/AlGaAsSb/
		AlAsSb/AlInAsSb/	GaInAsP, TlGaInP,	AlInAsSb/
		InP/AlGaPSb	TlInP,TllGaInAs,
			InP, AlGaInAs
GaAs/	Metamorphic	AlGaSb/AlAsSb/	GaSb, Ga.9In.1Sb,	AlSb/AlGaSb/
InP		AlInAsSb/	InAs, InAsSb	GaAs
		AlGaAsSb
GaN	Lattice	AlGaN/AlN	GaN, GaAsN, InN,	AlGaN/AlN
	Matched		GaInN
Saphire/	Metamorphic	AlGaN/AlN	GaN, GaAsN, InN,	AlGaN/AlN
SiC/Si			GaInN