Semiconductor Stress Sensor MESFET or MESFET Array

1. Field of the Invention

The present invention relates to a semiconductor sensor, and more particularly to a semiconductor sensor for detecting an external force such as a pressure, an acceleration, or the like.

2. Description of the Relevant Art

It is known in the art from Japanese laid open patent publication No. 53-153537, for example, that when a stress is applied to a field-effect transistor made of a piezoelectric semiconductor such as GaAs, the drain current of the field-effect transistor varies.

The applicant has previously proposed a cantilever-type semiconductor sensor based on the above nature of a field-effect transistor, as disclosed in Japanese laid-open patent publication No. 2-194343.

FIG. 1 of the accompanying drawings shows, in perspective, the external physical structure of the proposed cantilever-type semiconductor sensor. FIG. 2 of the accompanying drawings shows the circuit arrangement of the proposed semiconductor sensor.

As shown in FIG. 1, the cantilever-type semiconductor sensor, generally designated by the reference numeral 101, comprises a semiconductor substrate 102 and a crystalline layer 103 deposited on the semiconductor substrate 102 by way of epitaxial growth. The crystalline layer 103 includes an FET (field-effect transistor) 104. The semiconductor substrate 102 includes a flexible region 105 of reduced thickness defined by a recess in the surface thereof remote from the crystalline layer 103.

The semiconductor substrate 102 includes a fixed region 106 on one side of the flexible region 105 and a weight region 108 on the other side of the flexible region 105. The flexible region 105 has an end 107 adjacent to the fixed region 105 and an end 110 adjacent to the weight region 108.

The electric circuit of the semiconductor sensor 101 includes the FET 104 for detecting a stress developed by an external force that is applied to the weight region 108 at its center of gravity 109, a current-to-voltage (I-V) converter 3 for generating a voltage output corresponding to a drain current ID from the FET 104, a pair of resistors R1, R2 for applying a gate bias voltage VG to the gate G of the FET 104, and a power supply VDD.

The FET 104 has a drain D connected to the power supply VDD. The gate bias voltage VG, which is produced by dividing the voltage from the power supply VDD with the resistors R1, R2, is applied to the gate G of the FET 104.

The current-to-voltage converter 3 comprises an operational amplifier 3a and a feedback resistor 3b. The FET 104 has a source S connected to an inverting input terminal 3c of the operational amplifier 3a. The operational amplifier 3a has a noninverting input terminal 3d grounded. The FET 104 is driven at a constant voltage with the potential difference between the input terminals 3c, 3d of the current-to-voltage converter 3 being substantially zero.

When a stress is applied to the FET 104, the drain current ID of the FET 104 varies, and the variation in the drain current ID is detected and outputted as a voltage output 3e by the current-to-voltage converter 3.

It has been confirmed by a simulation process that a strain or deformation which occurs in the crystalline layer 103 when an external force is applied perpendicularly to the weight region 108 is maximum at the end 107 of the flexible region 105, and is almost zero at the end 110 thereof.

Therefore, the FET 104, which is in the form of a single unit and has a small area, is positioned on the end 107 of the flexible region 105, as shown in FIG. 1, in order to maximize the variation in the drain current ID of the FET 104 thus positioned, i.e., the sensitivity of the semiconductor sensor 101, at the time an external force is imposed on the semiconductor sensor 101.

However, the FET 104 of the type described above is problematic in that the drain current ID suffers irregular fluctuations (noise) even when no stress is applied to the semiconductor sensor 101, and hence no sufficient signal-to-noise ratio is provided in a range of small stresses applied.

It is an object of the present invention to provide a semiconductor sensor which is of high sensitivity and has a high signal-to noise ratio.

According to the present invention, there is provided a semiconductor sensor comprising a semiconductor substrate, a linear array of field-effect transistors disposed on the semiconductor substrate at spaced intervals along a direction, the field-effect transistors having respective drains electrically connected parallel to each other, respective sources electrically connected parallel to each other, and gates electrically connected parallel to each other, means for applying a gate bias voltage to each of the field-effect transistors, and means for detecting a stress applied to the semiconductor substrate based on a change in a combined output of the field-effect transistors.

According to the present invention, there is also provided a semiconductor sensor comprising a semiconductor substrate, a single field-effect transistor disposed on the semiconductor substrate, the field-effect transistor having a comb-shaped drain, a comb-shaped source, and a comb-shaped gate, the comb-shaped drain, source, and gate having respective teeth spaced along a direction, means for applying a gate bias voltage to the field-effect transistor, and means for detecting a stress applied to the semiconductor substrate based on a change in an output of the field-effect transistor.

According to the present invention, there is further provided a semiconductor sensor comprising a semiconductor substrate, an annular array of comb-shaped field-effect transistors disposed on the semiconductor substrate at spaced intervals along a circumferential direction, the field-effect transistors having respective drains electrically connected parallel to each other, respective sources electrically connected parallel to each other, and gates electrically connected parallel to each other, means for applying a gate bias voltage to each of the field-effect transistors, and means for detecting a stress applied to the semiconductor substrate based on a change in a combined output of the field-effect transistors.

The above and further objects, details and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof, when read in conjunction with the accompanying drawings.

FIG. 1 is a perspective view of the external physical structure of a previously proposed semiconductor sensor;

FIG. 2 is a circuit diagram of the electric circuit of the previously proposed semiconductor sensor;

FIG. 3 is a circuit diagram of an electric circuit of a semiconductor sensor according to the present invention;

FIG. 4(a) is a plan view of the external physical structure of a cantilever-type semiconductor sensor according to an embodiment of the present invention;

FIG. 4(b) is a side elevational view of the external physical structure of the cantilever-type semiconductor sensor shown in FIG. 4(a);

FIGS. 5(a) and 5(b) show the relationship between FETs, strains or sensitivity, and distances from an end of a flexible region adjacent to a fixed region, with the cantilever-type semiconductor sensor shown in FIG. 4(a);

FIG. 6 is a graph showing the relationship between the numbers of parallel-connected FETs, sensitivities, and signal-to-noise ratios with the cantilever-type semiconductor sensor shown in FIG. 4(a);

FIG. 7(a) is a plan view of the external physical structure of a cantilever-type semiconductor sensor according to another embodiment of the present invention;

FIG. 7(b) is a side elevational view of the external physical structure of the cantilever-type semiconductor sensor shown in FIG. 7(a);

FIG. 8(a) is a plan view of the external physical structure of a diaphragm-type semiconductor sensor according to still another embodiment of the present invention; and

FIG. 8(b) is a side elevational view of the external physical structure of the diaphragm-type semiconductor sensor shown in FIG. 8(a).

Like or corresponding parts are denoted by like or corresponding reference characters throughout views.

FIG. 3 shows the electric circuit of a semiconductor sensor according to the present invention.

As shown in FIG. 3, a semiconductor sensor 1 comprises a plurality of FETs 2a.about.2n having drains, sources, and gates electrically connected parallel to each other, a current-to-voltage (I-V) converter 3 for producing a voltage output corresponding to a drain current ID from the FETs 2a.about.2n, a pair of resistors R1, R2 for applying a gate bias voltage VG to the gates Ga.about.Gn of the respective FETs 2a.about.2n, and two power supplies VDD1, VDD2.

The FETs 2a.about.2n have respective drains Da.about.Dn electrically connected to the power supply VDD1 and respective sources Sa.about.Sn electrically connected to the current-to-voltage converter 3 to keep a constant voltage applied between the drains and sources of the FETs 2a.about.2n, so that the output signal of the semiconductor sensor 1 remains unaffected by changes in the ambient temperature around the semiconductor sensor 1.

The power supply VDD2 and the resistors R1, R2 are adjusted to control the potential between the gate and source of each of the FETs 2a.about.2n for selecting the gate bias potential VG in the vicinity of the threshold voltage of the FETs 2a.about.2n thereby making the semiconductor sensor 1 highly sensitive.

When a stress is applied to the FETs 2a.about.2n, the respective drain currents IDa.about.IDn thereof vary depending on the applied stress, and the combined drain current ID is converted by the current-to-voltage converter 3 into a voltage output 3e that is available as an output signal of the semiconductor sensor 1.

FIGS. 4(a) and 4(b) show the external physical structure of a cantilever-type semiconductor sensor according to an embodiment of the present invention.

The semiconductor sensor 1 is of the cantilever type, and comprises an elongate semiconductor substrate 4 of GaAs which is etched on its reverse side in a final stage of fabrication, providing a thin flexible region 5, a fixed region 6 disposed on one side of the flexible region 5, and a weight region 7 disposed on the other side of the flexible region 5. The flexible region 5 has an end 5a adjacent to the fixed region 6, and an end 5b adjacent to the weight region 7. Although the flexible region 5 is defined by an opening defined completely through the substrate 4 in FIG. 4(b), such region could alternatively comprise a reduced thickness portion of the substrate 4 similar to the flexible region 105 in FIG. 1.

A layer 8 of AlGaAs is deposited on the semiconductor substrate 4 according to the MBE process. The AlGaAs layer 8 is thick enough to serve as a selective etching stopper layer when the reverse side of the semiconductor substrate 4 is etched to form the flexible region 5 according to a wet etching process based on jet polishing.

The etching solution used when the semiconductor substrate 4 is etched to form the flexible region 5 should preferably comprise a mixture of NH.sub.4 OH and H.sub.2 O.

Then, a semiconductor or P-type buffer layer 9 of GaAs is formed by way of crystalline growth on the AlGaAs layer 8 according to the VPE process. Thereafter, an N-type GaAs layer is formed as an active FET layer 10 by way of crystalline growth on the GaAs buffer layer 9 according to the VPE process.

The active FET layer 10 is then selectively etched away according to the wet etching process, thus providing a plurality of mesa active FET layers 10 in a range of about 2/3 of the full length L of the flexible region 5. The mesa active FET layers 10 are spaced at intervals in the longitudinal direction of the semiconductor substrate 4.

Thereafter, source, drain, and gate electrodes are formed on each of the active FET layers 10 according to the liftoff method.

The gate electrodes should preferably comprise a lower portion of titanium (Ti) and an upper portion of tungsten (W). The gate electrodes of such a structure are advantageous in that since the titanium adheres intimately to the active FET layers 10 and the tungsten has a large modulus of direct elasticity, the FETs thus formed are highly sensitive to stresses applied thereto.

Actually, an upper wiring layer and an insulating film are provided to connect the FETs parallel to each other, but are omitted from illustration for the sake of clarity.

The range in which the FETs are positioned will be described below with reference to FIGS. 5(a) and 5(b). FIGS. 5(a) and 5(b) show the relationship between the FETs, strains or sensitivity, and distances from the end of the flexible region adjacent to the fixed region, with the cantilever-type semiconductor sensor shown in FIG. 4(a).

When an external force is applied to the weight region 7 at its center of gravity 7G, a strain or deformation which occurs in the semiconductor substrate 4 is maximum at the end 5a of the flexible region 5, and is almost zero at the end 5b thereof.

The drain currents of the FETs disposed over the flexible region 5 vary depending on the magnitude of the strain of the semiconductor substrate 4. Therefore, a variation (hereinafter referred to as a sensitivity) which is developed in the drain current when a predetermined load is applied to the weight region 7 at the center of gravity 7G in the direction indicated by the large arrow decreases as the FET is shifted toward the weight region 7, as shown in FIG. 5(b).

Now, it is assumed that the FET which is located on the end 5a of the flexible region 5 has a sensitivity A, and that n FETs of substantially the same configuration and characteristics are disposed at substantially equal intervals within a range LF (FIGS. 5(a) and 5(b)) from the end 5a, the n FETs being connected parallel to each other. The total sensitivity S of the n FETs corresponds to the hatched area shown in FIG. 5(b). The area of the hatched trapezoidal shape is expressed by the following equation (1): ##EQU1##

In the above equation, the area of the hatching trapezoidal shape is determined using a solid line K1 that represents zero sensitivity at the end 5b of the flexible region 5 adjacent to the weight region 7. However, it is more accurate to determine the hatched area using a dotted line K2 that represents zero sensitivity at the center of gravity 7G.

Even when no stress is applied to the semiconductor sensor 1, the drain currents of the respective FETs vary irregularly, and such fluctuations (AC component) of the drain currents are produced as noise outputs from the FETs. The combined noise output is increased if the number of parallel-connected FETs is increased. Since the noise of each of the FETs is produced at random, the combined noise output is not simply equal to the multiple of the noise output of a single FET and the number n of FETs used, but the multiple of the noise output of a single FET and the square root of the number n.

Therefore, the combined signal-to-noise ratio of the n FETs that are located within the range or distance LF from the end 5a, as show in FIG. 5(a), is given by the following equation (2): ##EQU2##

Insofar as the number n of FETs is proportional to the distance LF, the number n may be replaced with LF. The equation (2) is modified into the following equation (3), which is then differentiated into the equation (4): ##EQU3##

Then, the range in which the FETs are disposed for a maximum signal-to-noise ratio is determined by equalizing the equation (4) to zero. More specifically, the righthand side of the equation (4) is equalized to zero, as indicated by the following equation (5): ##EQU4##

Both sides of the equation (5) are multiplied by the square root of LF, providing the following equation (6): ##EQU5##

The equation (6) is then modified into the following equation (7): ##EQU6##

Therefore, the signal-to-noise ratio is maximum if the FETs are disposed in the range which is approximately 2/3 of the full length L of the flexible region 5.

A specific arrangement of the FETs for the maximum signal-to-noise ratio will be described below with reference to FIGS. 5(a), 5(b), and 6 and the following table:

As shown in FIG. 5(a), a total of six FETs, including those indicated by the imaginary lines, each disposed in an area having a width of about L/6, are formed on the buffer layer 9.

The FETs, which are indicated respectively by Q1.about.Q6, produce respective noise output voltages (converted from respective currents) when no stress is applied, respective output voltages (converted from respective currents) when a predetermined stress is applied, and have respective sensitivities, as shown in the above table. The FET Q1 singly has a signal-to-noise ratio of 3. When the FETs Q1, Q2 are connected parallel to each other, their combined signal-to-noise ratio is 3.89 as indicated by the following equation (8): ##EQU7##

The combined signal-to-noise ratios of three, four, five, and six parallel-connected FETs are shown in FIG. 6.

As shown in FIG. 6, when the number of FETs used is increased, the increase in the combined sensitivity thereof is gradually reduced, and the combined noise is reduced in proportion to the square root of the number of FETs used. The signal-to-noise ratio is large if the FETs are disposed in the distance which is about 2/3 of the total length of the flexible region 5.

Since the semiconductor sensor 1 has a plurality of FETs connected parallel to each other, the output signals produced respectively by the FETs in response to an applied stress are added into a large output voltage. Since the combined noise level of the FETs is lower than the sum of noise levels of the respective FETs because of their irregularities, there is a resultant increase in the signal-to-noise ratio.

FIGS. 7(a) and 7(b) show the external physical structure of a cantilever-type semiconductor sensor according to another embodiment of the present invention.

In the embodiment shown in FIGS. 7(a) and 7(b), the semiconductor sensor, generally designated by the reference numeral 20, has a single FET 21 including a source S, a drain D, and a gate G each of a comb shape, for the detection of an applied stress. More specifically, the comb-shaped source S, the comb shaped drain D, and the comb-shaped gate G have respective teeth that are spaced in the longitudinal direction of the semiconductor substrate 4.

Since the single FET 21 is employed, the semiconductor sensor 20 does not require interconnections and electrode terminals which would otherwise be necessary to connect a plurality of FETs. The comb-shaped source S, drain D, and gate G of the single FET 21 are functionally equivalent to an array of small FET cells with respect to stresses applied, located within a desired range on the flexible region 5. With the single FET 21 being comb-shaped, and also with no interconnections and no electrode terminals being employed, more FET cells can be disposed in the desired range than if individual FETs were used. Therefore, the semiconductor sensor 20 has a high signal-to-noise ratio based on the aforesaid principle.

FIGS. 8(a) and 8(b) show the external physical structure of a diaphragm-type semiconductor sensor according to another embodiment of the present invention.

The semiconductor sensor, generally designated by the reference numeral 30, is designed for detecting an applied pressure. The semiconductor sensor 30 includes a diaphragm-shaped circular flexible region 31 having an radially outer end 31a that is spaced a distance or radius L from the center of gravity GD of the flexible region 31. A plurality of radial comb-shaped FETs that are circumferentially spaced are disposed in a range that is 2/3 of the distance L radially from the end 31a. The FETs have respective electrodes, and like electrodes are connected parallel to each other.

Each of the comb-shaped FETs may be of a sectorial configuration such that the gate is longer radially outwardly toward the outer circumference of the flexible region 31 and progressively shorter toward the center thereof, and the drain, source, and gate are disposed in arcuate areas. The sectorial comb-shaped FETs may extend fully circumferentially over the flexible region 31.

Each of the drain, source, and gate of each FET may alternatively be of a pattern composed of concentric elements.

While each of the FETs is formed on the substrate 4 of GaAs in the above embodiments, it may be formed on a silicon substrate. The active FET layers 10 may be of a piezoelectric semiconductor such as InP or the like.

The device configuration of each of the FETs is not limited to the mesa MESFET, but may be a planar thermally insulative gate MESFET or HEMT. The crystalline growth layers may be formed according to the MO-VPE process, rather than the MBE or VPE process.

Although there have been described what are at present considered to be the preferred embodiments of the invention, it will be understood that the invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description.