METAL-SEMICONDUCTOR FIELD EFFECT TRANSISTOR DEVICE

Abstract: A metal-semiconductor field effect transistor device comprises an active layer (24a) formed on a surface of a semiconductor substrate (25), a first gate electrode (20a) provided on the active layer so as to make a direct contact therewith, source and drain electrodes (22a, 23a) provided on the active layer at both sides of the first gate electrode (20a) so as to make ohmic contact with the active layer, a second gate electrode (21a, 21b) provided on said active layer between the first gate electrode and the source electrode and between the first gate electrode and the drain electrode, a terminal (10) for applying an input signal to the first gate electrode, and means (32) for applying a constant voltage to the second gate electrode (21a, 21b) in order to inhibit the formation of depletion regions in the active layer. A protective coating of silicon nitride (29) can further be provided over the entire structure. The device can be used in radio-frquency or microwave amplifiers.

Description: BACKGROUND OF THE INVENTION The present invention generally relates to so-called metal-semiconductor field effect transistors (MESFET) having a gate electrode which makes a Schottky contact with a semiconductor layer underneath, and more particularly to a high power MESFET device for operation in a high frequency region. A MESFET is a type of junction field effect transistor comprising a gate electrode, a source electrode and a drain electrode making contact with a semiconductor layer underneath, wherein the gate electrode makes Schottky contact and the source and drain electrodes make ohmic contact. Such a MESFET has a simple structure which is easily manufactured. Further, MESFET is suited for miniaturization as the gate length of such a device is easily decreased. A MESFET based on gallium arsenide (GaAs) which provides high electron mobility is used for various high power devices for high frequency use as well as high speed integrated circuits in the field of microwave or satellite telecommunications. In a typical MESFET structure, a semi-insulating semiconductor material such as intrinsic GaAs is used as a substrate and the GaAs substrate is doped to the n-type along its top surface to form an active layer in which a channel of electron is formed. On the active layer, there is provided a gate electrode so as to make Schottky contact therewith, and on the both sides of the gate electrode a source electrode and a drain electrode are provided so as to make ohmic contact with the active layer underneath. In such a MESFET structure, there is formed a depletion region in the active layer in correspondence to the gate electrode which limits the channel formed in the active layer. By applying a negative voltage to the gate electrode, the depletion region extends further into the active layer and the electrical current flowing through the channel from the drain electrode to the source electrode is reduced or shut down. By reducing the magnitude of the negative gate voltage or by applying a positive voltage to the gate electrode, on the other hand, extension of the depletion region in the active layer is reduced and the channel in the active layer is extended. Responsive thereto, the electrical current flowing through the channel is increased. In such a simple MESFET structure, there is formed another depletion region in the active layer in a surface part between the gate electrode and the source as well as between the gate electrode and the drain electrode as a result of surface states. Such a depletion region also limits the extension of the channel in the active layer and as a result, the maximum electrical current flowing through the channel is limited. It should be noted that such a depletion region is not controlled in contrast to the case of the depletion region formed underneath the gate electrode. Thus, the simple MESFET has a problem in that the maximum source-drain current is limited at a level lower than the level which the device is potentially capable of providing. In order to avoid this problem, there is proposed a MESFET structure having a recessed gate in which a groove is provided in the active layer between the source electrode and the gate electrode by isotropic etching, and the gate electrode is provided on the bottom of the groove thus formed. By providing the gate electrode at a level substantially lower than the level of the source and drain electrodes on the surface of the substrate, the channel extending between the source electrode and drain electrode passes through a part of the active layer away from its surface and the deteriorative effect of the depletion region at the surface of the active layer is avoided. However, formation of the groove in the active layer by etching is undesirable as it is difficult to control the extent of etching exactly. Further, such a process complicates the manufacturing of the device. It should be noted that the etching of the groove determines the thickness of the active layer which is critical to the operation of the device, especially the drain current of the device which is determined by the channel. Further, there is proposed another type of MESFET structure designed to avoid the deteriorative effect of the surface depletion region as described in Japanese Laid-open Patent Application No.58-50780 in which the surface of the active layer including the source, drain and gate electrodes is covered by an insulator layer and that another electrode is provided on the insulator layer so as to cover the latter uniformly. Referring to FIG.1 showing this prior art MESFET structure, the MESFET structure is formed on a substrate 1 of GaAs having a surface region 2 doped to the n-type. On the surface region 2, there are formed a pair of conductive layers 2a and 2b doped to the n⁺-type and an active layer 2c doped to the n-type. In correspondence to the layers 2a, 2b and 2c, metal electrodes 3a, 3b and 3c forming the source, drain and gate electrodes are deposited as usual. Further, an insulator layer 4 is deposited on the surface region 2 so as to cover the electrodes 3a - 3c and the active layer 2c. Furthermore, an electrode 5 is deposited so as to cover the insulator layer 4. According to this prior art MESFET structure, a depletion region 6 caused by the surface states of the active layer 2c is compensated or eliminated by applying a positive voltage to the additional electrode 5. Thus, one can eliminate the unnecessary limitation of the channel in the active layer 2c and the maximum source-drain current is increased. In this MESFET structure, however, there arises a problem in that a substantial junction capacitance appears between the gate electrode 3c and the electrode 5 as well as between the source or drain electrode 3a, 3b and the electrode 5 as a result of the use of the insulator 4. Any of these capacitances causes decrease in the gain of the device particularly in the high frequency range and the high frequency operational characteristic of the MESFET is deteriorated accordingly. Further, the provision of such an insulator layer tends to invite unstable operation of the device such as oscillation particularly when silica (SiO₂) or other oxide is used as the insulator layer. In such a case, oxygen in the SiO₂ layer 4 tends to penetrate into the GaAs active layer 2c and form a deep impurity level. Further, the provision of the insulator layer 4 is not easy as compared to the case of silicon transistors as the layer 4 cannot be formed by simple oxidation of the surface of the substrate. When the insulator layer 4 is formed by deposition of silicon nitride (Si₃N₄) which is another material commonly used for insulating layer, on the other hand, there is a problem of mechanical stress accumulated in the layer 4 particularly when the thickness of the layer 4 is increased. When the thickness of the layer 4 is decreased so as to decrease the mechanical stress, on the other hand, the capacitance across the electrodes is increased. Such an increase in the capacitance, especially those across the gate electrode 3c and the electrode 5 significantly deteriorates the operation of the MESFET at high frequency region. In a commonly used Si₃N₄ layer of about 100 nm (1000 Å) thickness, there is accumulated a stress which is ten times as large as the stress caused in the corresponding SiO₂ layer. Further, the structure of FIG.1 is inconvenient for connection of wiring conductors to the gate, source or drain electrodes as these electrodes are located underneath the electrode 5. SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide a novel and useful metal-semiconductor field effect transistor device wherein the aforementioned problems are eliminated. Another and more specific object of the present invention is to provide a high power metal-semiconductor field effect transistor device in which device-by-device fluctuation of maximum current capable of flowing through the device is eliminated. Another object of the present invention is to provide a high power metal-semiconductor field effect transistor device wherein decrease of gain in a high frequency range is eliminated. Another object of the present invention is to provide a metal-semiconductor field effect transistor having a high withstand voltage. Another object of the present invention is to provide a metal-semiconductor field effect transistor device having a first gate electrode for application of an input signal provided on an active surface layer of a semiconductor substrate so as to make a Schottky contact therewith, wherein a second and third gate electrodes are provided similarly on the active surface layer at both sides of the first gate electrode such that a surface region extending between the first gate electrode and a drain electrode and between the first gate electrode and a source electrode is covered by these second and third electrodes and that there is further provided biasing means for applying a biasing d.c. voltage to said second and third electrodes such that a depletion region formed in the active layer underneath these electrode is eliminated. According to the present invention, the depletion region formed in the active layer between the first gate electrode and the source electrode or the drain electrode and causing decrease in the channel region is effectively suppressed or eliminated. Thus, the device of the present invention provides a large source-drain current. Another feature of the device of the present invention is that the second and third gate electrodes make a direct contact with the active layer similarly to the case of the first gate electrode. In other words, there is no insulator or dielectric layer between the active layer and the second or third electrode in contrast to the case of the prior art device. Thus, parasitic capacitance formed across the first electrode and the second or third electrode is eliminated and the problem of decrease in gain at a high frequency range pertinent to such a prior art device is successfully eliminated. Further, the device of the present invention provides a high withstand voltage as the second gate electrode to which a small positive voltage is applied is interposed between the first gate electrode and the drain electrode to which a large drain voltage is applied. Furthermore, the device of the present invention is suitable for use of a thin protective film. The parasitic capacitance decreases with decreasing thickness of the protective film and the device is particularly suited for use of the commonly used silicon nitride protective film having a thickness of about 100 nm (1000 Å) or less. Other objects and further features of the present invention will become apparent from the following description for preferred embodiments when read in conjunction with attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a cross sectional view showing a prior art MESFET device; FIG.2 is a plan view showing a first embodiment of the MESFET device of the present invention; FIG.3 is a cross sectional view showing a part of the device of FIG.2; FIG.4 is a circuit diagram showing an example of application of the MESFET device of FIG.2; and FIG.5 is a cross sectional view similar to FIG.3 showing a second embodiment of the MESFET device of the present invention. DETAILED DESCRIPTION FIG.2 shows a plan view of the MESFET device of the present invention, and FIG.3 shows a part of the device of FIG.2 in cross section taken along a III - III′ line. In the drawings, corresponding parts are given identical reference numerals. Referring to FIG.2, there is provided a comb shaped gate conductor 20 of a metal such as aluminium (Al) or tungsten silicide (WSi) on an n-type surface layer 24 formed at the top of an intrinsic type GaAs substrate 25 such that there is established a Schottky contact between the gate conductor 20 and the active layer 24. The gate conductor 20 has a shape similar to a comb and is branched into a plurality of gate electrodes 20a corresponding to the teeth of the comb. Further, a drain conductor 22 and a source conductor 23 having a corresponding comb shape is provided on the active layer 24 such that there is formed a drain electrode 22a and a source electrode 23a in correspondence to the teeth part of the comb so that the electrodes 22a and 23a are located at both sides of the gate electrode 20a. The conductors 22 and 23 form an ohmic contact with the active layer 24 underneath and comprises stacked layers of gold (Au), nickel (Ni) and Au (Au/Ni/Au structure) or stacked layers of Au, germanium (Ge) and Au (Au/Ge/Au structure). In correspondence to a part of the conductors 22 and 23 forming the source and drain electrodes, a conductive layer of the n⁺-type (not shown in FIG.2, see FIG.3) is formed in the active layer 24 so that the conductors make ohmic contact therewith. Further, the part of the source electrode 23a crossing the gate conductor 20 forms an airbridge structure in which a part of the electrode 23a corresponding to an intersection part of the conductors is lifted from the surface of the substrate 25 and passes above the gate conductor 20. Further, there is provided an additional gate electrode by a conductor 21 having a corresponding comb shape on the active layer 24 in which the conductor 21 is splitted into two additional gate electrodes 21a and 21b in correspondence to the teeth part of the comb and the gate electrodes 21a and 21b extend on both sides of the gate electrode 20a in a region defined by a pair of the source and drain electrodes 23a and 22a. This additional conductor 21 may be made of a same material as that of the conductor 20. Further, an airbridge structure similar to the one already described is also formed at an intersection between the electrode 22a and the conductor 21. Referring now to the cross section of FIG.3 taken along the III-III′ line of FIG.2 showing the MESFET device together with associated circuit parts also forming the device, it can be seen that the gate electrode 21a and the gate electrode 21b are disposed on the surface of the active layer 24 at a region between the drain electrode 22a and the source electrode 23a in a manner such that the gate electrode 21a and the gate electrode 21b extend along both sides of the gate electrode 20a. Further, there are provided a conductive layer 24a and a conductive layer 24b both doped to the n⁺-type in the active layer 24 in correspondence to the drain electrode 22a and the source electrode 23a. As a result, an n-type active region 24c is left between the pair of conductive layers 24a and 24b and it is this region 24c that makes Schottky contact with the gate electrode 20a and the additional gate electrodes 21a and 21b. In the illustrated example, the thickness of the active layer 24 is about 200 nm (2000 Å) and the gate length L1 of the gate electrode 20a forming the gate of the MESFET is set to about 0.2 - 0.5 µm. Further, the gate electrode 21a or 21b forming the additional gate has a gate length of about 1 - 5 µm. Further, the separation t between the electrodes 20a and 21a, 21a and 22a, 20a and 21b, and 21b and 23a is set to about 1 - 5 µm. In the drawing, the distance t is illustrated only for the gap between the gate electrode 21a and the source electrode 22a for avoiding the complexity of the drawing. In the present embodiment, the distance t is made identical for all of the above mentioned gaps. However, the present invention is not limited so but the distance t may be changed in each of the gaps. The thickness of the electrodes may be several microns. From FIG.3, it can be seen that the gate electrode 20a is connected to an input terminal 10 of the MESFET device. Further, there is provided another electrode 21c at the bottom of the substrate 25 and this electrode 21c is connected to the ground. Further, there is provided a d.c. voltage source 32 having a plus terminal connected to the gate electrodes 21a and 21b and a minus terminal connected to the electrode 21c. Next, the operation of the MESFET device of the present invention shown in FIG.3 will be described. Referring to the drawing, it can be seen that there are depletion regions 26, 27 and 28 formed in the active region 24c in correspondence to the electrodes 21a, 20a and 21b respectively, in which the depletion region 28 under the gate electrode 20a is controlled responsive to the high frequency, i.e. radio frequency signal applied to the input terminal 10. On the other hand, the depletion regions 26 and 27 are formed as a result of the surface states as well as the Schottky contact between the gate electrode 21a or 21b and the the active region 24c and is not controlled by the input signal. Thus, when the conductive layer 24b underneath the source electrode 23a supplies electron to the active region 24c and the drain region 24a collects the electron passed through the active region 24c along a channel formed therein, this channel of electron in the active layer 24 is narrowed with extension of the depletion regions 26 - 28 extend into the active layer 24 and the source-drain current is decreased accordingly. As the depletion region 28 is controlled by the signal at the input terminal 10 and controls the source-drain current, the depletion layer 28 is essential to the operation of the MESFET. On the other hand, the depletion regions 26 and 27 is not related to the operation of the device and acts as an unwanted obstacle for the electron. In the MESFET device of the present invention, extension of these unwanted depletion regions 26 and 27 are reduced by applying a suitable positive voltage to the gate electrodes 21a and 21b by connecting the the electrodes to the plus terminal of the d.c. voltage source 32. In correspondence to the gate electrodes 21a and 21b, there is provided an opposing electrode 21c at the bottom of the substrate 25 to which the minus terminal of the voltage source 32 is connected. By applying a biasing voltage of about + 0.4 to + 0.6 volts, which is substantially lower than the voltage usually applied to the drain electrode 22a (about 10 volts), to the additional gate electrodes 21a and 21b so as to compensate the surface potential formed between these gate electrodes and the drain or source electrode which is typically in the rage of - 0.4 to - 0.6 volts, one can reduce or eliminate the depletion region as is represented in FIG.3 by numerals 26′ and 27′. As a result, the channel for electron is wide open and the MESFET can provide a large source-drain current. Further, it should be noted that, as a result of application of the small positive d.c bias voltage of about + 0.4 to 0.6 volts to the gate electrodes 21a located between the gate electrode 20a and the drain electrode 22a to which a large positive drain voltage is applied, the concentration of electrical field at opposing edges of the gate electrode 21a and the drain electrode 22a is avoided. Thus, the damage to the electrodes due to the concentration of the electrical field is avoided and the withstand performance of the device is improved. FIG.4 shows an example of the use of the MESFET device of the present invention in which the MESFET device is used as an amplifying circuit operating in a radio frequency or microwave frequency range. Referring to the drawing, the MESFET device is supplied with an input r.f. signal at the input terminal 10 from a high frequency signal source 30 through a high pass filter circuit comprising a capacitor 30a and an inductance 30b with a state such that the signal is biased with a negative d.c. voltage produced by a d.c. voltage source 31 having a minus terminal connected to the input terminal 10. Further, it can be seen that the d.c. voltage source 32 applies the positive d.c. bias of + 0.4 to + 0.6 volts to the electrodes 21a and 21b as already described so as to eliminate the depletion region underneath these electrodes. The MESFET is further supplied with drain voltage V DD through a choke coil 33a and operates as an amplifying device in which an output signal is supplied to a load 33 such as antenna via a capacitor 33b. As the unwanted depletion regions 26 and 27 in the channel region is eliminated by applying a compensating voltage to the gate electrodes 21a and 21b as already described, a large output power can be obtained at the load 33. FIG.5 shows a second embodiment of the device of the present invention wherein a protective layer of silicon nitride 29 is provided so as to cover the entire surface of the structure of FIG.3 including the electrodes 20, 21a, 21b, 22a and 23a as well as surface regions of the substrate 25 located between these electrodes. In the drawing, only essential parts are illustrated. In this embodiment, the thickness of the protective layer should be as small as possible and preferrably about 50 - 100 nm (500 - 1000 Å) or less which coincides with the thickness of silicon nitride film commonly used in the art for protecting the surface of the semiconductor material. As the increase in the thickness in such a silicon nitride layer invites accumulation of mechanical stress as well as increase in the junction capacitance, the thin silicon nitride layer is quite suited for the MESFET structure of the present invention. Further, the material for the substrate 25 is not limited to GaAs but indium gallium arsenide (InGaAs) may also be used. Further, the present invention is not limited to these embodiments but various variations and modifications may be made without departing from the scope of the present invention.

Claims: A metal-semiconductor field effect transistor device comprising an active layer (24a) formed on a surface of a semiconductor substrate (25), a first gate electrode (20a) provided at said active layer so as to make a direct contact therewith for making a Schottky barrier contact with said active layer, source and drain electrodes (22a, 23a) provided on the active layer at both sides of the first gate electrode so as to make ohmic contact with the active layer, characterized by a second gate electrode (21a, 21b) provided on said active layer in direct contact therewith for making a Schottky barrier contact with said active layer so as to cover a part of the active layer between said first gate electrode and the source electrode and a part of the active layer between said first gate electrode and the drain electrode, first means (10) for applying an input signal for a transistor operation to said first gate electrode, and second means (32) for applying a constant d.c. voltage for suppressing development of depletion region in said active layer to said second gate electrode. A metal-semiconductor field effect transistor device as claimed in claim 1 characterized in that said constant voltage is in a range of + 0.4 volts to + 0.6 volts. A metal-semiconductor field effect transistor device as claimed in claim 1 characterized in that said input signal is a radiofrequency signal. A metal-semiconductor field effect transistor device as claimed in claim 3 characterized in that said substrate (25) comprises gallium arsenide and said first and second gate electrodes (20a, 21a, 21b) comprise aluminium. A metal-semiconductor field effect transistor device as claimed in claim 3 in which said substrate (25) comprises gallium arsenide and said second gate electrode (21a, 21b) comprises tungsten silicide. A metal-semiconductor field effect transistor device as claimed in claim 1 characterized in that the second gate electrode (21a, 21b) has a gate length substantially larger than that of the first gate electrode (20a). A metal-semiconductor field effect transistor device as claimed in claim 3 characterized in that the device further comprises a protective layer (29) of insulator provided so as to cover the surface of the active layer (24a) including the first and second gate electrodes (20a, 21a, 21b) as well as the source and drain electrodes (22a, 23a) with a thickness of less than about 100 nm (1000 Å). A metal-semiconductor field effect transistor device as claimed in claim 7 characterized in that said insulator (29) comprises silicon nitride.

Inventors: KAWAI, TAKAHISA 1134-3-201, SHIMOKODANAKA
Applicants: FUJITSU LIMITED
Application year: 1989
Main classification: H01L29
IPC codes: H01L29, H03F3, H01L23