Abstract:
A heterojunction semiconductor field effect transistor HFET having a pair of layers of different semiconductor materials forming a quantum well within the structure to support the 2DEG. Source, drain and gate electrodes are disposed above the channel. The HFET has a predetermined transconductance. A transconductance control electrode varies an electric field within the structure under the channel to vary the shape of the quantum well and thereby the transconductance of the FET in accordance with a variable control signal fed to the transconductance control electrode.

Description:
TECHNICAL FIELD 
     This disclosure relates to heterojunction field effect transistor (HFET) based variable gain amplifiers having variable transconductance. 
     BACKGROUND 
     As is known in the art, microwave systems, including microwave radar systems, can benefit from an amplifier whose gain can be adjusted in a predictable manner. For example, Group III-V (such as Gallium Nitride (GaN)) HFET amplifiers used in radars may have oscillation problems under certain conditions (e.g. temperature) due to excessive gain at a given condition (e.g. temperature). 
     As is known in the art, HFETs generally are formed by providing semiconductor layers of different materials forming a heterojunction. One such layer may be, for example, GaN and the other AlGaN to provide a high-electron mobility transistor (HEMT). The heterojunction supports a two-dimensional electron gas (2DEG) confined in a triangular quantum well (a potential well with only discrete energy values) at the heterojunction. This confinement of the 2DEG leads to quantized energy levels for motion along the channel of the HFET. Electrons confined to the heterojunction of HEMTs exhibit higher mobilities than those in MOSFETs, since the former utilizes an intentionally undoped channel thereby mitigating the deleterious effect of ionized impurity scattering. 
     As is also known in the art, the gain of a GaN HFET amplifier is set by the HFET transconductance (g m , the change in drain current divided by change in gate voltage) having a value fixed by the geometry and construction of the device and set by a fixed gate bias voltage applied at the gate above the channel of the transistor. One attempt to provide a variable gain of an FET amplifier uses two separate transistors in a cascode arrangement, such as described in a paper entitled “AlGaN/GaN-based Variable Gain Amplifiers for W-band Operation” by Diebold et al., Microwave Symposium Digest (IMS), 2013 IEEE MTT-S International DOI:10.1109/MWSYM.2013.6697340 publication year 2013 pages 1-4. However, the use of two separate transistors is relatively costly, lower yielding, and occupies a relatively large surface area. 
     SUMMARY 
     In accordance with the present disclosure, an HFET having a heterojunction semiconductor structure is provided. The heterojunction semiconductor structure includes: a pair of layers of different semiconductor materials forming a quantum well within the channel of the structure to support the 2DEG; source, drain and gate electrodes above the channel with the HFET having a predetermined transconductance; and a transconductance control electrode for varying an electric field within the structure under the channel to vary the shape of the quantum well and thereby the transconductance of the HFET in accordance with a variable control signal fed to the transconductance control electrode. 
     In one embodiment, an HFET is provided having: a source electrode in ohmic contact with a first portion of a surface of a heterojunction semiconductor structure having a pair of layers of different semiconductor materials forming a quantum well within the channel of the structure to support the 2DEG; a drain electrode in ohmic contact with a second portion of the surface of the structure; and a gate electrode in Schottky contact with a third portion of the surface of the structure disposed between the first portion and the second portion for controlling a flow of carriers between the source contact and the drain contact as such carriers pass through the channel. The source electrode, drain electrode and gate electrodes are disposed above the channel on a first one of the pair of layers. A fourth electrode is provided for varying an electric field within the structure to vary the shape of the quantum well in accordance with a variable control signal fed to the fourth electrode. 
     In one embodiment, an HFET is provided having: a source electrode in ohmic contact with a first portion of a surface of a heterojunction semiconductor structure having a pair of layers of different semiconductor materials forming a quantum well within the channel of the structure to support the 2DEG; a drain electrode in ohmic contact with a second portion of the surface of the structure; and a gate electrode in Schottky contact with a third portion of the surface of the structure disposed between the first portion and the second portion for controlling a flow of carriers between the source contact and the drain contact as such carriers pass through the channel. The source, drain and gate electrodes are disposed above the channel on a first one of the pair of layers. The HFET has a predetermined transconductance. A transconductance control electrode is provided for varying an electric field within the structure to vary the shape of the quantum well and thereby the transconductance of the FET in accordance with a variable control signal fed to the transconductance control electrode. 
     In one embodiment, the transconductance control electrode is disposed in the second one of the pair of layers for varying the electric field within the structure. 
     In one embodiment, the transconductance control electrode is disposed in a region of the second one of the pair of layers structure under the channel for varying the electric field within a region. 
     In one embodiment, an insulating layer is disposed between the transconductance control electrode and the region of the second one of the pair of layers structure under the channel. 
     In one embodiment, the transconductance control electrode is in ohmic contact with the region of the second one of the pair of layers structure under the channel. 
     In one embodiment, the transconductance control electrode is in Schottky contact with the region of the second one of the pair of layers structure under the channel. 
     In one embodiment, an HFET structure is provided, comprising: a heterojunction semiconductor structure having a pair of layers of different semiconductor materials forming a quantum well within the channel of the structure to support the 2DEG, such structure having a predetermined nominal transconductance; a source electrode in ohmic contact with a first portion of a surface of a semiconductor; a drain electrode in ohmic contact with a second portion of the surface of the semiconductor structure; a gate electrode in Schottky contact with a third portion of the surface of the structure, the third portion being disposed between the first portion and the second portion for controlling a flow of carriers between the source contact and the drain contact as such carriers pass through the channel. The source, drain and gate electrodes are disposed above the channel. A transconductance control electrode is for varying an electric field within the semiconductor under the channel to varying the shape of the quantum well and thereby the transconductance of the transistor in accordance with a variable control signal fed to the transconductance control electrode. 
     In one embodiment, a system is provided, comprising: an HFET, comprising: a heterojunction semiconductor structure having a pair of different semiconductor layers forming a quantum well within the channel of the structure to support the 2DEG, such structure having a predetermined nominal transconductance; a source electrode in ohmic contact with a first portion of a surface of a semiconductor; a drain electrode in ohmic contact with a second portion of the surface of the semiconductor structure; and a gate electrode in Schottky contact with a third portion of the surface of the structure, the third portion being disposed between the first portion and the second portion for controlling a flow of carriers between the source contact and the drain contact as such carriers pass through the channel. The source, drain and gate electrodes are disposed above the channel. A transconductance control electrode is provided for varying an electric field within the semiconductor under the channel to varying the shape of the quantum well and thereby the transconductance of the transistor in accordance with a variable control signal fed to the transconductance control electrode. The system includes a variable control signal generator for producing the variable control signal. 
     In one embodiment, the HFET structure and the variable control signal generator are disposed on a common semiconductor. 
     In one embodiment, the variable control signal generator senses temperature of the semiconductor and the control signal varies in accordance with variations in the sensed temperature. 
     With such arrangement, varying the shape of the quantum well and thereby the transconductance (g m ) of an HFET is provided by adding a fourth electrode (the transconductance control electrode in addition to the source, gate, drain) to provide an electric field under the channel to confine and modulate the 2DEG, thereby varying drain current flow and hence varying the transconductance of the device. 
     Thus, the transconductance g m  of the HFET is varied by adding a 4th electrode (in addition to the source, gate, drain) to provide an electric field under the 2DEG channel to confine and restrict the 2DEG channel, thereby restricting drain current flow and hence varying the transconductance of the device (since transconductance is defined as change in drain current divided by the change in gate voltage). 
     The details of one or more embodiments of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of circuit having an HFET structure with a 2DEG in the channel having a predetermined nominal transconductance and a transconductance control electrode for varying an electric field under the channel to vary the shape of the quantum well and thereby the transconductance of the FET in accordance with a variable control signal fed to the transconductance control electrode in accordance with the disclosure; 
         FIG. 1A , is an analog circuit used as a temperature sensing compensation section of the circuit of  FIG. 1 ; 
         FIG. 1B , is a digital circuit used as a temperature sensing compensation section of the circuit of  FIG. 1 ; 
         FIG. 2  is a cross section of an HFET structure having a fourth electrode implemented as an metal-insulator-semiconductor contact used in  FIG. 1  in accordance with the disclosure; 
         FIG. 3  is a cross section of an HFET structure having a fourth electrode implemented in ohmic or Schottky contact used in  FIG. 1  in accordance with another embodiment of the disclosure. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a system  10  is shown formed on a single crystal substrate  12 , here for example, silicon carbide (SiC). The system  10  includes a HFET amplifier  14  having an HFET  16 . The HFET  16  has a gate  18  (G) fed to an RF input signal (V in ) through a coupling capacitor C in  and to a V g  bias voltage (−V g ) through an RF blocking inductor L 1 , as shown. The source electrode S of the HFET  16  is connected to ground, as shown. The drain D is connected to a (+V d ) bias through an RF blocking inductor, L 2 , as shown and provides the amplified output (V out ), after passing through a dc blocking capacitor C 2 , as shown. 
     The HFET  16  is provided with a transconductance control electrode  20  for varying the shape of the quantum well and thereby the transconductance of the HFET  16  in a manner to be described in more detail below. Suffice it to say here that the transconductance control electrode  20  is a control signal from a variable control signal generator, here, for example, a temperature sensing section  24 , formed on the substrate  12 , to be described. The temperature sensing section  24  generates the control signal in accordance with variations in the sensed temperature of the substrate  12 . 
     Here, for example, absent the temperature sensing section  24 , the HFET  16  has an unwanted oscillation when the substrate  12  is at room temperature; however, the oscillation reduces as the temperature of the substrate  12  increases because the gain of the HFET  16  reduces with increasing temperature as correspondingly the unwanted oscillation reduces. Here, the temperature sensing section  24  includes a temperature sensing device TS, here, for example, a diode D-(or mesa resistor or thermistor), as a part of either analog circuitry ( FIG. 1A ) or digital circuitry ( FIG. 1B ), to reduce the transconductance, and hence the gain, of the HFET  16  at room temperature to reduce or remove the unwanted oscillation and as the temperature of the substrate  12  increases above room temperature, the temperature sensing section  24  increases the transconductance, and hence the gain, of the HFET  16  in such a way so as not to reintroduce the unwanted oscillation. 
     Thus, in  FIG. 1A , here the temperature sensing device TS, is here, for example, a diode D or mesa resistor or thermistor, formed on the substrate  12 , serially connected between a predetermined negative voltage source −VREF and ground through a pair of resisters R 1  and R 2 , as shown. The output of the temperature sensing device TS is fed to the fourth electrode  20 , as shown. Thus, a voltage divider network is formed having in addition to the serially connected resistors R 1  and R 2  the temperature sensing device TS. The voltage at the junction between temperature sensing device TS and the resistor R 1  is fed to the fourth electrode  20 . When the substrate  12  is at room temperature the values of R 1 , R 2  and −VREF of the temperature sensing section  24  are selected to produce a voltage at the fourth electrode  20  that results in removal or reduction of the unwanted oscillation and, as the temperature of the substrate  12  increases above room temperature, the temperature sensing section  24  adjust the voltage at the fourth electrode  20  so that it becomes more positive to thereby increase the transconductance of the HFET  24  without reintroducing the unwanted oscillation. 
     For example, a measurement is made of the voltage drop V x  across of the temperature sensor TS, for example diode D (or mesa resistor or thermistor), at room temperature with a predetermined current passing through it, for example, 3 mA. Assume V x =2 Volt is measured with 3 mA current passing through it at room temperature. Next, the value of R 1  is set to a convenient value, for example, R 1 =500 ohms. Next, the −V REF  is set to a convenient negative voltage, for example, −5 Volts. With the voltage at the fourth electrode  20  at 0 Volts, the voltage at the gate electrode G, V G , is selected for the desired drain current Id and/or the desired peak transconductance g m ; for example V G =−2V. The value of the resistance of R 2  is adjusted to yield a voltage applied to the fourth electrode  20  such that the oscillation at room temperature stops. For example, R 2 =500 ohms, and the voltage of the fourth electrode  20  is =−1.5 V 
     In  FIG. 1B  the temperature sensing section  24 ′ has the temperature sensing device TS, again, for example, the diode D (or mesa resistor or thermistor) and voltage at the junction between the anode of the temperature sensing device TS and the resistor R 1  is first converted into a corresponding digital signal by an analog to digital converter (A/D). The corresponding digital signal is fed to a microprocessor  40 , as shown. As a result of an a priori calibration process which produces a relationship between the voltage produced at the output of the diode D (or thermistor or mesa diode) (and hence a measure of substrate  12  temperature) and proper voltage at the fourth electrode  20  (and hence the transconductance of the HFET  24 ) to reduce or remove unwanted oscillations at room temperature while not reintroducing the unwanted oscillation above room temperature, the produced relationship is stored as data in a table of the microprocessor  40 . The microprocessor is programmed to use the stored data to produce a proper voltage for the fourth electrode  20  at room temperature and above room temperature in accordance with the voltage produced by the diode D (or thermistor or mesa diode). 
     Referring now to  FIG. 2 , the HFET  16  is shown to include: a heterojunction semiconductor structure  30  having the single crystal substrate  12 , here for example, silicon carbide (SiC), a III-V buffer or nucleation layer  34 , here for example, Aluminum Nitride (AlN) on the substrate  12 , a gallium nitride (GaN) layer  36  on the nucleation layer  34 ; and an Aluminum Gallium Nitride (AlGaN) layer  38  formed on the gallium nitride (GaN) layer  36  in any conventional manner to form a heterojunction between the gallium nitride (GaN) layer  36  and the Aluminum Gallium Nitride (AlGaN) layer  38  to thereby produce a quantum well to support the 2DEG  40  within the structure  30 . Once fabricated, the HFET has a predetermined nominal transconductance. 
     The HFET  16  has: a source electrode, S, in ohmic contact with a first portion of a surface of a source contact region  42  of the Aluminum Gallium Nitride (AlGaN) layer  38 ; a drain electrode, in ohmic contact with a drain contact region  44  of the Aluminum Gallium Nitride (AlGaN) layer  38 ; and a gate electrode, G, in Schottky contact with a Schottky contact region  46  of the Aluminum Gallium Nitride (AlGaN) layer  38 , the gate contact,  18 , being disposed between the source S and drain D for controlling the flow of carriers between the source S and the drain D as such carriers pass through the 2DEG  40 . It is noted that the source electrode, S, the drain electrode D and gate electrode  18  (G) are in contact with the AlGaN layer  38  above the 2DEG  40 . 
     The structure  30  includes a fourth electrode  20 , here a transconductance control electrode. More specifically, a via  54  is formed through the back side  52  of the substrate  12  using any conventional technique, such as photolithographic etching or laser drilling. The via terminates in a bottom portion  56  disposed in the GaN layer  36 , under the portion of the 2DEG  40  in a region between, and under, the Schottky region  46  and drain contact region  44  as shown. After forming the via  54 , the sidewalls of the via  54 , including the bottom portion  56  of the via  54  are coated with a thin dielectric layer  58 , here, for example, silicon nitride (SiN) having a thickness in the range of 5 to 100 nm. 
     Next, a conductive layer  60 , here a metal, for example gold, is deposited over the bottom surface  52  of the substrate  12  and is then selectively removed from the bottom  52  of the substrate  12  using any conventional photolithographic etching technique to form the fourth electrode  20 , as shown. It is noted that the bottom of the fourth electrode  20  is separated from the GaN layer  36 , as well as from the AlN layer  34  by underlying portions of the dielectric layer  58 . With a variable voltage applied to the fourth electrode  20 , a varying electric field will be produced within the GaN under the 2DEG  40  varying the shape of the quantum well and thereby the transconductance of the HFET  16  in accordance with a variable control signal fed to the transconductance control electrode, as for example, from the temperature sensing section  24  of  FIG. 1 . 
     Referring now to  FIG. 3 , another embodiment is shown for the HFET  16 ′. Here, after coating the sidewalls of the via  54 , including the bottom portion  56  of the via  54  with the dielectric layer  58 , here for example. SiN, the portion of the dielectric layer  58  on the bottom portion  56  is removed to expose an underlying portion of the GaN layer  36 . Next, a conductive layer  60 ′ is deposited over the bottom surface  52  of the substrate  12  and is then selectively removed from selected portions of the bottom  52  of the substrate  12  to form the fourth electrode  20 ′, as shown. It is noted that here the fourth electrode  20 ′ may be formed either in ohmic or Schottky contact with the portion  37  of the GaN layer  36  at the bottom portion  56  of the via  54 . 
     A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, other control circuits may be used instead of the temperature sensing compensation section, such as a section that detects output power and produces a control signal for the fourth electrode to set the gain necessary for the desired output power level. Accordingly, other embodiments are within the scope of the following claims.