Double heterojunction AlGaAs/GaAs bipolar junction transistors (DHBJT’s) grown by molecular beam epitaxy (MBE) were fabricated and tested. The incorporation of a graded base‐collector junction improved the dc performance of a DHBJT over that obtained with an abrupt base‐collector junction. Maximum current gains of 500, 900, and 1650 were obtained using graded collector and emitter junctions and base widths of 0.05, 0.2, and 0.1 μm, respectively. The reduction in current gain in going from a 0.1‐μm base width to a 0.05‐μm base width is attributed to a lowering of the electron lifetime resulting from the high base doping level used. The value of 1650 is comparable to the current gains obtained from liquid phase epitaxy (LPE)‐grown HBJT’s and is the best obtained from MBE‐grown HBJT’s. Common‐emitter transistor turn‐on voltages were found to correspond to a difference in the collector and emitter junction turn‐on voltages. High collector growth temperature improved dc performance with 700 °C as the optimum g...

Double heterojunction Al0.35Ga0.65As/GaAs bipolar junction transistors (DHBJTs) grown by molecular beam epitaxy (MBE) were fabricated and tested. Devices with 2000 Å and 500 Å base widths exhibited common emitter current gains of about 325 and 500, respectively, in a wide range of base and collector currents. The use of such high Al mole fraction and double heterojunctions placed stringent require...

Modulation-doped FETs grown by MBE exhibit very high currents and transconductances per unit gate width. In short gate FETs the high transconductances are a result of large electron velocities; typically 2×107 cm/s at 300 K and 3×107 cm/s at 77 K. Since the low field mobility does not strongly affect device performance, improvements were obtained by adjusting the parameters of the heterostructure to maximize charge transfer across the heterojunction. An improved calculation of the maximum charge transfer is used to derive closed expressions for the maximum current and transconductance. Predictions of increasing current and transconductance as the undoped (Al,Ga)As spacer layer thickness is decreased have been verified by experimental results. For a N-off FET with a 20 Å spacer layer transconductances as high as 250 mS/mm at 300 K and 400 mS/mm at 77 K have been obtained. Maximum currents as high as 200 and 300 mA/mm were obtained in N-off and N-on devices, respectively, at 300 K. In this paper the crystal growth, device fabrication, and design considerations for modulation-doped FETs are discussed.

The performance of camel gate GaAs FET's and its dependence on device parameters has been described. In particular, the dependence of the performance on the doping-thickness product of the p + layer was examined. Theoretical calculations indicate that using large p + doping-thickness products provides relatively voltage-independent transconductances and large reverse breakdown voltages, both of which are desirable in large signal applications. Decreasing the p + doping increases the transconductance, which is desirable in logic applications. Comparison with performance of fabricated devices indicates good agreement between theory and experiment over a wide range of structural parameters. Microwave measurements on CAMFET's have yielded a gain of 10 dB at 9 GHz.

The dc characteristics of modulation-doped AlxGa1−xAs/GaAs field-effect transistors have been studied experimentally and theoretically to determine the effect of the thickness of the undoped AlxGa1−xAs spacer layer commonly left at the heterointerface. Increasing the thickness of the spacer layer decreases charge transfer and increases mobility. Current transport in short channel transistors, however, is limited by the electron saturation velocity which is independent of the spacer thickness. Due to increased charge transfer, decreasing the spacer thickness from 100 to 20 Å doubled the maximum saturation current and transconductance. This should allow faster switching speeds to be obtained. A maximum current of 24 mA was obtained for a gate width of 145 μm with a 40-Å spacer and a transconductance of 250–275 mS/mm was obtained for a device with a 20-Å spacer. Theoretical results indicate that intrinsic transconductances greater than 900 mS/mm are possible. Preliminary small-signal rf measurements indicate a maximum available gain of about 9 dB at 8 GHz.

Double heterojunction AlGaAs/GaAs bipolar junction transistors (DHBJT's) grown by molecular beam epitaxy (MBE) were fabricated and tested. Devices with 0.2-µm and 0.1-µm base thicknesses exhibited common emitter current gains of up to 325 and 1650, respectively, in a wide range of collector currents. To obtain such high current gains, growth conditions had to be optimized and controlled. These high current gains, compared with the previous best value of 120 obtained in a MBE-grown transistor, make the HBJT's very promising for low-power high-speed logic application.

High-performance normally-off and normally-on field-effect transistors have been fabricated from modulation-doped (Al,Ga)As/GaAs heterostructures grown by molecular-beam epitaxy. At 300 K, transconductances of 210 and 240 mS/mm were obtained, respectively, for normally-off and normally-on transistors with a 1 µm gate length. The mode of operation was determined by the depth to which the gate was r...

High-performance normally-off modulation-doped (Al,Ga)As/GaAs field-effect transistors with a 1 μm gate length were fabricated and characterised. The transconductance obtained was 225 mS/mm at 300 K and 400 mS/mm at 77 K, leading to intrinsic transconductances (zero source resistance) of 305 and 565 mS/mm at 300 and 77 K, respectively. Since the device performance in short-gate transistors is limi...

Normally-off and normally-on camel gate GaAs field effect transistors have been fabricated from structures grown by molecular beam epitaxy. These new devices use a camel diode gate formed from n+ and p+ layers instead of a Schottky barrier gate as used in metal-semiconductor field effect transistors. The camel diode gate provides these devices with several advantages over metal-semiconductor field effect transistors, including elimination of the metallurgical difficulties of the metal-semiconductor contact, relatively easy adjustment of the built-in voltage, and the potential for improved reliability in adverse environments and under conditions of high power dissipation. Fabrication of these devices does not require precision etching, making them compatible with large scale integrated circuit technology. Fabricated devices have yielded transconductances of 80 ms/mm in long channel (3 μm gate length) normally-on and normally-off field effect transistors and of 120 ms/mm in short channel (1 μm gate length) normally-on field effect transistors. Significantly improved gate-drain breakdown voltages and, in devices with AlxGa1−xAs buffer layers, excellent saturation characteristics have been observed. A simple theory providing analytical expressions describing the performance of normally-on and normally-off camel gate field effect transistors has been developed and good agreement with experiment has been obtained.

A new Al0.3Ga0.7As/GaAs modulation-doped FET fabricated like a MESFET but operating like a JFET was successfully fabricated and tested. This new device replaces the Schottky gate of the MESFET with an n+/p+ camel diode structure, thereby allowing problems associated with the former to be overcome. The devices, which were fabricated from structures grown by molecular beam epitaxy (MBE), had a 1µm gate length, a 290µm gate width, and a 4µm channel length. The room temperature transconductance normalized to the gate width was about 95 mS/mm, which is comparable to that obtained in similar modulation-doped Schottky barrier FET's. Unlike modulation-doped Schottky barrier FET's, fabrication of this new device does not require any critical etching steps or formation of a rectifying metal contact to the rapidly oxidizing Al0.3Ga0.7As. Relatively simple fabrication procedures combined with good device performance make this camel gate FET suitable for LSI applications.

Inverted structure (Al, Ga)As/GaAs modulation doped Schottky barrier FETs were fabricated. The structures from which the FETs were prepared were grown by molecular beam epitaxy. These FETs are easier to fabricate than normal MD FETs, because the rectifying metal contact is made to GaAs rather than to (Al, Ga)As. Having the GaAs on top of the (Al, Ga)As also makes these devices less susceptible to degradation. Although the device structure is not yet fully optimized, normalized transconductances of 70 mS/mm and excellent saturation characteristics have been obtained.

The purpose of this work is to determine the optimum design parameters of modulation doped AlxGa1-xAs/GaAs field effect transistors (MODFETs). An analytical model for MODFETs was developed and used to characterize several 1 µm gate transistors. Extremely high transconductances were obtained and are attributed to large electron saturation velocities in the undoped GaAs. At 300 K transconductances of 250 mS/mm and 235 mS/mm have been obtained for normally-off and normally-on devices respectively. At 77 K a transconductance of 400 mS/mm was obtained for a normally-off MODFET. Using our model to characterize these devices requires the electron saturation velocity to be about 2 × 107cm/s at 300 K and 3 × 107cm/s at 77 K.

GaAs field effect transistors (FET's) having submicron gate lengths (0.7 µm) and Al0.3Ga0.7As buffer layers were fabricated. The saturation, and particularly the pinch-off characteristics, showed a considerable dependence on the growth conditions used during preparation by molecular beam epitaxy (MBE). The structures grown at high substrate temperatures exhibited an excellent pinch-off characteris...

A novel device utilizing the "camel diode" in place of a Schottky barrier gate has been demonstrated in GaAs grown by molecular beam epitaxy (MBE). The devices have a 7.5 µm channel length, 3 µm gate length, and a 280 µm gate width. The layers from which the devices are fabricated consist of a 0.15 µm GaAs layer doped to a level of 1.5 × 10 17 cm -3 to form the channel, and a 100 Å p+GaAs and a 400 Å n+ region to form the gate. Because of the long gate length, the electron velocity does not reach saturation, thus a transconductance of 80 mS/mm is obtained. A simple theory describing the device operation has also been developed.

The effect of a GaAs smoothing layer on the performance of GaAs/AlxGa1−xAs field-effect transistors prepared by molecular beam epitaxy was investigated. The GaAs smoothing layer was inserted between the channel layer and the AlxGa1−xAs buffer layer in an attempt to reduce the dependence of interface quality on growth conditions. Current-voltage characteristics of field-effect transistors with 1-μm gate lengths were used to characterize the properties of the heterointerface. Without the GaAs smoothing layer, extremely sharp interfaces, as indicated by the electron velocity, were obtained when the structures were grown at 700 °C. However, the interface sharpness was very sensitive to growth conditions and in particular to the substrate temperature, decreasing from 50 Å at 700 °C to 70 Å at 640 °C and to 260 Å at 580 °C. Incorporation of a 200-Å-thick undoped GaAs smoothing layer at the heterointerface decreased this sensitivity to growth conditions. With the smoothing layer, interface sharpnesses of 150, 60, and 50 Å were obtained for substrate temperatures of 580, 630, and 700 °C, respectively.