MODFETs have been fabricated using heterojunctions consisting of AlGaAs and pseudomorphic InGaAs, grown on GaAs substrates. The large conduction band discontinuity (about 0.46 eV for 25% In and Al concentration) leads to a 2-D electron density as high as 2.3*10/sup 12/ cm/sup -2/, with electron mobilities of 7000 and 16000 cm/sup 2//V-s at 300 and 77 K, respectively. Such a high electron density in combination with reasonable transport properties leads to MODFETs with exceptional characteristics. Devices with 0.15-0.25- mu m gate length have room-temperature drain currents as high as 600 mA/mm and room-temperature transconductance as high as 500 mS/mm. The f/sub T/ is as high as 98 GHz, as determined by 20-dB/decade extrapolation of microwave data taken to 25 GHz. A comparison of the effect of bias on the total delay through standard and pseudomorphic MODFETs suggests that the excellent microwave performance exhibited by the pseudomorphic device arises from a reduction in parasitic and drain delays and not from a higher electron velocity under the gate. >

Pulse-doped AlGaAs/InGaAs pseudomorphic MODFETs

In the past few years, a new transistor has appeared on the scene, made of GaAs and AlGaAs, which now holds the record as the fastest logic switching device, switching at speeds of close to ten trillionths of a second (10 ps). The device evolved from the work on GaAs-AlGaAs superlattices (thin alternating layers of differing materials sharing the same crystalline lattice) pioneered by L. Esaki and R. Tsu at IBM in the late 1960's. They realized that high mobilities in GaAs could be achieved if electrons were transferred from the doped and wider band-gap AlGaAs to an adjacent undoped GaAs layer, a process now called modulation doping. R. Dingle, H. L. Stormer, A. C. Gossard, and W. Wiegmann of AT&T Bell Labs, working independently, were the first to demonstrate high mobilities obtained by modulation doping in 1978, in a GaAs-AlGaAs superlattice. Realizing that such a structure could form the basis for a high-performance field-effect transistor (Bell Labs Patent 4163237, filed on April 24, 1978), researchers at various labs in the United States (Bell Labs, University of illinois, and Rockwell), Japan (Fujitsu), and France (Thomson CSF) began working on this device. In 1980, the first such device with a reasonable microwave performance was fabricated by the University of Illinois and Rockwell, which they called a modulation-doped FET or MODFET. The same year Fujitsu reported the results obtained in a device with a 400-µm gate which they called the "high electron mobility transistor" or HEMT, in the open literature. Thomson CSF published shortly thereafter calling their realization a "two-dimensional electron gas FET" or TEGFET, and Bell Labs followed, using the name "selectively doped heterojunction transistor" or SDHT. These names are all descriptive of various aspects of the device operation as we will discuss in the text. For the sake of internal consistency will call it MODFET, hereafter. In this paper we review the principals of MODFET operation, factors affecting its performance, optimization of the device, and comparison with other high-performance compound and elemental semiconductor devices. Finally, the remaining problems and future challenges are pointed out.

Modulation-doped GaAs/AlGaAs heterojunction field-effect transistors (MODFET's), ultrahigh-speed device for supercomputers

Recently developed modulation-doped field-effect transistors (MODFETs) now hold the record for high-speed logic. In this device structure only the larger bandgap (Al,Ga)As layer is doped with donors and the GaAs layer is left undoped. Electrons minimize their energy by diffusing out of the (Al,Ga)As into the lower potential GaAs where they form a two-dimensional electron gas near the heterointerface. Since the electrons and donors are spatially separated, ionized impurity scattering is avoided making it possible to obtain extremely high electron mobilities. Even at electron densities of 1019cm-3or ∼ 1 × 1012cm-2per interface, transport properties of these heterostructures are comparable to pure bulk GaAs. Modulation-doped FETs using this heterojunction system offer many advantages among which are a small gate to conducting channel separation (∼ 300 A) leading to extremely high transconductances, a large current-carrying capability (∼ 500 mA/mm per interface), a small source resistance, and a small saturation voltage. The benefits improve quite substantially at 77 K where the transconductance increases to ∼ 500 mS/mm. Improved device performance has been obtained in both high-speed and digital applications. Modulation-doped FETs used as low-noise amplifiers exhibit noise figures as low as 0.4 dB with 14-dB gain at 10 GHz at 77 K and a noise temperature of 3.5 K at 3.3 GHz with the MODFET cooled to 15 K. The rate of increase in noise temperature is about 1 K/GHz. Quarter micrometer devices have exhibited a current-gain cutoff frequency of 70 GHz. When used as inverters in logic circuits, propagation delays of 12 ps and under 10 ps have been obtained at 300 and 77 K, respectively. Delay times as low as 5 ps are quite possible at 77 K. Static RAMs with 4-kbit complexity exhibited access times of about 2 ns. These results are far superior to any other three-terminal device which is the primary reason why numerous university, industrial, and government laboratories in the U.S., Europe, and Japan have sizable MODFET programs.

Modulation-doped GaAs/(Al,Ga)As heterojunction field-effect transistors: MODFETs

Semiconductor heterojunctions with ideal lattice matching, well-controlled in fabrication, yield devices that cannot be achieved in any other way. These devices include modulated-doped high-speed field-effect transistors, ultra-high-gain and high-speed bipolar transistors, efficient injection lasers and light-emitting diodes and sensitive photo-detecting structures. Atomic reconstructions take place at heterojunction interfaces and are process-fabrication-dependent and not adequately understood. The barrier discontinuities observed are therefore scattered in value and also somewhat dependent on the determination method. Many papers in this Special Issue contain review aspects of these matters. Others, however, are specific contributions on very particular heterojunction topics. Not all aspects of heterojunctions are dealt with by the papers that follow, and the present article is intended for newcomers to the field as a brief commentary on topics that are not adequately represented.

Semiconductor heterojunction topics: Introduction and overview

We report the effect of negative differential resistance (NDR) in the drain circuit of a new type of selectively doped AlGaAs/ GaAs heterojunction transistor. The key new element of our structure is the presence of a subsidiary GaAs conducting layer, separated from the FET channel by an AlGaAs graded barrier. In this work the subsidiary layer is realized by the conducting substrate. The NDR effect arises due to the heating of channel electrons by the source-to-drain field, and the subsequent charge injection over the barrier. This effect is strongly influenced by the gate and substrate voltages. In a floating-substrate arrangement the current-voltage characteristics exhibit memory effects associated with retention of injected charge in the substrate. In this mode, the NDR is seen only at low temperatures with the peak-to-valley ratios in current at 77 K reaching values as high as 30. On the other hand, when the substrate is biased positively, the NDR results from a peculiar effect of dynamical channel depletion by the injected space charge which drifts on the downhill slope of the graded barrier. In this case, the NDR is observed even at room temperature.

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A field-effect transistor with a negative differential resistance

The operation of high-speed divide-by-two circuit (binary counter) composed of selectively doped heterostructure logic gates is reported for the first time. These field-effect transistor circuits utilize the enhanced transport properties of high-mobility electrons confined near a heterojunction interface in a selectively doped AlGaAs/GaAs structure. The dividers are based on a Type-D flip-flop composed of six direct-coupled NOR-gates having 1-µm gate lengths and 4-µm source-drain spacings. They are fabricated by conventional optical contact lithography on a four-layer Al .3 Ga .7 As/GaAs structure grown by molecular-beam epitaxy. Successful operation is demonstrated at 5.9 GHz at 77 K for 1.3-V bias and 30-mW total power dissipation (including output buffers) and 3.7 GHz at 300 K for 1.4-V bias and 19-mW total power dissipation. Total power dissipation values as low as 3.9 mW at 0.65-V bias were also obtained for 2.85-GHz operation at 300 K. These preliminary results illustrate the promise of SDHT logic for ultrahigh-speed low-power applications.

Selectively doped heterostructure frequency dividers

Microwave generation in a NEgative Resistance Field-Effect Transistor (NERFET) is reported for the first time. This device is based on a GaAs/AlGaAs heterostructure which exhibits negative differential resistance due to a transfer of hot-electrons out of a source-drain channel and into a conducting substrate. In an untuned microwave circuit at 77 K, the NERFET was found to generate wide-band noise at frequencies up to 2.3 GHz. In a tunable resonant circuit, stable microwave oscillations were observed at frequencies as high as 1.45 GHz. While further experiments are needed to determine the performance limits of the NERFET, the preliminary results presented here demonstrate the potential of this new device as a high-frequency element.

Microwave generation in NERFET

Selectively-doped depletion-mode FET's which show full pinch-off at 295 K with transconductance in excess of 90 mS/mm and contact resistance less than 0.4 Ω-mm have been fabricated. The device properties reported represent a significant improvement over previously published work on an equivalent structure, because of improvements in growth technique and modifications in the device structure. Analysis of our measurement at room temperature suggests that the two-dimensional electron gas at the hetero-interface is not the dominant contributor to the total device current.

R.H. Hendel

Papers

Selectively doped heterostructure frequency dividers

Microwave generation in NERFET

Selectively-doped Al 0.48 In 0.52 As/Ga 0.47 In 0.53 As heterostructure field effect transistor

IIA-2 selectively doped heterostructure transistors for ultra high-speed integrated circuits

IVA-3 selectively doped heterostructure divided-by-two circuit