A high-transconductance self-aligned GaAs MESFET fabricated by through-AIN implantation

This paper describes a new technique for the fabrication of high-transconductance GaAs MESFET's. Tungsten-silicide gate, self-aligned GaAs MESFET's were fabricated on extremely thin channel layers formed by implantation through AlN layers on semi-insulating GaAs substrates. Transconductance of the through-implanted MESFET's showed 30- to 50-percent increase as compared with that of conventional self-aligned MESFET's and reached its maximum value at 300 mS/mm for 1-µm gate-length FET's. The uniformity of the threshold voltage across a 2-in wafer was also excellent with a standard deviation of 44 mV. Circuit simulation indicates that the advantage of these FET's becomes more crucial when used in a very large-scale integrated circuit (VLSI).     

GaAs MESFET's fabricated on monolithic GaAs/Si substrates

GaAs MESFET's have been fabricated for the first time on monolithic GaAs/Si substrates. The substrates were prepared by growing single-crystal GaAs layers on Si wafers that had been coated with a Ge layer deposited by e-beam evaporation. The MESFET's exhibit good transistor characteristics, with maximum transconductance of 105 mS/mm for a gate length of 2.1 µm.     

High-speed integrated logic with GaAs MESFET's

The feasibility of using GaAs metal-semiconductor field-effect transistors (GaAs MESFET's) in fast switching and high-speed digital integrated circuit applications is demonstrated. GaAs MESFET's with 1-/spl mu/m gate length are shown to have a current-gain-bandwidth product f/SUB T/ equal to 15 GHz. These devices exhibit a 15 ps internal delay in a large-signal switching test. A simple logic circuit consisting of MESFET's and Schottky diodes was monolithically integrated on a semiinsulating GaAs substrate. This logic circuit exhibits a propagation delay of 60 ps with no output load, and 105 ps when its output is loaded by three similar logic gates. A useful bandwidth of approximately 3 GHz is observed.     

A dc and microwave comparison of GaAs MESFET's on GaAs and Si substrates

In order to assess GaAs on Si technology, we have made a performance comparison of GaAs MESFET's grown and fabricated on Si and GaAs substrates under identical conditions and report the first microwave results. The GaAs MESFET's on Si with 1.2-µm gate length (290-µm width) exhibited transconductances (gm) of 180 mS/mm with good saturation and pinchoff whereas their counterparts on GaAs substrates exhibited gmof 170 mS/mm. A current gain cut-off frequency of 13.5 GHz was obtained, which compares with 12.9 GHz observed in similar-geometry GaAs MESFET's on GaAs substrates. The other circuit parameters determined from S-parameter measurements up to 18 GHz showed that whether the substrate is Si or GaAs does not seem to make a difference. Additionally, the microwave performance of these devices was about the same as that obtained in devices with identical geometry fabricated at Tektronix on GaAs substrates. The side-gating effect has also been measured in both types of devices with less than 10-percent decrease in drain current when 5 V is applied to a pad situated 5 µm away from the source. The magnitude of the sidegating effect was identical to within experimental determination for all side-gate biases in the studied range of 0 to -5 V. The light sensitivity of this effect was also very small with a change in drain current of less that 1 percent between dark and light conditions for a side gate bias of -5 V and a spacing of 5 µm. Carrier saturation velocity depth profiles showed that for both MESFET's on GaAs and Si substrates, the velocity was constant at 1.5 × 10⁷cm/s to within 100-150 Å of the active layer-buffer layer interface.     

Theoretical characterization and high-speed performance evaluation of GaAs IGFET's

Fundamental characterization of GaAs IGFET's is carried out utilizing two-dimensional numerical analysis, and high-speed operation performances are evaluated. Two-dimensional analysis shows that the operational mechanism of GaAs IGFET's is very similar to that of MESFET's, i.e., channel depletion-type operation due to control of the gate depletion layer. But normally-off-type current-voltage characteristics can be easily realized because of positive VFBfor GaAs dioxide films. Electrical characteristics are strongly dependent on the oxide film. Although the transconductance gmdeteriorates compared to MESFET's due to the potential drop through the oxide layer, the deterioration in gmis found to be small for oxide films with large dielectric constant εOX. Furthermore, it is clarified that cutoff frequency fTfor IGFET's is greater than for MESFET's because the input capacitance Cgsin depletion-type device operation is found to be much smaller than for MESFET's. In additional, a high-voltage swing is applicable in device operation because of the IG structure, and higher gm, is achieved by supplying high gate biases. These features give GaAs IGFET's wide application fields as high-performance devices and make them superior to MESFET's, especially in digital circuits.     

Surface-Oriented Transferred-Electron Devices

The application of MESFET technology to the manufacturing of surface-oriented transferred-electron devices (TED's) with parmeters close to GaAs MESFET's is discussed. The limitations related to the contact resistance, fringing capacitance, domain formation time, impact ionization, and heat sinking are analyzed for GaAs and InP devices. Our estimates show that the surface-oriented devices can be used as microwave LSA generators at higher frequencies than the conventional LSA diodes. In a domain mode, the surface-oriented TED's can yield low values of the power-delay product comparable to those of GaAs MESFET's at higher speeds. The analysis of impact ionization within a high-field domain leads to a conclusion that even InP logic devices with practical lengths of the active layer can be manufactured with doping densities up to 10/sup 17/cm/sup -3/. The estimate of the temperature rise indicates that a CW operation is possible for practical device parameters. Because the parameters of surface-oriented TED's are similar to those of GaAs MESFET's they may be manufactured using the rapidly developing GaAs integrated-circuit technology and used in combination with GaAs MESFET's.     

Monolithic integration of Si MOSFET's and GaAs MESFET's

Integration of Si MOSFET's and GaAs MESFET's on a monolithic GaAs/Si (MGS) substrate has been demonstrated. The GaAs MESFET's have transconductance of 150 mS/mm for a gate length of 1 µm, and the Si MOSFET's have transconductance of 19 mS/mm for a gate length of 5 µm and an oxide thickness of 800 Å. These characteristics are comparable to those for devices fabricated on separate GaAs and Si substrates.     

Bias dependence of GaAs and InP MESFET parameters

A comparative analysis of the bias dependence of critical RF parameters in GaAs and InP metal-semiconductor field-effect transistors (MESFET's) led to the following conclusions. 1) The drain-gate feedback capacitance in GaAs MESFET's is lower than in InP MESFET's, because of a stronger tendency in GaAs to form stationary Gunn domains at the typical drain bias levels employed. 2) The drain-source output resistance in InP MESFET's is lower than in GaAs MESFET's mainly for high drain current units, a fact which is linked to a substrate related softer pinch-off behavior in InP. 3) The current-gain cutoff frequency fT, in the current saturation range of the GaAs MESFET decreases strongly with drain bias as a result of the formation of the stationary Gunn domain. In the InP MESFET, this effect is weaker. At the optimum bias, fT is only 10-20 percent higher in InP MESFET's than in GaAs ones.     

Buried-channel GaAs MESFET's with frequency-independent output conductance

The output conductance of GaAs MESFET's is shown to be a function of frequency and to have a well-defined temperature dependence. This behavior is in agreement with other reports of the behavior of GaAs ion-implanted MESFET's. The output conductance of buried-channel GaAs MESFET's is shown to be independent of frequency and temperature. This is an important improvement in the output characteristics with many implications for GaAs analog and digital circuits.     

Effects of a buried p-layer on alpha-particle immunity of MESFET's fabricated on semi-insulating GaAs substrates

Measurements of alpha-particle-induced charge are carried out for the first time on both conventional MESFET's fabricated directly on semi-insulating GaAs substrates and MESFET's with a buried p-layer. The maximum collected charge is found to be 65 fC in the MESFET's with a buried p-layer, one order smaller than in conventional MESFET's.     

GaAs MESFET's fabricated on InP substrates

This paper reports the first successful MESFET fabrication in GaAs layers grown directly on InP substrates by molecular beam epitaxy (MBE). The fabricated GaAs MESFET's exhibit good I-V curves with complete pinch-off and saturation characteristics. About 100-mS/ mm transconductance was obtained for a 1.2-µm gate length device.     

Accurate modeling for submicrometer-gate Si and GaAs MESFET's using two-dimensional particle simulation

Device characteristics, including nonstationary carrier-transport effects such as velocity overshoot phenomena in submicrometergate Si and GaAs MESFET's, are presented in detail by two-dimensional full Monte Carlo particle simulation. Accurate current-voltage characteristics and transient current response are successfully obtained without relaxation time approximation. Moreover, the carrier dynamics influence on device operation is clarified in a realistic device model, compared with the conventional simulation. It can be pointed out that such nonstationary carrier transport is acutely important for accurate modeling of submicrometer-gate GaAs MESFET's, but is not as important for that of Si MESFET's.     

Improvement in GaAs MESFET performance due to piezoelectric effect

This paper describes the possibility of improving the performance of GaAs MESFET's by using piezoelectric effects. It is shown that piezoelectric charges, induced in the FET channel region due to the stressed dielectric overlayer, can be used to compensate for the deep tail of carrier distribution in the channel region. As a result, transconductance of short-channel GaAs FET's can be improved with a smaller shift in threshold voltage. The experimental results obtained for WSix- gate self-aligned MESFET's are qualitatively in good agreement with the theoretical values.     

A class-E power amplifier based on an extended resonance technique

In this paper, a class-E power amplifier using four 1-W GaAs MESFET's at 935 MHz is demonstrated using a new extended resonance power-combining technique. A microstrip amplifier based on this technique was designed and fabricated which combines four 1-W Siemens CLY5 GaAs MESFET's with 67% power-added efficiency at 935 MHz     

Performance comparison of highly integrated circuits: Silicon NMOS versus gallium arsenide normally-off MESFET technology

This study presents a performance comparison between highly integrated circuits on gallium arsenide and on silicon realized with normally-off MESFET's and n-channel MOSFET's, respectively. As a basis, a standard cell structure is chosen in order to obtain a realistic capacitive loading. This cell is scaled down twice from an area of 1 mm²to 0.38 mm²and to 0.13 mm². The corresponding effective gate length inside the cell is 1, 0.5, and 0.2 µm, respectively. The delay time of a loaded inverter, the power consumption as well as the power-delay product are calculated using device parameters deduced from experimentalI-Vcharacteristics. For MOSFET's good noise margins at low switching times are obtained at a supply voltage of 3 V. The GaAs circuit exhibits a lower power consumption by one order of magnitude and a smaller delay time by about a factor of 2. Since nonoptimized GaAs MESFET's with recessed gates were regarded for the comparison improvements are expected for self-aligned MESFET's. For a supply voltage of 1 V, the MOSFET circuit shows a comparable power consumption to the GaAs circuit but longer delay time (factor 2 to 5).     

Temperature dependence study of two-dimensional electron gas effecton the noise performance of high frequency field effect transistors

We present experimental evidence that the noise figure (NF) and associated gain equal to those achieved with GaAs pseudomorphic high electron mobility transistors (GaAs p-HEMT's) can also be accomplished by ion implanted GaAs metal-semiconductor field-effect transistors (GaAs MESFET's). These measured noise figure results as a function of low temperature for GaAs MESFET's and p-HEMT's clearly suggest that the transport properties of the two-dimensional electron gas in HEMT's and p-HEMT's do not make a significant contribution to the noise reduction at high frequency operation of these devices     

Microwave characterization of (Al,Ga)As/GaAs modulation-doped FET's: Bias dependence of small-signal parameters

Microwave characterization on 1-µm gate (Al, Ga)As/GaAs modulation-doped field-effect transistors (MODFET's) was done using a network analyzer. Equivalent circuit parameters were computed and compared to those of GaAs MESFET's with an identical geometry. The MODFET's had higher current-gain and power-gain cutoff frequencies (18 and 38 GHz versus 14 and 30GHz) and the circuit parameters gmo, Cgs, and Rdsdisplayed sharper pinchoff effects than those of the MESFET's. Cgsin the MODFET displayed a gate bias dependence due to widening of the potential well in the channel. This information should prove valuable in the development of MODFET computer models for circuit simulation.     

Origin of 1/f3/2noise in GaAs thin-film resistors and MESFET's

This paper addresses the problem of 1/f^3/2low-frequency noise in GaAs thin-film resistors and MESFET's. Experimental data seem to rule out the existence of the so-called "diffusion noise" usually invoked in GaAs devices. Therefore, we propose a new "surface thermal-noise" model based on the existence of lumped thermal-noise generators distributed along the semiconductor-air or semiconductor-dielectric protection interface. The observed dependence of the low-frequency noise on the surface resistance of different MESFET's supports our conclusions.     

Optimal noise figure of microwave GaAs MESFET's

The optimal value of the minimum noise figure Foof GaAs MESFET's is expressed in terms of either representative equivalent circuit elements or geometrical and material parameters in simple analytical forms. These expressions are derived on a semiempirical basis. The predicted values of Fofor sample GaAs MESFET's using these expressions are in good agreement with the measured values at microwave frequencies. The expressions are then applied to show design optimization for low-noise devices. This exercise indicates that shortening the gate length and minimizing the parasitic gate and source resistances are essential to lower Fo. Moreover, a simple shortening of the gate length may not bring an improved Founless the unit gate width is accordingly narrowed. The maximum value of the unit gate width is defined as the width above which the gate metallization resistance becomes greater than the source series resistance. Short-gate GaAs MESFET's with optimized designs promise a superior noise performance at microwave frequencies throughKband. The predicted values of Foat 20 GHz, for example, for a half-micrometer gate device and a quarter-micrometer gate device are 3 and 2 dB, respectively. These devices could be fabricated with the current technology.     

Beyond-punchthrough current in GaAs MESFET's

In a recent letter published in this journal, Patrick et al. reported on a maximum drain voltage for pinchoff Which varied exponentially with gate length in very short-gate GaAs MESFET's. The I-V characteristics given showed that this variation is associated with beyond-punchthrough drain current. Current flowing across a depleted region is an instance of the triode mode of FET operation described by other researchers in 1966. Triode-mode theory can help in the understanding of the behavior of GaAs MESFET's near pinchoff, including the devices of Patrick et al.