Modulation-doped FET Research Articles

Nonlinear charge control in AlGaAs/GaAs modulation-doped FET's

A new model for nonlinear charge control in normally on modulation-doped field-effect transistors (MODFET's) is proposed. It is shown that conventional charge control models are insufficient to describe MODFET's with large negative pinchoff voltages, and that the depletion approximation is inaccurate in circumstances where the layer dimensions become of the order of a Debye length. The new model is based upon a one-dimensional numerical solution of Poisson's equation and the drift-diffusion equation. It also takes into account the shift in the 2DEG position with gate bias, and parallel conduction in the doped AlGaAs layer. The effect of nonlinear charge control on MODFET transconductance is considered by combining the new model with a two-dimensional analytic representation of the MODFET.

Determination of mobility in modulation-doped FET's using magnetoresistance effect

A simple measurement technique based on the magnetoresistance effect is developed to obtain the differential and average mobilities of modulation-doped field-effect transistors (MODFET's) with respect to gate bias voltage. The effect of parasitic series resistances can be neglected by using a low magnetic field. The measurement is not affected by parasitic gate capacitance and therefore constitutes an effective tool for characterizing fully processed ultra-short gate-length MODFET's.

Pseudomorphic InGaAs/AlGaAs modulation-doped FET's with reduced low-frequency noise and thermally stable performance

A high-performance MODFET structure grown by MBE with the incorporation of a single quantum well In 0.15Ga 0.85As layer for the transport of 2D electron gas has been critically examined for its thermal stability at 80 K and low-frequency noise from 10 -2 to 10 8 Hz. Experimental results indicate that the behavior of this device in both these respects is much superior when compared with the same behavior of conventional MODFET's. A maximum low-field carrier mobility of 29,000 cm 2/s at 80 K and an average carrier saturation velocity of 2×10 7 cm/s at 300 K in a 1 μm gate device clearly indicate that the quality of the pseudomorphic quantum well (InGaAs) layer is either comparable or better than that of the usual GaAs buffer layer. The deep level spectra, obtained through photo-FET measurements, and the low-frequency noise spectra at different temperatures obtained for the new pseudomorphic and conventional MODFET's have clearly indicated that contributions from various deep levels present in the new structure are significantly reduced.

Ionizing Radiation Hardness of GaAs Technologies

The radiation response of several GaAs technologies to ionizing radiation has been investigated. Self-aligned gate (SAG) E/D GaAs Metal Semiconductor FET (MESFET), SAG AlGaAs/GaAs Modulation Doped FET (MODFET), and complementary-AlGaAs/GaAs Heterostructure Insulated Gate FET (C-HIGFET) devices and circuits all demonstrated minimal sensitivity to total dose effects to 250 Mrad(GaAs). The heterostructure based technologies showed superior tolerance to high dose rate exposures, with upset levels exceeding 1×1010 rads(GaAs)/s.

A self-aligned gate superlattice (Al,GA)As/n+-GaAs MODFET 5 × 5-bit parallel multiplier

A 5 × 5-bit parallel multiplier circuit has been demonstrated with self-aligned gate superlattice (Al,Ga)As/n+-GaAs modulation-doped FET's (MODFET's). Multiplication times (gate delays) and corresponding power dissipations of 1.80 ns (73 ps/gate) at 0.43 mW/gate and 1.08 ns (43 ps/gate) at 0.75 mW/gate were measured at room temperature and 77 K, respectively. These are the shortest gate propagation delays ever reported for parallel multiplier circuits at room temperature or 77 K using any semiconductor IC technology.

High-performance self-aligned gate (Al,Ga)As/GaAs MODFET's on MBE Layers grown on

Self-aligned gate (Al,Ga)As/GaAs modulation-doped FET's (MODFET's) have been demonstrated on MBE layers grown on

Low-noise behavior of InGaAs quantum-well-structured modulation-doped FET's from 10-2to 108Hz

Equivalent gate noise voltage spectra of 1-µm gate-length modulation-doped FET's with pseudomorphic InGaAs quantum-well structure have been measured for the frequency range of 0.01 Hz to 100 MHz and compared with the noise spectra of conventional AlGaAs/ GaAs MODFET's and GaAs MESFET's. The prominent generation-recombination (g-r) noise bulge commonly observed in the vicinity of 10 kHz in conventional MODFET's at 300 K does not appear in the case of the new InGaAs quantum-well MODFET. Instead, its noise spectra indicate the presence of low-intensity multiple g-r noise components superimposed on a reduced 1/f noise. The LF noise intensity in the new device appears to be the lowest among those we have observed in any MODFET or MESFET. The noise spectra at 82 K in the new device represent nearly true 1/f noise. This unusual low-noise behavior of the new structure suggests the effectiveness of electron confinement in the quantum well that significantly reduces electron trapping in the n-AlGaAs, and thus eliminates the g-r noise bulge observed in conventional MODFET's.

Characterization of extremely low contact resistances on modulation-doped FET's

The measurement and affects of extremely low contact resistances on GaAs/AlGaAs modulation-doped FET's (MODFET's) were studied. Ohmic contacts were characterized by a variety of test structures in close proximity to each other. Measurements of front, end, and interfacial contact resistance were used to calculate the specific contact resistance by the transmission line model (TLM) and the two-dimensional resistor network model. The validity of these models in describing contacts to GaAs is discussed. Normalized contact resistance as low as 0.06 Ω . mm in MODFET's exhibiting semiconductor sheet resistance of 536 Ω/ are reported, contributing only 10 percent to the correspondingly reduced parasitic resistances. This leads to high extrinsic transconductances of 240mS/mm at 300 K in devices with a gate length of 1.4 µm and to extraordinary small drain-source saturation voltages of about 0.6 V. Both effects lead to much improved MODFET performance which would be not only advantageous in small-signal, but especially in high-speed/low-power logic circuits.

Determination of 2-D Electron-gas carrier mobility in short gate-length MODFET's by direct elimination of parasitic resistance effects

Two-dimensional (2-D) electron-gas carrier mobiliy in 1µm gate-length modulation-doped FET's has been determined as a function of the gate bias voltage. The measurement technique utilizes a small-signal gate voltage excitation and probes the true channel conductance by directly eliminating the source and drain series resistance effects. The mobility is extracted by combining the channel conductance data with the CV data and its variation with gate bias voltage is indicative of the variation of the effectiveness of screening of the ionized impurities and the quality of the MBE-grown GaAs buffer layer.

Design calculations for submicron gate-length AlGaAs/GaAs modulation-doped FET structures using carrier saturation velocity/charge-control model

dc and small-signal ac characteristics of AlGaAs/GaAs modulation-doped FET's have been calculated by combining the conventional charge-control model with the carrier velocity saturation concept in realistic submicron gate structures. For the desired threshold voltage and the maximum gate voltage swing, the designer basically selects the thickness of the n-AlGaAs layer, for the preferred doping concentration, from the design graphs provided. Representative theoretical I/V characteristics and saturation current and transconductance versus gate voltage graphs are presented for the 0.25 μm and 0.8 μm gate-length structures with and without the presence of source and drain series resistances.

Noise behavior of 1-µm gate-length modulation-doped FET's from 10-2to 108Hz

Measured equivalent gate noise voltage spectra of 1-µm gate-length MODFET's, for the frequency range of 0.01 Hz-100 MHz, are presented. They indicate that the noise consists of several high-intensity trap-related generation-recombination (g-r) noise components superimposed on a background <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/f</tex> noise. The g-r noise is reduced when the Al mole-fraction is lowered. The same occurs when the gate reverse bias is increased. At 100 K the g-r noise bulge moves towards subaudio frequencies clearly revealing the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/f</tex> noise.

Magnetoresistance method to determine GaAs and AlxGa1−xAs mobilities in AlxGa1−xAs/GaAs modulation-doped field-effect transistor structures

Charge carrier mobilities are conveniently measured in simple, homostructure field-effect transistors (FET’s) by means of the geometric magnetoresistance (GMR) technique. Heterostructure FET’s, however, are more complicated because of multiple conducting regions, as well as multiple conducting bands within a given region. We apply a multilayer GMR mobility model to a frequently used heterostructure FET design, namely, the Al0.3Ga0.7As/GaAs modulation-doped FET (MODFET). By analyzing the results at different magnetic fields, we can separate the contributions of the various GaAs subbands and the Al0.3Ga0.7As conduction band. In the particular MODFET structure studied here, the lowest GaAs subband mobility ranges from 5.7×103 cm2/Vs at threshold to 6.9×103 cm2/Vs at saturation, while the Al0.3Ga0.7As mobility is about 5×102 cm2/Vs. This is the first time that the various mobilities in MODFET structures have been separately measured.

A new model for modulation-doped FET's

A new study showing that Lehovec and Zuleeg analysis for silicon FET's can be extended to the GaAs FET's is presented. An analytical model that effectively predicts the characteristics of the MODFET is introduced. The agreement between the model and the experimental data appears to be very good.

Model for the temperature dependence of the threshold voltage of modulation-doped field-effect transistors

A model for the temperature dependence Of the threshold voltage of modulation-doped FET's caused by traps in doped AlGaAs is presented. The model takes into account the charge distribution in the depletion region determined by the temperature and time-dependent occupation of traps. The theory shows excellent agreement with experiment in the temperature range 77 to 400 K.

Summary Abstract: Electron traps in MBE AlxGa1−xAs and the temperature dependence of the threshold voltage of modulation-doped FET’s

Summary Abstract: Electron traps in MBE AlxGa1−xAs and the temperature dependence of the threshold voltage of modulation-doped FET’s

Modulation-doped FET DCML comparator

A differential current mode logic (DCML) comparator has been inplemented with modulation-doped FET technology. The dynamic input/output hysteresis is low enough to realise a flash quantiser with up to 5-bit accuracy.

Complementary p-MODFET and n-HB MESFET (Al,Ga)As transistors

A complementary transistor structure that utilizes a p-channel modulation-doped FET (p-MODFET) to produce improved p-channel characteristics if reported for the first time. The structure also includes a new type of n-channel MESFET in which the electrons are confined by an n-p heterojunction barrier (n-HB MESFET). The complementary transistor pair is fabricated on a novel MBE-grown n-GaAs/p-(Al,Ga)As/i-GaAs heterostructure. The experimental current-voltage characteristics are presented and demonstrate that this is a promising approach for the development of GaAs-based high-speed complementary logic circuits.

Determination of carrier saturation velocity in short-gate-length modulation-doped FET'S

Based on a combined carrier saturation velocity/charge-control model, measured drain saturation current, small-signal transconductance and channel conductance data have been analyzed with consistency for the determination of the effective carrier saturation velocity using 1-µm gate-length MODFET's. The results demonstrate the validity of the model in the mid-range of gate bias voltage and indicate the extent of deviations that occur due to different physical processes in the lower and higher gate bias ranges.

A new technique for characterization of the "End" resistance in modulation-doped FET's

We propose a new extended "end" resistance measurement technique to determine the series source and drain resistance of a field-effect transistor. In this method the small-signal "end" resistance, defined as a derivative of the drain-source voltage, with respect to the gate current, is measured as a function of the drain current. The "end" resistance includes the contributions from the source series resistance and from the resistance related to the conducting channel under the gate. As the drain current increases, the drain side of the channel becomes more reverse biased with respect to the gate. This shifts the gate current distribution towards the source, decreasing the channel contribution to the "end" resistance. By extrapolation to infinite drain current the channel contribution can be eliminated and the series resistance of the undepleted source and drain regions can be accurately determined. The technique is applied to a long-gate modulation-doped field-effect transistor and is shown to Yield reasonable values of the series resistances.

On the collapse of drain I-V characteristics in modulation-doped FET's at cryogenic temperatures

The collapse of the drain current-voltage characteristics of modulation-doped field-effect transistors (MODFET's) at cryogenic temperatures, previously thought to be unavoidable, has been investigated. The results indicate that the mechanism responsible for the collapse is dependent on both the device fabrication steps and the parameters of crystal growth. Bulk Al x Ga 1 - x AsFET's fabricated in our laboratory exhibited little or no collapse in the I-V characteristics at 77 K in the dark, demonstrating that the mechanism responsible for this pheonomenon is not related to problems associated with contacting Al x Ga 1 - x As. MODFET's with proper fabrication and growth procedures showed no collapse. In those devices exhibiting no collapse, the source resistance exhibited a substantial decrease upon cooling. At 300 K source resistances slightly over 1.0 Ω . mm with a transconductance of 170 mS/mm were obtained. Upon cooling, the source resistance decreased to 0.5 Ω . mm with a transconductance of 280 mS/mm. These results demonstrate that MODFET's will exhibit enhanced performance at 77 K without exposure to light. Specific contact resistivities measured at room temperature ranged from 2 × 10-7to 2 × 10-6Ω cm2depending on the structural parameters.