M. Hurt

Enhancement of Schottky barrier height in heterodimensional metal-semiconductor contacts

We report on the measurements of the heterodimensional Schottky barrier height in two-dimensional metal-semiconductor field effect transistors (2D MESFETs). Our experimental data indicate approximately 0.1 eV greater barrier height compared to conventional metal-semiconductor contacts of the same materials. The enhancement is explained in terms of two effects—quantization of energy levels of the carriers in the quantum well and broadening of the corresponding wave functions. The increased barrier height leads to a substantial reduction of the gate leakage current in 2D MESFETs.

Narrow channel 2-D MESFET for low power electronics

A 2-D MESFET utilizing sidewall Schottky contacts on either side of a very narrow 2-d electron gas channel is described. Record transconductance of 295 and 130 mS/mm have been achieved at room temperature in 1.0 and 0.5 micron wide devices, respectively. We also present accurate 2-D MESFET current-voltage and capacitance-voltage models. These models have been implemented into AIM-Spice which was used to simulate DCFL inverter and ring oscillator circuits. The ring oscillator simulations predict a power-delay product of less than 0.1 fJ/gate at room temperature, suggesting that the 2-D MESFET may be useful for ultra low power electronics applications. >

Novel heterodimensional diodes and transistors

Abstract We describe novel heterodimensional devices which utilize Schottky barriers to a two-dimensional (2D) electron gas. These devices include a 2D–3D Schottky diode, an AlGaAs GaAs Schottky Gated Resonant Tunneling Transistor (SGRTT), an AlGaAs InGaAs 2D Metal Semiconductor Field Effect Transistor (2D MESFET), and a Coaxial MESFET. These devices hold promise of ultra low power, high speed operation. The 1 micron wide 2D MESFET, which has a very low output conductance and a steep subthreshold slope, exhibited the highest transconductance of any 1 μm wide device.

Sub-half-micrometer width 2-D MESFET

Two-dimensional (2-D) MESFETs having sub-half-micron channel widths have been fabricated on double-/spl delta/-doped Al/sub 0.24/Ga/sub 0.76/As/In/sub 0.18/Ga/sub 0.82/As/GaAs heterostructures. The 2-D MESFET operates like a normal transistor at room temperature but uses very few electrons in the channel (about 500 at peak current and 5 at threshold). Also, the Narrow Channel Effect (NCE) and Drain-Induced Barrier Lowering (DIBL) (two effects which limit the minimum power operation in conventional devices) have been practically eliminated. The 0.4 micron wide device had an ON/OFF current ratio of 10/sup 5/, a peak transconductance of 100 mS/mm, a threshold voltage of 0.3 V, a saturation voltage of 0.2 V, and a subthreshold ideality factor of 1.1. The 2-D MESFET DCFL inverter had a switching voltage and noise margin of 0.35 V and 0.26 V, respectively, at 0.8 V supply. These room temperature results suggest that the 2-D MESFET is an excellent candidate for future low power digital electronics applications.

Heterodimensional field effect transistors for ultra low power applications

We describe a new class of field effect transistors (FETs) based on a heterodimensional contact between a three-dimensional gate (metal or semiconductor) and a two-dimensional electron gas. The heterodimensional FET family (2D MESFET, 2DI MESFET, and 2D JFET) has shown significant promise for future high speed, ultra low power applications. We review the recent developments, and report on a new fully ion implanted quasi-heterodimensional FET, the coax-2D JFET.

High-temperature characteristics of 2-D MESFETs

Experimental data of 2-D MESFETs, which utilize sidewall Schottky contacts to degenerate two-dimensional electron gas, indicate a much weaker temperature dependence of the drain current compared to conventional MESFETs in the temperature range from 25-150/spl deg/C. Measured drain current characteristics show that the 2-D MESFET structure exhibits negligible threshold voltage shift with temperature in this temperature range. The negligible threshold voltage shift can be explained in terms of a nearly temperature independent built-in voltage related to the degeneracy of the two-dimensional electron gas. Furthermore, the low-field mobility extracted from the measured transconductance exhibits a smaller degradation with increasing temperature compared to conventional MESFETs. For our devices, the mobility drops by approximately 25% over the temperature range 25-125/spl deg/C, compared to 40-50% for conventional MESFETs. The smaller temperature variations of the low-field mobility are linked to a more effective screening of impurity scattering by the two-dimensional electron gas.

Quasi-three-dimensional modeling of a novel 2-D MESFET

We present a quasi-three-dimensional method for modeling novel semiconductor devices. The first step of our method is a two-dimensional numerical simulation of the device cross-section under different gate biases. Next, we use the interpolated results of the two-dimensional numerical simulation as input to the analytical theory describing the potential distribution and current in the third dimension. As an example, this method is applied to a novel two-dimensional AlGaAs/InGaAs Metal-Semiconductor Field Effect Transistor (2D MESFET).

Heterodimensional technology for ultra low power electronics

We describe new heterodimensional technology suitable for ultra low power applications. This technology uses Schottky barrier contacts between three-dimensional metal and two-dimensional electron gas. The low power performance is due to the following: the small capacitance of the 2D-3D junction; the concentration of the depletion layer electric field streamlines in the active channel; suppression of parasitic resistance; small leakage current; and, most of all, due to the total elimination of the narrow channel effect which allows us to scale the device width to submicron dimensions. We present, compare, and discuss measured and simulated I-V and C-V characteristics for the 2D-3D Schottky diode, 2D MESFET and Schottky Gated 2D-3D RTT.

Scaling of two dimensional MESFETs for ultra low power applications

Presents new experimental data and simulations of AlGaAs/InGaAs/GaAs two dimensional MESFETs (2D MESFETs) which utilize sidewall Schottky contacts on either side of a very narrow 2D electron gas channel. These devices demonstrate excellent scaling characteristics down to submicron dimensions in both the channel length and the width, which are attributed to the special geometry of the 2D-3D contacts suppressing both the narrow channel effect (NCE) and the drain induced barrier lowering (DIBL). Specifically, when the device was scaled from 1.0/spl times/1.0 /spl mu/m/sup 2/ to 0.8/spl times/0.5 /spl mu/m/sup 2/, output conductance was reduced from 40 mS/mm to less than 1 mS/mm, knee voltage was reduced from 0.75 V to 0.25 V, and the ideality factor was reduced from 1.3 to 1.08, while the threshold voltage became less negative from -0.5 V to 0.3 V as expected. An excellent source-drain breakdown voltage over 10 V, and a current ON/OFF ratio over 105 were also observed. The gate leakage current remains small up to 0.6 V gate bias, demonstrating a good Schottky barrier between the side gates and the 2D electron gas. These characteristics compare favorably with those of a conventional HFET with similar dimensions.

Breakdown behavior of low-power pseudomorphic AlGaAs/InGaAs 2-D MESFET's

We report the breakdown behavior of the two-dimensional (2-D) MESFET, which is a low-power heterodimensional transistor having dual side gates that contact the edge of the two-dimensional electron gas (2-DEG) channel in a double-side planar-doped pseudomorphic Al/sub 0.24/Ga/sub 0.76/As/In/sub 0.17/Ga/sub 0.83/As material structure. Low output conductance (less than 6 mS/mm for V/sub GS/=0 V) and low gate leakage current (less than 100 nA) are measured out to a drain-source bias of 20 V, indicating that the effects of impact ionization are reduced in the 2-D MESFET. Excellent off-state drain-source and drain-gate breakdown voltages are experimentally measured to be 20 and 21 V, respectively. We attribute these high breakdown values to the electric and geometric properties of the heterodimensional Schottky metal/2-DEG junction.