Researchers are following two approaches in an effort to improve the low-noise performances of InP-based high-electron-mobility transistors (HEMTs) that could be used as front-end amplifiers in millimeter-wave receivers. These devices are designed to operate while cooled to temperatures ≤ 20 K, where they function with high gain, low leakage, and the lowest noise levels of any transistors operating in the millimeter-wavelength range. However, the low-noise performances of these devices are still slightly below those of nontransistor state-of-the-art devices.

A Typical InGaAs/InAlAs/InP HEMT is depicted here in cross section (not to scale). Research directed toward improving low-noise performance has followed two approaches: (1) optimizing the indium content and thickness of the channel layer and (2) shortening the gate.

The two approaches to improvement are best explained with the help of a cross-sectional view of a typical InGaAs/InAlAs/InP HEMT (see figure). The first approach involves increasing the indium content, x, of the InxGa1 – xAs channel layer, which is grown by molecular-beam epitaxy. Increasing x results in enhanced electron-transport properties, which translate to higher cutoff frequency and higher gain. Unfortunately, increasing x also increases the strain in the InxGa1 –xAs layer with respect to the InP substrate; as a result, the thickness of the InxGa1 – xAs must not be allowed to grow beyond a maximum critical thickness that decreases with increasing x. The thickness of the channel layer is important because the thickness of the quasi two-dimensional gas in this layer determines the quantum electron states and the confinement and density of electrons in the channel. If the channel is too thin, then the performance of the device is degraded.

In this research, devices are fabricated with channels of x = 0.60, 0.65, and 0.80 and corresponding thicknesses. In the case of the x = 0.80 channel, the indium content is not uniformly 0.80; instead, it is graded to a maximum of 0.80 to minimize the strain and thereby make it possible to grow the thickest possible channel layer while benefitting from the transports with the highest feasible x.

The second approach involves reduction of the gate length from 0.1 µm to ≤ 0.07 µm. Reduction of the gate length causes a reduction of internal capacitances, leading to lower noise, higher cutoff frequency, and higher gain. For example, decreasing the gate length to 0.07 µm can increase the cutoff frequency by as much as 25 percent.

There are two challenges to reduction of gate length. The first challenge is the difficulty of developing a high-yield manufacturing process to build 0.07-µm gates. Even electron-beam state-of-the-art electron lithography has thus far been limited to features with dimensions ≥ 0.1 µm. The second challenge is a short-channel effect; if the gate is too short, it cannot effectively pinch off the channel. This short-channel effect can severely limit device yield in mass production.

Both approaches have been followed in the fabrication of HEMTs in a dedicated 8-wafer lot split for the various indium compositions and gate lengths. The information gained from the devices on these wafers has provided insight on how to build the ultimate cryogenic millimeter-wave solid amplifier.

This work was done by Richard Lai of TRW Inc. for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Electronics & Computers category. NPO-20341


This Brief includes a Technical Support Package (TSP).
Two approaches to improvement of InGa/InA1As/InP HEMTs

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This article first appeared in the February, 2000 issue of NASA Tech Briefs Magazine.

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