The epHEMT gate at microwave frequencies

This paper examines the high-frequency behavior of the enhancement-mode pseudomorphic high electron-mobility transistor (epHEMT) gate. During this study, no bias was applied between the drain and source. Rather, the gate was forward biased with either the drain, source, or channel (drain and source connected together) grounded. While applying positive voltage V/sub g/ to the gate, one-port S-parameters were measured from 0.1 to 10 GHz and then converted to Z-parameters. Plotting the real part R of the impedance reveals two sharp peaks. The first peak occurs near the device threshold voltage for conduction in the InGaAs well. A second peak occurs at higher voltages where conduction begins to occur in the surface AlGaAs layer. An equivalent-circuit model is proposed to account for the epHEMT gate's high-frequency behavior and the proposed model is shown to be in good agreement with the experimental data.

I. Introduction

Enhancement-Mode pseudomorphic high electron-mobility transistors (epHEMTs) are increasingly popular in circuits other than amplifiers. The gate of a high electron-mobility transistor (HEMT) can be used in place of a diode for designing mixers [1], [2], downconverters [3], detectors [4], and voltage-controlled oscillators (VCOs) [5]. To achieve the performance for the application requires a thorough understanding of the gate at high frequency. Measurements of an epHEMT gate have been made to understand the physical mechanisms at RF and microwave frequencies.