Current collapse, memory effect free GaN HEMT

Current collapse (drain current dispersion, gradual power saturation, or memory effect) encountered during microwave GaN HEMT power amplifier operation remains to be a major reliability and stability issue for the highly promising, emerging III-nitride, polar semiconductor based technology. Current collapse leads to bias condition induced memory effect, which is particularly detrimental to broad band RF amplifiers operating with large peak-to-average signal level. Loss of polarization induced, surface mobile holes, accompanied by the reduction of transistor channel electron density have recently been identified as the root cause of these undesirable, transient effects exhibited by the polar semiconductor based transistors, in which charge carriers are primarily originated from the built-in electric dipoles of the nitride material. A novel ohmic metal contact scheme, presenting low energy barrier for both the transistor channel electrons, and the surface mobile holes, is found to be effective in eliminating the undesirable, time and signal strength dependence behavior of the transistor performance characteristics. A GaN MISHEMT with such compound ohmic contacts is found to be capable of operating at higher saturation current density, compared to that of a GaN HEMT with traditional ohmic contacts. The new finding represents a major breakthrough in polar semiconductor device technology for microwave power amplification.