LOW-TEMPERATURE ELECTROLUMINESCENCE SPECTROSCOPY OF HIGH-ELECTRON-MOBILITY TRANSISTORS ON INP

Electroluminescence spectroscopy of short gate high-electron-mobility transistors (HEMTs) on InP substrates is performed at cryogenic temper atures. Electroluminescence is a reliable tool to investigate impact i onization as compared to studies based on gate current which depend on the weakness of the intrinsic gate current intensity. In on-state bia sed devices, a low energy (0.7-0.9 eV) recombination band is observed which is related to radiative recombination of carriers created by imp act ionization in the low band gap InGaAs channel. The evolution of th e luminescence intensity versus bias applied to the device shows that the electroluminescence intensity and impact ionization depend on two competing parameters: the electric field in the gate-drain access area and the drain current intensity. We show that the so-called ''kink'' effect, which is a noticeable increase of the output conductance and w hich is observed at relatively moderate drain bias (600-750 mV) in our devices, is not correlated with impact ionization. The electrolumines cence of the device in the off state is also investigated. This study allows the direct observation of-impact ionization in the off-biased c hannel of InP-based HEMTs. In this low current regime, the electrolumi nescence intensity follows the electric field, i.e., the drain-gate vo ltage until breakdown of the device occurs. The voltage breakdown of t he device in the off state is discussed in terms of impact ionization in the InGaAs channel due to hot carriers originating from the gate le akage current. Finally, a method for reducing or avoiding impact ioniz ation in these devices, i.e., for increasing the device reliability, i s discussed. (C) 1996 American Institute of Physics.