Emitter-metal-related degradation in InP-based HBTs operating at high current density and its suppression by refractory metal

Abstract

We investigated the reliability of InP-based HBTs with a ledge structure, focusing on emitter-metal-diffusion-induced degradation. Bias-temperature accelerated tests under high temperatures and high current densities of up to 5 mA/μm 2 were conducted for HBTs with conventional emitter electrodes, whose metal configuration was Ti/Pt/Au, and for HBTs with refractory metal emitters of Ti/Mo–Ti/Pt/Au, Ti/W–Ti/Pt/Au, or W–Ti/Pt/Au. The emitter contact layer was analyzed by transmission electron microscopy, energy dispersive X-ray spectroscopy, and transmission electron diffraction. Severe damage and disruption of uniformity of the atomic composition were observed due to diffusion of Ti and Au in HBTs with conventional emitter, whereas suppression of those degradations was observed in HBTs with refractory emitter. Refractory metals were found to be advantageous for blocking upper metal diffusion. Interstitials of host species generated due to metal diffusion must cause a shift of atomic composition. The time-wise change in the emitter resistance was estimated to compare the speed of the contact layer degradation between different emitter electrode metals. The critical time, which we determined as an emitter resistance increases of 3% from the initial value, increased by one order for HBTs with refractory metal than for HBTs with conventional metal at the same junction temperature, despite the same activation energy of 2.0 and 1.65 eV for J c of 2 and 5 mA/μm 2, respectively, for all types of emitter electrode. The advantages of refractory metal for improving the reliability of InP HBTs were confirmed, especially for operation at high current densities.