The role of setback layers on the breakdown characteristics of AlGaAs/GaAs/GaN HBTs

Abstract

Breakdown voltages (VCBO) of AlGaAs/GaAs/GaAs and AlGaAs/GaAs/GaAs‐setback/GaN HBTs have been compared both theoretically and experimentally with respect to setback layer thicknesses and the doping type. VCBO was calculated using a non‐local energy model. The hard breakdown voltage was measured on as‐grown GaAs homojunctions and AlGaAs/GaAs/GaN HBTs formed by direct wafer fusion. The calculation showed an increase of VCBO from ∼ 20 V to ∼ 325 V by replacing the GaAs collector with GaN, and VCBO ∼90 V was indeed measured. The smaller than predicted VCBO is attributed to the large leakage current currently present in the fused junctions. Insertion of a lightly doped GaAs setback layer has resulted in improved current gain of the fused HBTs, but it also degrades the breakdown characteristics. 20‐30 nm setback layers were found to maintain VCBO significantly higher than that of GaAs homojunctions and likely reasonable current gains. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)