Mechanism and enhancement of the near-threshold low OFF-state breakdown voltage in gallium nitride high electron mobility transistors

Abstract

We studied AlGaN/GaN high electron mobility transistors grown on a SiC substrate. They had a hard breakdown voltage V BR ≥ 70 V under a deep OFF-state with a much less and softer V BR ∼ 27 V for near-threshold OFF-state gate bias conditions. We established that the low, soft breakdown is due to space charge limited current (SCLC) from drain to source via the GaN buffer, which has deep traps. Previous works had established the presence of SCLC from bulk to gate and drain to gate. For the first time we establish drain to source SCLC. We achieved this by analyzing the I DS-V DS data extracted from the measured I S-V DS. We determined the following by numerical simulation: (a) the density and energy level of traps causing the observed SCLC; (b) the minimum trap density required to suppress the SCLC and raise the V BR up to the avalanche limit for the whole OFF-state regime for devices with a channel length down to 0.15 μm.