Measurement of the hot electron mean free path in GaN

We present a method for measuring the mean free path (MFP) and relaxation time of hot electrons in GaN using the hot electron transistor (HET). In this device electrons are injected over a high energy emitter barrier into the base where they experience quasi-ballistic transport well above the conduction band edge. After traversing the base, high energy electrons either surmount the base-collector barrier and become collector current or reflect off the barrier and become base current. We fabricate HETs with various base thicknesses and measure the common emitter transfer ratio ($\alpha$) for each device. The MFP is extracted by fitting a decaying exponential to $\alpha$ as a function of base width and the relaxation time is computed using a suitable injection velocity. For current devices with an injection energy of $\sim$1eV, we measure a hot electron MFP of 14nm and calculate a relaxation time of 16fs. These values are in agreement with theoretical calculations where longitudinal optical (LO) phonon scattering is the dominant relaxation mechanism.