Ballistic transport at GHz frequencies in ungated HEMT structures

By measuring the ac impedance, we measure for the first time the crossover from diffusive (ωτ<1) to ballistic (ωτ>1) transport as a function of frequency (dc to 5 GHz) in a dc contacted 2d electron gas in the low electric field limit, where τ is the momentum scattering time. The geometry is an "ungated HEMT", meaning current flows through an ohmic contact into a 2d electron gas, laterally through the 2d electron gas, and out into a second ohmic contact; the 2d electron gas itself is not gated. At the measurement temperature (4.2 K), the low field mobility is 3.2×10 ⁶ cm²/Vs. We also measure for the first time the frequency dependent contact impedance in this ballistic limit.