An N-polar Al0.65Ga0.35N/Al0.85Ga0.15N high electron mobility transistor (HEMT) was demonstrated on an AlN substrate. The epitaxial layers were grown by plasma-assisted molecular beam epitaxy (PAMBE), and a 3 nm in situ GaN cap was employed to prevent surface oxidation of the AlGaN channel. To reduce contact resistance, highly Si-doped, compositionally graded AlGaN was selectively regrown by PAMBE in the contact regions. Peak drain currents of 0.12 mA/mm at room temperature and 0.8 mA/mm at 300 °C were measured. Transconductance also increased from 53 μS/mm at room temperature to 262 μS/mm at 300 °C. Temperature-dependent Hall measurements indicated a reduction in channel resistance with increasing temperature, attributed to enhanced carrier density and mobility. A breakdown voltage of 95.8 V was obtained, along with a corresponding critical electric field of 2.74 MV/cm. While further improvements in current density are necessary for practical applications, this paper reports the first N-polar AlGaN channel HEMTs grown by PAMBE.
Kosanovic et al. (Sun,) studied this question.