Normally-off Al0.25Ga0.75N/GaN MOSHEMT with stack gate dielectric structure

A Stack Gate Metal Oxide Semiconductor Field Effect Transistor (MOSHEMT) structure having HfO₂ and SiO₂ as gate dielectric is proposed. For achieving enhancement mode operation, very thin i.e. only 3nm AlGaN layer is considered as barrier. This is not sufficient to form adequate amount of two dimensional electron gas (2DEG) density at the hetero interface at zero gate bias for current conduction. In order to investigate the effect of oxide/semiconductor interface charge on device performance, the dielectric arrangement is interchanged to construct HfO₂/SiO₂/AlGaN/GaN (HS-MOSHEMT) and SiO₂/HfO₂/AlGaN/GaN (SH-MOSHEMT). The HS-MOSHEMT device shows comparatively better transconductance of 180mS/mm and a positive threshold voltage (V_th) of 0.5V due to negative oxide/barrier interface charge. Therefore this device can be used as a constituent of cascode combination in Power electronic applications, whereas SH-MOSHEMT possesses cut-off frequency (f_T) of 16GHz and maximum oscillation frequency (f_MAX) of 60GHz, thus it is very much suitable for RF applications.