Measurement of unity gain cutoff frequency and saturation velocity of a GaN HEMT transistor

The measured intrinsic saturation velocity (vsi) of carriers in a gallium nitride (GaN) high electron mobility transistor (HEMT) is very much lower than that predicted using Monte Carlo simulation. A novel method of extraction of the intrinsic saturation velocity (vsi) of carriers has been developed utilising the deembedded s-parameters, thus enabling the calculation of vsi over a wide range of bias conditions. The method is equally applicable for gallium arsenide (GaAs) and indium phosphide (InP) based transistors. The measurements indicate for GaN-based HEMT a maximum deembedded saturation velocity of 1.1×105 m/s close to the pinchoff voltage (VP). It was found that self-heating had only a weak effect on the saturation velocity up to junction temperatures approaching 140°C above ambient.