Investigation of Coulomb scattering centers using a substrate bias effect in n -channel metal–oxide–semiconductor field-effect transistors with AlSiO/AlN/ p -type GaN gate structures

Electron scattering factors are analyzed for GaN metal–oxide–semiconductor field-effect transistors (MOSFETs) using gate-biased Hall-effect measurements. The fabricated MOSFETs and Hall devices have AlSiO/AlN/p-type GaN gate structures formed on bottom p+/n+ GaN junctions. Because the capacitance of the bottom p+/n+ junction is much greater than the gate capacitance, the p-type body layer and the substrate electrode are short-circuited. Thus, the body potential is controlled by the substrate bias (Vsub). To investigate Coulomb scattering due to Mg dopants, MOSFETs with Mg-doped and undoped body layers are prepared. Comparing the sheet carrier densities Ns estimated from a capacitance–voltage curve and the Hall effect reveals that almost all electrons contribute to conduction in both channels. In the plot of Hall-effect mobility as a function of Ns, Coulomb scattering components are proportional to Nsγ. If Coulomb scattering is mainly caused by bulk impurities, γ should be unity. However, γ < 1 is obtained, indicating the existence of Coulomb scattering centers other than Mg atoms. The effective electric field perpendicular to the channels is essentially different between both channels, resulting in differences in the scattering magnitudes. Vsub allows control of the effective channel field independent of the gate bias. The effective channel field in the Mg-doped channel at Vsub = 0 V is almost equivalent to that in the undoped channel at −40 V. Then, the Hall-effect mobilities are almost equal. We conclude that Coulomb scattering centers are mainly located near the interface between the gate insulator and the channel.