A quantitative approach for trap analysis between Al_0.25Ga_0.75N and GaN in high electron mobility transistors

The characteristics of traps between the Al_0.25Ga_0.75N barrier and the GaN channel layer in a high-electron-mobility-transistors (HEMTs) were investigated. The interface traps at the Al_0.25Ga_0.75N/GaN interface as well as the border traps were experimentally analyzed because the Al_0.25Ga_0.75N barrier layer functions as a dielectric owing to its high dielectric constant. The interface trap density D_it and border trap density N_bt were extracted from a long-channel field-effect transistor (FET), conventionally known as a FATFET structure, via frequency-dependent capacitance-voltage (C-V) and conductance-voltage (G-V) measurements. The minimum D_it value extracted by the conventional conductance method was 2.5 × 10¹² cm^-2·eV^-1, which agreed well with the actual transistor subthreshold swing of around 142 mV·dec^-1. The border trap density N_bt was also extracted from the frequency-dependent C-V characteristics using the distributed circuit model, and the extracted value was 1.5 × 10¹⁹ cm^-3·eV^-1. Low-frequency (1/f) noise measurement provided a clearer picture of the trapping-detrapping phenomena in the Al_0.25Ga_0.75N layer. The value of the border trap density extracted using the carrier-number-fluctuation (CNF) model was 1.3 × 10¹⁹ cm^-3·eV^-1, which is of a similar level to the extracted value from the distributed circuit model.