Correlation between sheet carrier density-mobility product and persistent photoconductivity in AlGaN/GaN modulation doped heterostructures

High quality Al0.25Ga0.75N/GaN modulation-doped heterojunction field-effect transistor (MOD-HFET) structures grown on sapphire substrates with high sh eet carrier density and mobility products (n(s)mu > 10(16)/Vs room temperat ure) have been grown by metal organic chemical vapor deposition (MOCVD). Th e optimized structures were achieved by varying structural parameters, incl uding the AlGaN spacer layer thickness, the Si-doped AlGaN barrier layer th ickness, the Si-doping concentration, and the growth pressure. In these str uctures, the persistent photoconductivity (PPC) effect associated with the two-dimensional electron gas (2DEG) system was invariantly observed. As con sequence, the characteristic parameters of the 2DEG were sensitive to light and the sensitivity was associated with permanent photoinduced increases i n the 2DEG carrier mobility (mu) and sheet carrier density (n(s)). However, we observed that the magnitude of the PPC and hence the photoinduced insta bility associated with these heterostructures were a strong function of onl y one parameter, the product of n(s) and mu, which is the most important pa rameter for the HFET device design. For a Fixed excitation photon dose, the ratio of the low temperature PPC to the dark conductivity level was observ ed to decrease from 200% to 3% as the n(s)mu (300 K) product was increased from 0.048 x 10(16)/Vs to 1.4 x 10(16)/Vs. Based on our studies, we suggest that the magnitude of the low temperature PPC can be used as a sensitive p robe for monitoring the electronic quality of the AlGaN/GaN HFET structures .