Vertical GaN n-channel MISFETs on ammonothermal GaN substrate: Temperature dependent dynamic switching characteristics

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Switching of GaN-based vertical-trench gate MISFET on ammonothermal n-type GaN substrate are studied as a function of temperature. Pulsed on-state IV characterization revealed three unexpected independent effects. First, drain current increases with temperature, although electron mobility decreases with temperature. Second, drain current decreases with pulse length, which cannot be explained by heating. Third, dynamic Ron increases with off-state drain bias, which usually is considered as an issue of lateral GaN HFETs only. The drain current temperature dependence is most likely dominated by the low electron mobility in the inversion layer beneath the gate. The field effect channel mobility is estimated as < 10 cm2/V s and can thus be considered as trap limited and thermally activated, Ron decreases from 133 Ω mm at 20 °C to 62 Ω mm at 110 °C. Longer on-state pulses lead to negative charging of the ALD-Al2O3 gate oxide and shift the threshold voltage (DC-Vth ~8 V) positively. With the maximum applicable Vgs = 10 V, the drain current is not saturated yet and Ids thus drops by a factor 2 when increasing the on-state pulse width from 0.2 µs to 20 µs due to the Vth-shift. The observed 10-times dynamic Ron increase with off-state drain bias up to 30 V can be related to possible charging mechanism in the gate oxide as well. Activation energies for different off-state stress voltage are ranged linear between 0.08 eV and 0.26 eV for drain bias stress of Vds = 0 V and 30 V respectively.