Reduced dynamic on-state resistance for GaN power transistors
GaN-based high-voltage switching transistors enable particularly efficient power converters due to their low area-specific on-state resistance and low gate capacitance. Due to lower losses per switching cycle, GaN-based converters can operate at higher frequencies than converters with Si-based switches, enabling more compact and light-weighted converters. However, the on-state resistance is increased for short times after some 100 V off-state drain stress. This so called increased dynamic on-state resistance may counteract the GaN-HFET-related fast switching opportunity for high off-state voltages. Charging of the GaN semiconductor and of the semiconductor passivation under high electric fields typically present for the transistor off-state condition are the root causes. The injected charges remain inside the device structure after the transistor is switched to on-state and reduce the electron concentration in the transistor channel – the on-state resistance is increased.
The magnitude of increased dynamic on-state resistance is determined by the GaN buffer composition. Carbon doping is used to elevate the voltage blocking strength of the semiconductor, but it is also a major source for increased dynamic on-state resistance. Target is high electric blocking strength combined with low dynamic on-state resistance.
Recently developed GaN buffer compositions using AlGaN or iron-doped GaN feature electrical blocking strengths comparable to carbon-doped GaN buffer with low doping concentrations. But they show a significantly reduced dynamic on-state resistance. In addition to improved semiconductor material quality, a homogeneous electric field distribution inside the device may reduce the dynamic on-state resistance. Its increase gets reduced to 1/3 when applying gate-connected field plates to the devices.
O. Hilt, E. Bahat-Treidel, E. Cho, S. Singwald, J. Würfl, "Impact of Buffer Composition on the Dynamic On-State Resistance of High-Voltage AlGaN/GaN HFETs", Proc. Int. Symp. on Power Semiconductor Devices & IC's (ISPSD), Bruges / Belgium, 3-7 June, pp. 345-348 (2012).