(Invited) GaN Photoconductive Semiconductor Switches for Efficient High-Voltage Power Conversion Applications

Abstract

Gallium nitride (GaN) photoconductive switch (PCSS) technology can revolutionize high-voltage, efficient power-switching capabilities. PCSSs are optically driven, which advantageously isolates sensitive control circuitry from electromagnetic interference (EMI) resulting from rapidly switching high voltages and currents (high dI/dt or dV/dt). Fast switching times and extremely low on-resistances are achievable in PCSSs. The conductive channel region during on-state (under illumination) also serves as the voltage hold-off region in the off-state (dark) and does not require an additional drift layer that limits conventional unipolar device performance. In addition, the on-state resistivity can be further reduced by increasing the driving optical power density. Carbon-doped GaN (GaN:C) can result in extremely low dark currents, high blocking voltages, as well as support very low on-resistances.Lateral and vertical GaN PCSSs are fabricated. Lateral GaN:C PCSS structures consist of a 4 µm thick GaN:C layer grown by metal organic chemical vapor deposition (MOCVD) on a SiC substrate. Vertical GaN:C PCSS structures consist of a 100 µm thick GaN:C layer grown by hydrive vapor phase epitaxy (HVPE) on a native GaN substrate. Contacts were formed by evaporation of Ti/Au (25 nm /250 nm). The PCSS devices are driven with a 3 W commercial off-the-shelf (COTS) ultraviolet (UV) light emitting diode (LED) with peak output at 365 nm and a 10 nm full width half maximum.Vertical GaN PCSSs demonstrate exceptional on-state characteristics with an on-state current density of ~150A/cm2 , or an on-state resistivity of 66 mΩ-cm2. The off-state (dark) blocking capability exceeds 1 kV (maximum value tested). The transient switching characteristics of GaN:C PCSS will be characterized, and discussion will be provided relating to device design and optical driving conditions, including GaN:C carbon doping density, GaN:C layer thickness considerations, design for reduced contact resistance, illumination power density, and illumination wavelength.