Semiconductor power devices are widely used to carry large currents and support high voltages. Conventional power devices are generally fabricated using silicon semiconductor material. One widely used power device is the power MOSFET. In a power MOSFET, a gate electrode provides turn-on and turn-off control upon the application of an appropriate gate bias. For example, turn-on in an n-type enhancement-mode MOSFET occurs when a conductive n-type inversion-layer channel (also referred to as “channel region”) is formed in a p-type base region in response to application of a positive gate bias. The inversion-layer channel electrically connects the n-type source and drift/drain regions and allows for majority carrier conduction therebetween.
The power MOSFET's gate electrode is separated from the base region by an intervening insulating layer, typically silicon dioxide. Because the gate is insulated from the base region, little if any gate current is required to maintain the MOSFET in a conductive state or to switch the MOSFET from an on-state to an off-state or vice-versa. The gate current is kept small during switching because the gate forms a capacitor with the MOSFET's base region. Thus, only charging and discharging current (“displacement current”) is required during switching. Because of the high input impedance associated with the gate electrode, minimal current demands are placed on the gate drive circuitry. Moreover, because current conduction in the MOSFET occurs through majority carrier transport using an inversion-layer channel, the delay associated with recombination and storage of excess minority carriers is not present. Accordingly, the switching speed of power MOSFETs can be made orders of magnitude faster than many minority carrier devices, such as bipolar transistors. Unlike bipolar transistors, power MOSFETs can be designed to withstand high current densities and the application of high voltages for relatively long durations, without encountering the destructive failure mechanism known as “second breakdown.” Power MOSFETs can also be easily paralleled, because the forward voltage drop across power MOSFETs increases with increasing temperature, thereby promoting an even current distribution in parallel connected devices.
Efforts to develop power MOSFETs have also included investigation of silicon carbide (SiC) as a substrate material. Silicon carbide has a wider bandgap, a lower dielectric constant, a higher breakdown field strength, a higher thermal conductivity and a higher saturation electron drift velocity compared to silicon. Accordingly, silicon carbide power devices may be made to operate at higher temperatures, higher power and voltage levels and/or with lower specific on-resistance relative to silicon power devices. Nonetheless, the voltage rating of silicon carbide power devices may still be limited if the voltage supporting drift regions therein are insufficiently thick. Thus, notwithstanding the preferred electrical characteristics of silicon carbide, there continues to be a need for silicon carbide power devices having thicker voltage supporting regions therein.
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