GaN has a higher critical electric field strength than silicon. Its higher electron mobility enables a GaN device to have a smaller size for a given on-resistance and breakdown voltage than a silicon semiconductor. Compared to silicon devices, this also allows devices to be physically smaller and their electrical terminals closer together for a given breakdown voltage requirement.

What are the two types of GaN power semiconductors?
The two types are the depletion mode and enhancement mode. The depletion mode transistor is normally on and is turned off with a negative voltage relative to the drain and source electrodes. In contrast, the enhancement mode transistor is normally off and is turned on by positive voltage applied to the gate. Depletion mode transistors are inconvenient because at start-up of a power converter, a negative bias must first be applied to the power devices or a short circuit will result. Enhancement mode devices do not have this problem: with zero bias on the gate, an enhancement mode device is off and will not conduct current.

To allow normally off operation of a depletion mode GaN HEMT, it is often packaged in cascode with a low voltage silicon MOSFET to allow normally off operation.  The cascode configuration provides the ruggedness of a silicon gate, coupled with the improved voltage blocking characteristics of a high voltage GaN HEMT (Fig. 2).


Fig. 2 - EPC GaN transistors employ the Texas Instruments’ LM5113 half-bridge gate driver IC.