Below is a recent question submitted by a Power Electronics Reader, answered by Alex.
Q. How do GaN transistors compare with power MOSFETs for power switching applications in terms of efficiency?
Alex Lidow  To effectively compare the potential performance of a power MOSFET and an enhancement mode GaN transistor in a power conversion circuit, some figures of merit (FOM) need to be defined first.
A FOM that has been used by MOSFET manufacturers to show both generational improvements and to compare their products to other competitive devices is the product of the gate charge, QG, and the RDS(ON) for a given device. What makes this so useful is that no matter the size of the die, this FOM is almost constant for a given technology or ‘Generation’ of device. This FOM is related to device performance and can be useful in predicting power loss improvements with improved technologies, but it is less sensitive to differences when a device is used more as a switching element than as a conducting element. We will therefore discuss three distinct FOMs. The first of these is the traditional FOM. We will call that the “Rectifier FOM” because it is most applicable when a FET is used as a rectifier element such as in the lower transistor of a buck converter. The second FOM we will call the “Switching FOM” because it best describes relative performance of devices used mostly as switching elements such as in the upper transistor in a classic buck converter. Of these two FOMs, the switching performance is more important in ‘hard switching’ converter circuits. The third FOM we will call the “Resonant FOM” because it best describes relative performance of devices used in resonant circuits where the switching occurs when there is zero current through the device (ZCS).
The rectifier FOM is shown in Fig. 1 and plots RDS(ON)vs. QG for the eGaN FET as well as for different equivalent silicon MOSFETs. From this we can draw a number of conclusions:

40 V eGaN FETs are comparable to the best 25 V lateral silicon devices.

100 V eGaN FETs are comparable to 25 V vertical silicon devices

200 V eGaN FETs are comparable to 40 V vertical silicon devices
Fig. 2 plots RDS(ON) vs. QGD (Switching FOM) for eGaN FETs as well as for various equivalent silicon MOSFETs. We can see that, based on switching FOM, eGaN FETs offer a distinct advantage over any equivalent voltage rated silicon device. As with the Rectifier FOM:

40 V eGaN FETs are comparable to 25 V lateral silicon devices.

100 V eGaN FETs are comparable to 40 V vertical silicon devices

200 V eGaN FETs are comparable to 100 V vertical silicon devices
Fig. 3 plots RDS(ON) vs. QG+ QOSS (Resonant FOM) for eGaN FETs as well as for various equivalent silicon MOSFETs. When a device is switched at zero current, the major elements of switching loss come from charging and discharging the device output (QOSS) and the input gate (QG). We can see that, based on the resonant FOM, eGaN FETs again offer a distinct advantage over any equivalent voltage rated silicon device.
Whether the designer of a power conversion system is using a hard switched topology or a soft switched topology, GaN transistors can reduce the power losses and increase power density. The higher the conversion frequency, the greater the gain by upgrading to the new generation of power transistor made with a remarkable material, GaN.
Glossary 
Parameter 
Description 
RDS(ON) 
Drain source onresistance 
QG 
Total gate charge 
QOSS 
Output charge 
QGD 
Gatetodrain charge 
FOM 
Figureofmerit 
eGaN 
Enhancement mode GaN transistor 
Figures

Rectifier FOM (RDS(ON)vs. QG) for eGaN FETs

Switching FOM (RDS(ON) vs. QGD) for various eGaN FETs and MOSFETs

Resonant FOM (RDS(ON) vs. QG+ QOSS) for eGaN FETs
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