GaN HEMT or GaN FET power devices are quickly emerging as an alternative to other power devices such as silicon-based MOSFETs, and IGBTs owing to their better performance in terms of speed, efficiency, form factor, operating temperature, and other parameters. With conventional devices such as MOSFETs reaching their theoretical limits in terms of speed, efficiency, and performance, GaN FETs provide a spectrum of better performance characteristics and hence are rapidly being adopted as the next-gen power-switching device by the industry. 

Working Principle:

The remarkably high electron mobility of gallium nitride (GaN) enables the fabrication of transistors that have a low value of ON resistance combined with a very high switching frequency capability. Typically, GaN is deposited on either silicon or silicon carbide to give GaN on Silicon, often referred to as Gan on Si, or GaN on Silicon carbide referred to as GaN on SiC. GaN HEMT is a high electron mobility transistor, and it utilizes the HEMT structure with its two-dimensional electron gas (2DEG) region providing the fundamental mode of operation for the device. The AlGaN layer acts as the barrier.

GaN HEMT Structure

GaN HEMT is available in enhancement and depletion modes. Depletion mode GaN (dGaN) HEMT works in always-on mode and a negative voltage is applied across the gate relative to the drain and source to turn it off whereas Enhancement mode GaN (eGaN) HEMT  works in always-off mode. It works like a MOSFET, with a positive bias across its gate relative to source turns on the device. 

Depletion mode GaN Transistor Structure:

Just like any FET structure, dGaN has three electrodes; Gate, source, and drain. The source and drain connect directly to the GaN layer, while the gate sits on the AlGaN layer. Since it is in only on mode, a short circuit is created between the drain and the source, so a negative potential is applied to the gate relative to the drain and source which in turn depletes the electrons from the 2DEG layer, stopping the conduction. 

While depletion mode is used in many electronic circuits, due to its configuration and always ON mode, this structure is not commonly used in power circuits. The enhancement mode is used mostly in power-switching circuits. 

Enhancement mode GaN Transistor Structure:

The always-OFF mode is used for most power-switching applications. There are several ways of fabricating eGaN FETs. The most commonly used methods are implanted gates and recessed gate eGaNs.

Implanted gate eGaN: Fluorine atoms are implanted on the AlGaN barrier layer which creates a negative layer that in turn depletes the electrons from the 2DEG plane so, when a positive bias is applied at the gate, electrons are attracted back to the 2DEG layer which starts the conduction. 

Recessed gate eGaN: The AlGaN layer positioned above the 2DEG layer is thinned. This reduces the voltage produced in the region. A point is reached where the voltage generated by the stress in the crystal structure is less than the built-in voltage of the Schottky metal gate and with zero bias the 2DEG plane is eliminated here. When the gate is positively biased, the electrons are attracted back to the 2DEG layer which initiates the current flow in the device. 

GaN FETs find their use in high-power and high-speed applications. A high-voltage power device mapping of various PE devices is shown below and it shows the versatility of GaN PE devices in high-speed switching and high-power applications such as electric vehicles, power grids, renewable energy sector, etc.

High Voltage Power Device Mapping

GaN FETs are actively replacing IGBTs and other PE devices in power factor correction setups, and multi-level AC/DC power converters. They provide, better efficiency, higher dv/dt immunity, faster switching, and more power density. A graph representing switching losses at different frequencies between GaN and SiC devices is shown below:

GaN FETs vs Conventional MOSFETs

It has been established in the industry that the new GaN FETs dramatically reduce the size of power converters while giving better performance metrics. Their performance differs in many ways as compared to conventional MOSFETs, which have been the gold standard of the power electronics industry till now. GaN turn-on times are about four times faster than MOSFETs with the same R_DS(ON) rating, while turn-off time is about twice as fast. A table indicating the key performance differences between the two types of transistors has been shown below:

Equivalent results are observed for other power electronic devices such as thyristors, Bipolar transistors, IGBT, etc and GaNs have shown better performance parameters consistently. However, GaN FETs come with certain drawbacks too and have their limitations. However, the speed and capabilities of these GaN devices mean that even more attention and sophistication are needed to effectively manage their turn-on/off characteristics to gate drive, voltage, and current slew rate, current levels, noise sources and coupling, layout considerations, and many other factors. The parasitic elements of the PCBs have a very vulnerable effect on the switching performance of the GaN FET. This fact should be considered during the design, to avoid oscillations and EMI problems.

There is also the cost factor to consider. A unit of GaN FET is significantly costlier than a typical MOSFET or an IGBT. A 600V GaN device costs approximately twelve times a typical IGBT and five times a typical MOSFET in the same voltage range. However, with expected improvements in the manufacturing and fabrication process in the future, the per-unit cost of GaN FET is expected to go down. A graph representing relative cost projection by FET technology is shown below:

Relative Cost Projections

In conclusion, GaN is quickly becoming the industry favorite in <10 kW applications such as electric vehicle chargers, telecom converters, solar inverters, robotics motor drives, and so on. GaN FETs allow designers to build greener, cost-effective, and lighter products that the industry desperately needs. 

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