Gallium nitride (GaN) is a mechanically stable wide bandgap semiconductor. Power devices based on GaN outperform silicon-based devices in terms of breakdown strength, switching speed, thermal conductivity, and on-resistance.
Crystals of gallium nitride can be grown on a variety of substrates, including sapphire, silicon carbide (SiC), and silicon (Si). By growing a GaN epi layer on top of silicon, the existing silicon manufacturing infrastructure can be used, eliminating the need for expensive specialized production sites and leveraging cheap large diameter silicon wafers.
GaN is used in the manufacture of semiconductor power devices, light-emitting diodes (LEDs, and RF components. It’s also proven to be a viable replacement technology for silicon semiconductors in power conversion, RF, and analog applications.
How does gallium nitride work?
Gallium nitride (GaN) is a wide bandgap semiconductor used in high-efficiency power transistors and integrated circuits. When a thin layer of aluminum gallium nitride (AlGaN) is grown on top of a GaN crystal, a strain is created at the interface that induces a compensating two-dimensional electron gas (2DEG). This 2DEG is used to efficiently conduct electrons when an electric field is applied across it.
The electrons are constrained to a very small region at the interface, which contributes to the 2DEG’s high conductivity. The mobility of electrons increases from about 1000 cm2/Vs in unstrained gallium nitride to between 1500 and 2000 cm2/Vs in the 2DEG region due to this confinement. Because of their high mobility, transistors and integrated circuits have higher breakdown strength, faster switching speed, higher thermal conductivity, and lower on-resistance than comparable silicon solutions.
Basic GaN FET (Field Effect Transistor) Structure
There are gate, source, and drain electrodes on this power FET, as with any other power FET. The source and drain electrodes pierce the top AlGaN layer to make an ohmic connection with the underlying 2DEG. This creates a short circuit between the source and drain until the 2DEG “pool” of electrons is depleted and the semi-insulating GaN crystal can block current flow.
A gate electrode is placed on top of the AlGaN layer in order to deplete the 2DEG. This gate electrode was formed as a Schottky contact to the top surface in many of the first GaN transistors. When a negative voltage is applied to this contact, the Schottky barrier becomes reverse biased, depleting the electrons underneath.
As a result, a negative voltage between the drain and source electrodes is required to turn this device off. This type of transistor is known as a “depletion mode,” or d-mode, high-frequency electronic device (HFET). AIGaN/GaN HFET structure in depletion mode with three metal-semiconductor contacts for the source, gate, and drain.
D-mode devices are inconvenient in power conversion applications because a negative bias must be applied to the power devices first at the startup of a power converter or a short circuit will result. An enhancement mode (e-mode) device, on the other hand, is not subject to this constraint. An e-mode device is turned off and does not conduct current when the gate bias is zero.
What is GaN HEMT?
High electron mobility transistors (HEMTs) are transistors that make use of a 2-dimensional electron gas (2DEG) created by a junction between two materials with different band gaps. Gallium nitride (GaN) HEMTs have a faster switching speed, higher thermal conductivity, and lower on-resistance than comparable silicon-based solutions.
These characteristics enable gallium nitride transistors and integrated circuits to be used in circuits to improve efficiency, shrink size, and lower the cost of a wide range of power conversion systems.
Since the dawn of the electronic age over a century ago, power design engineers have been on the lookout for the ideal switch, one that can convert raw electrical energy into a controlled, useful flow of electrons quickly and efficiently.
The vacuum tube was the first, but its inefficiency, as evidenced by the heat it generates, as well as its large size and high cost, limited its ultimate use. Following that, in the late 1950s, the transistor became widely used; with its small size and improved efficiency, it appeared to be the “holy grail,” rapidly displacing tubes while creating enormous new markets unattainable by vacuum tube technology.
The Age of GaN has Begun
With the improvement in transistor and IC performance made possible by GaN materials, now is the time for innovative power design engineers to take advantage of GaN attributes:
- Lower on-resistance translates to lower conductance losses.
- Faster devices result in lower switching losses.
- Less capacitance means less loss when charging and discharging devices.
- Less power is required to power the circuit.
- Smaller devices occupy less space on the printed circuit board.
- More affordable than its counterparts.