Gallium nitride (GaN) is a binary III/V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it special properties for applications in optoelectronic, high-power and high-frequency devices. For example, GaN is the substrate which makes violet (405nm) laser diodes possible, without requiring nonlinear optical frequency-doubling.
Its sensitivity to ionizing radiation is low (like other group III nitrides), making it a suitable material for solar cell arrays for satellites. Military and space applications could also benefit as devices have shown stability in radiation environments.
Because GaN transistors can operate at much higher temperatures and work at much higher voltages than gallium arsenide (GaAs) transistors, they make ideal power amplifiers at microwave frequencies. In addition, GaN offers promising characteristics for THz devices. Due to high power density and voltage breakdown limits, GaN is also emerging as a promising candidate for 5G cellular base station applications.
GaN Transistors and Power ICs
The very high breakdown voltages, high electron mobility and saturation velocity of GaN has also made it an ideal candidate for high-power and high-temperature microwave applications, as evidenced by its high Johnson's figure of merit. Potential markets for high-power/high-frequency devices based on GaN include microwave radio-frequency power amplifiers (such as those used in high-speed wireless data transmission) and high-voltage switching devices for power grids.
In 2010, the first enhancement-mode GaN transistors became generally available. Only n-channel transistors were available. These devices were designed to replace power MOSFETs in applications where switching speed or power conversion efficiency is critical. These transistors are built by growing a thin layer of GaN on top of a standard silicon wafer, often referred to as GaN-on-Si by manufacturers. This allows the FETs to maintain costs similar to silicon power MOSFETs but with the superior electrical performance of GaN. Another seemingly viable solution for realizing enhancement-mode GaN-channel HFETs is to employ a lattice-matched quaternary AlInGaN layer of acceptably low spontaneous polarization mismatch to GaN.
How Does GaN Improve Efficiency?
Power transistors are one of the primary contributors to power loss in a switching power supply. Losses in the transistors are generally separated into two categories; conduction and switching. Conduction losses are those caused by current flow when the transistor is on and switching losses occur in the transition between on and off states.
When on, GaN transistors (like those made of silicon) resemble a resistance between drain and source, often referred to as Ron, and the conduction losses are proportional to this resistance. A key benefit of GaN and other WBG materials is their relationship between breakdown voltage and Ron. For a given breakdown voltage, the Ron of the WBG devices is much lower than that of silicon, with GaN being the lowest of the three. As silicon is nearing its theoretical limit, the use of GaN and other WBG materials becomes necessary if improvements to Ron are to continue.
In addition to improvements in conduction losses, the use of GaN also leads to a reduction in switching losses. There are multiple factors that contribute to switching losses, several of which are improved through the use of GaN. One loss mechanism results from the fact that the current in a transistor begins to flow before the drain-source voltage begins to fall. During this time, the losses (equal to the volt-amp product) are very large. Increasing the speed at which the switch turns on will reduce the losses incurred during this transition. Because GaN transistors are able to turn on faster than silicon transistors, they are able to reduce the losses caused by this transition.
Another way that GaN reduces switching loss is through the absence of a body diode. To avoid a short circuit condition, a period of time exists when both switches of a half-bridge are off, known as the ”dead-time”. During this period, current continues to flow, but because both switches are off, it is forced through the body diode. The body diode is much less efficient than the Ron resistance of a silicon transistor when it is on. For a GaN transistors there is no body diode. Current that would flow through the body diode of a silicon transistor instead flows through the Ron resistance.
This significantly reduces the losses incurred during the dead-time. Because the body diode of a silicon transistor conducts during the dead-time, it must be turned off when the other switch turns on. During this time, current flows in the reverse direction as the diode turns off, causing additional losses. In a GaN transistors, the absence of a body diode results in near zero reverse recovery losses.
How Does GaN Decrease Form Factor?
While switching losses occur in short periods within the switching period, it is useful to look at them averaged over time. While the losses during a single switching transition may be large, if the time period between switches is large (meaning a low switching frequency), the average value can be kept at a safe level. Because the switching losses are lower in GaN, the time between switches is able to be reduced, increasing the switching frequency. The increased switching frequency allows the size of many large components (such as the transformer, inductors, and output capacitors) to be reduced.
GaN and other WBG devices also have better thermal conductivity and can withstand higher temperatures than silicon. Both help to reduce the need for thermal management components such as bulky heatsinks, frames, or fans. The absence of these devices (along with the shrinking of the powertrain components mentioned earlier) all lead to significant reductions in the overall size of the power supply.
We also integrate GaN in our latest product and thanks to its application, we manage to make our power strip smaller while obtains high efficiency of power delivery with better safe guarantee. Image a power strip of a billiard ball size, yet able to deliver more juice to all your devices in a shorter time. Not only wall outlet extender, but also a cord version to extend your use range. So wait no more to bring this back your home and enjoy the benefit of technological development.
