Gallium nitride is a semiconductor material from which most types of modern LEDs are made. Products made from it are also used in mobile communication base stations. But recently, a new application has opened up for gallium nitride — it is used to make power transistors used in alternative energy, electric vehicles, and even in household chargers.
An important characteristic of any semiconductor is the band gap. What does this indicator mean and how is it related to the use of devices made from this material in the energy sector?
There are two zones in which the energy levels of electrons in semiconductor materials can be located: valence or conduction. These zones do not intersect; the gap between them is called the forbidden zone. The energy levels located there cannot be occupied by electrons. The presence of a band gap is a characteristic feature not only of semiconductors, but also of dielectrics (in conductors, the valence and conduction bands overlap). The band gap is measured in electron volts (eV). It is generally accepted that if this indicator for a material is less than 5 eV, then we have a semiconductor, otherwise it is a dielectric.
The most common semiconductor used in electronics today is silicon. Its band gap is 1.12 eV. But now experts are more interested in semiconductors with a band gap of more than 2 eV (otherwise known as wide-gap), they are better suited for applications involving switching high currents and voltages. The reason for this interest is the following. The higher the temperature, the more active electrons are in spontaneously moving from one energy level to another. Moreover, the wider the band gap, the lower the probability of such a transition. As a result, the larger the band gap of a semiconductor, the higher, in general, the maximum permissible temperature for products based on it and the lower the leakage current.
If we imagine a MOS transistor in the form of a switch (and this is precisely how it is used in the electric power industry), then when wide-gap semiconductors are used for its manufacture, the switch resistance in the open state will be very large (on the order of tens of MOhms). And this resistance will weakly depend on heating, which is inevitable when switching significant currents. The parameters of the semiconductor materials most commonly used in electronics today are given in the table.
We already talked about transistors made from silicon carbide (SiC), which belongs to the category of wide-gap semiconductors. At the same time, gallium nitride (GaN) transistors began to be introduced into power equipment. In some ways, these transistors compete with solutions based on SiC; in some ways, these two branches of electronics development occupy their own niches.
Gallium nitride has been known as a material for making transistors since the 90s. But for transistors used in electrical power equipment, its intensive implementation began around 2018. This is due to the development of electric vehicles and solar generation. Among the companies producing gallium nitride power transistors are GaN Systems (Canada), EPC (Taiwan), Infineon (Germany), Nexperia (Netherlands) and many others.
Operation in saturation mode
In power supply installations, MOS transistors are usually used (the abbreviation stands for “metal-oxide-semiconductor”; the term MOSFET is used abroad), working as switches that interrupt the current. In this case, the transistor during operation should ideally be in only one of two modes — cutoff or saturation.
In cutoff mode, the current is interrupted, the resistance between source and drain is tens of megohms. In this case, the transistor is similar to a switch with open contacts. Saturation mode is when the voltage between source and drain is practically independent of the current flowing through the transistor. In this case, the resistance can be considered close to zero, that is, the transistor in saturation mode is similar to a switch with closed contacts. In both of these modes, the power dissipated by the transistor is very small and does not cause significant heating.
In addition to the cutoff and saturation modes, the MOSFET also has a third mode — active (otherwise called linear). In this mode, the relationship between the drain-source voltage and the current through the transistor channel remains close to the lines. In active mode, power is dissipated, the load increases, resulting in heating of the semiconductor device and loss of electricity.
The transition from cutoff mode to saturation mode and back again in real-life MOSFETs always occurs through the active mode. This is due to the finite speed of semiconductor devices. After the transistor “receives the command” to exit the saturation mode, it takes some time for the charge carriers to be absorbed from the channel.
The goal is to increase the rate of charge dissolution. The higher it is, the higher the row in which the transistor operates in active mode, irrationally conducts electricity for heating. There are two ways to reduce this parameter. First, reduce the crystal size. And secondly, a semiconductor material with higher electron mobility was used. Both methods can be used individually or together. For example, power transistors based on SiC may have a shorter resorption time compared to their silicon counterparts, although the survivability of charges in silicon carbide is lower than in pure silicon. The fact is that the high thermal stability of SiC allows the creation of crystals of smaller sizes than in devices made of pure silicon, at the same nominal speed.
What if you choose a material with greater mobility of chargers? Let’s turn to the table where the parameters of semiconductor materials are compared. The record holder for electron mobility is gallium arsenide (GaAs). But for power electronics it is reduced due to low thermal conductivity (almost 3 times lower than that of silicon), which makes it difficult to remove heat from the crystal. In addition, GaAs is not a wide-gap semiconductor.
At the same time, GaN combines both high electron mobility and good resistance to heat. In terms of the band gap, this material is even slightly superior to silicon carbide. Thus, the saturation time can be reduced both by increasing the mobility of charges and by reducing the size of the crystal.
The charge absorption rate for a high-power GaN transistor made using E-HEMT technology is about 6 C/s versus 0.6 C/s for a SiC transistor and approximately 0.2 C/s for a sleeve silicon IGBT.
GaN Transistor Control
As silicon assemblies progress, gallium nitride MOSFETs come in normally open and normally off types. A normally open version is when, at zero voltage at the transistor’s gate, it is completely open, and a negative control voltage is required to close it. Usually the closed version — at zero voltage the gate of the transistor is closed, a positive voltage is applied to open. In the case of GaN, devices are normally closed to provide more advanced regulation. The opening voltage for GaN transistors is 6 V.
In comparison, most SiC transistors require both positive and negative polarity signals. To open such a transistor to the gate, a voltage of 20 to 25 V relative to the source is required. But the closure, i.e. e. Switching to the cutoff mode, we will need to apply a voltage of -5 V to the gate. As a result, the driver — the unit that controls the powerful transistor — for SiC the design turns out to be expensive and bulky. In 2020, the American company UnitedSiC began serial production of the fourth generation of silicon carbide MOS transistors, in which the opening voltage is +12 V, and the closing voltage is equally excluded. But for a number of reasons, such transistors are still not widely used at the time of writing. Note that even power MOSFETs made of silicon require an opening voltage of at least 10 V, and for GaN transistors this value is lower. The result is a cheaper and more compact driver, which is an advantage over SiC. Another design is that, unlike MOS transistors made from other materials, the opening voltage of GaN devices depends very little on the temperature of the crystal. Therefore, a complex temperature circuit in the driver is not required.
Advantages and Disadvantages of GaN
In addition to a simpler control principle, GaN power transistors, when launched into mass production, may turn out to be more technologically advanced compared to SiC devices. The production of gallium nitride itself has already been well mastered using the example of LEDs. In addition, the substrate of SiC transistors is usually made of artificial sapphire. And for GaN devices, ordinary silicon is used as a substrate.
The disadvantages of GaN transistors include lower operating voltage. Thus, mass-produced gallium nitride transistors can switch voltages up to 650 V. Devices for 1200 V are produced in small batches. At the same time, SiC transistors are mass-produced for voltages up to 3000 V, prototypes can withstand up to 15 kV.
The thermal conductivity of GaN is 15% less than that of silicon and almost 4 times lower than that of silicon carbide. This means that in case of overheating, quickly removing excess heat from the crystal becomes problematic.
Application
The main use of GaN transistors is in all kinds of inverters, as well as voltage converters (including DC-DC type). High performance allows mass production of powerful inverters operating at frequencies up to 250 kHz. There are known prototypes of such inverters operating at a frequency of 1 MHz. For comparison, inverters based on silicon transistors operate at frequencies up to 50 kHz, SiC — up to 150 kHz. The higher the frequency, the more compact the inverter, since the sizes of transformers and chokes are reduced. Therefore, GaN transistors are predicted to have a great future in electric vehicles.
GaN devices are also convenient for use in solar power plants installed in private homes. The compact inverter can be placed in close proximity to solar panels installed on the roof.
The most well-known household application of GaN power transistors is a charger, similar in size to such a device for smartphones, the power of which (up to 100 W) is enough to charge a laptop.
The reduction in the size of the voltage converter when using GaN relative to SiC solutions can be estimated by 1.5–2 times, and relative to pure silicon by 2–3 times.
GaN-based devices have significant advantages over other semiconductor technologies: lower energy costs because GaN is more efficient than silicon, so less energy is dissipated as heat, resulting in lower cooling costs and fewer cooling systems (e.g. , radiators, fans).
The fairly rapid adoption of GaN-based power transistors and integrated circuits has been driven by the speed advantage of GaN over silicon. GaN-on-Si transistors switch approximately 10 times
faster than MOSFET and 100 times faster than IGBT. RF envelope tracking applications for 4G/LTE base stations and light detection and ranging (LIDAR) systems for autonomous cars, robots, drones and security systems were the first mainstream applications to take full advantage of high-speed GaN switching capabilities.
GaN transistors were not only faster than Si MOSFETs and IGBTs, but also much smaller—by a factor of about 5 to 10. This has given impetus to the development of many applications in robotics and medical electronics, space satellites and drones.
Gallium nitride-based devices are widely used in space because gallium nitride is inherently radiation resistant. Unlike silicon, which requires special manufacturing techniques and special packaging to protect semiconductors from radiation, GaN’s natural properties make it fairly immune to these harmful rays. GaN transistors are used in ion thrusters, to convert energy from satellite solar panels, in high-precision ruggedized BLDC motors to drive reaction wheels, and in robotics and automated instruments used in space missions for ranging using lidar.
The use of gallium nitride contributes to the development of renewable energy. To accelerate the adoption of renewable energy sources, it is necessary to achieve more efficient conversion, increased energy storage capacity and lower costs without compromising long-term reliability. GaN-based energy solutions enable solar microinverters, optimizers and energy storage systems used for solar power to increase efficiency and reduce size and cost, while providing unmatched reliability.
Conclusions
The main advantage of GaN transistors is the compactness of equipment built using them. Therefore, they will find their application in electric vehicles and personal devices (solar panels, energy storage devices, chargers). Corporate applications are also possible where power supply equipment must be built into strictly defined volumes (mobile base stations, charging stations for electric vehicles, control systems for three-phase electric motors).
At network infrastructure facilities, large power plants powered by solar and wind, silicon carbide transistors are still more suitable due to their better overload resistance. This division of applications between the two semiconductor materials is unlikely to change due to technological progress, since it is based on the physical properties of the materials. At the same time, GaN transistors that are easier to control could potentially find application in the niche currently occupied by silicon electronics.