What is GAN Technology

Gallium nitride (GaN) technology is not a new semiconductor technology. It has been commonly used in light-emitting diodes since 1990, but it is expensive.

It can be seen from the chemical name that this is a semiconductor material composed of two ions, nitrogen and gallium. In terms of physical characteristics, its band gap is greater than 2.2eV, and it is also called a wide band gap semiconductor material, which is the domestic It is often said that one of the third-generation semiconductor materials, in fact, the market is not only concerned about gallium nitride, but the third-generation semiconductor materials.

The third-generation semiconductors include silicon carbide (SiC), gallium nitride (GaN), aluminum nitride (ALN), gallium oxide (Ga2O3), etc. Their bandgap is above 2.3eV, and they are represented by SiC silicon carbide and GaN gallium nitride.

The bandgap determines the electric field a material can withstand.

GaN has a larger band gap than traditional silicon materials, so that it has a very narrow depletion region, so that a device structure with a very high carrier concentration can be developed, and the carrier concentration directly determines the conductivity of the semiconductor. The simple explanation is that gallium nitride can work at a higher temperature than silicon-based semiconductor devices.

Advantages of Gallium Nitride Technology

As a third-generation semiconductor, gallium nitride is similar in principle to the first-generation semiconductor (Si). Its main advantages lie in the following three points:

1. High performance: It has the advantages of high output power, high power density, high working bandwidth, high efficiency, small size and light weight. Excellent thermal stability, easy to achieve high working pulse width and high working ratio;

2. High reliability: with high temperature junction and high thermal conductivity, it can significantly improve the adaptability and reliability of devices at different temperatures;

3. Low cost: Excellent material performance means that less material is needed to achieve the same function, which can effectively reduce costs.

Gallium Nitride Transistor Structure Principle

Unlike power semiconductors made of silicon, gallium nitride transistors conduct electricity through the two-dimensional electron gas (2DEG) formed by the piezoelectric effect of two different bandgap materials (usually AlGaN and GaN) at the interface, as shown in the figure 1 . Since the two-dimensional electron gas only conducts electricity with a high concentration of electrons, there is no problem of minority carrier recombination (ie, body diode reverse recovery) in silicon MOSFETs.

Figure 1: Schematic diagram of gallium nitride conduction principle

The structure of the basic GaN transistor shown in the figure is a depletion-mode high electron mobility transistor (HEMT), which means that no voltage is applied between the gate and source (VGS= In the case of 0V), the drain of the gallium nitride transistor and the element are turned on, that is, it is a normally open device. This is completely different from the traditional normally closed MOSFET or IGBT power switch, which is very difficult to use for industrial applications, especially in the field of switching power supplies. In order to deal with this problem, there are usually two solutions in the industry, one is to use a cascode structure, and the other is to use P-type gallium nitride at the gate to form an enhancement mode (normally closed) transistor. Both structures are shown in Figure 2.

Figure2: GaN transistors with two structures

GaN in a cascaded structure is depletion-type GaN cascaded with a low-voltage silicon MOSFET. The advantage of this structure is that its drive is exactly the same as that of a traditional silicon MOSFET (because it drives a silicon MOSFET), However, this structure also has great disadvantages. First, the silicon MOSFET has a body diode, and there is a reverse recovery problem of the body diode when gallium nitride conducts current in reverse. Secondly, the drain of the silicon MOSFET is connected to the source of the depletion-type gallium nitride. The oscillation of the drain to the source during the turn-on and turn-off of the silicon MOSFET is the oscillation of the source of gallium nitride to the gate. Inevitably, then there is the possibility that GaN transistors may be turned on and off by mistake. Finally, because the two power devices are cascaded together, the possibility of further reducing the on-resistance of the entire gallium nitride device is limited.

Due to the above problems in the cascaded structure, the mainstream technology of GaN transistors in the power semiconductor industry is enhancement mode GaN transistors. Taking the Gallium Nitride transistor CoolGaN of Infineon Technologies Co., Ltd. as an example, its detailed structure is shown in the figure 2.

Figure 3: Schematic diagram of CoolGaN structure

As shown in the figure 3, the current gallium nitride transistor products in the industry have a planar structure, that is, the source, gate and drain are in the same plane, which is different from the vertical structure of silicon MOSFET represented by super junction technology (Super Junction) . The P-GaN structure below the gate forms the enhancement-mode GaN transistor described earlier. Another p-GaN structure next to the drain is to solve the current collapse problem that often occurs in GaN transistors. The substrate (Substrate) of CoolGaN products of Infineon Technologies Co., Ltd. uses silicon materials, which can greatly reduce the material cost of gallium nitride transistors. Since the thermal expansion coefficients of silicon materials and gallium nitride materials are very different, many transition layers (Transition layers) are added between the substrate and GaN, so as to ensure that gallium nitride transistors can withstand harsh conditions such as high and low temperature cycles, high and low temperature shocks, etc. In this case, there will be no failure problems such as wafer delamination.

For the application of gallium nitride, now we search for chargers, and many brands can see gallium nitride. In addition to many well-known digital accessory brands, there are also many mobile phone manufacturers such as Huawei, Xiaomi, and Nubia.

Through all kinds of information, we should be able to find that the gallium nitride charger is now almost on par with the ordinary charger market, and the gallium nitride charger is newly developed in recent years, from the 30W that just appeared to the current 140W, 200W, I believe there will be more possibilities in the future.

The Future of Gallium Nitride

Although the third-generation semiconductors are in the early stages of development, domestic and international giants are basically on the same starting line. This is an opportunity for China to catch up with foreign countries. In addition, the third-generation semiconductor process production line has low requirements for equipment, so the investment amount of the third-generation semiconductor factory is only about one-fifth of that of the first-generation silicon-based semiconductor. The difficulty lies not in the equipment or logic circuit design, but in the process. However, process development is accidental, and compared with logic chips, it is less difficult, which is good news for local companies.