From Principles to Examples: Why GaN?

PublishTime： 2023-10-08 Article Source：WAYON News

Power semiconductors are the core of electrical energy conversion and circuit control in electronic devices, mainly discrete semiconductor devices that can withstand high voltage or high current. Primarily, they are used to change voltage and frequency, DC-AC conversion, etc. In the development path of power semiconductors, they have been fully upgraded from various aspects such as structure, manufacturing process, technology, craft, integration, materials, etc. The main direction of its evolution is higher power density, smaller size, lower cost as well and lower loss. Especially the material iteration, from silicon (Si) gradually to gallium nitride (GaN) and another wide-band material upgrade, making the power device significantly improved in both volume and performance.

So what is the third-generation semiconductor GaN? It is a semiconductor material composed of nitrogen and gallium and is also known as a broadband semiconductor material because its forbidden bandwidth is greater than 2.2eV.

Table 1 compares several physical parameters of GaN and Si. It is undeniable that GaN shows better performance advantages in the following four areas:
1. Wide bandwidth: The wide band allows the material to withstand higher temperatures and higher electric field strengths. The concentration of intrinsically excited carriers is not as high when the device is operating at elevated temperatures, making it possible to use it in special environments at higher temperatures.

2. High breakdown field: The breakdown field strength of GaN itself is 3.3E+06, about 11 times of Si. Under the same withstand voltage conditions, the GaN depletion zone spreading length can be reduced to 0.1 times Si, which greatly reduces the drift zone resistivity to obtain lower Ron and higher power performance.

3. High electron saturation drift rate: In the working process of semiconductor devices, most of them use electrons as carriers to implement the current transmission. High electron saturation drift rate can ensure that semiconductor devices working in high electric field materials can still maintain high mobility, and thus have a large current density, which is the key to the device to obtain a large power output density. This is also the most obvious advantage of GaN materials.

As you can see, the electron mobility of GaN in the table is not high, Why is it called high electron mobility transistor? The reason is that GaN & AlGaN are formed by two-dimensional electron gas (2DEG) due to the material properties at the interface induction, 2DEG exists in a thin layer of 2-4nm and is confined in a small range, this confinement makes the electron mobility increase to 1500~2000cm²/(V-s).

4. Good temperature tolerance: As can be seen, the difference in thermal conductivity between GaN and Si is not significant, GaN can however have a higher junction temperature. Therefore, the combination of good thermal conductivity and higher thermal tolerance enhances the lifetime and reliability of the device. The superior performance of GaN devices also has great relevance to their device structure. Currently, there are two types of GaN devices in industrialization: P-GaN enhanced devices and common source/common gate devices.

Since GaN devices are extremely sensitive to parasitic parameters, the driving requirements of GaN are more stringent than those of traditional Si-based semiconductor devices, so it is interesting to study their driving circuits. In the actual application of high-voltage power GaN devices, we made a comparison between GaN devices and SJ MOSFETs in terms of switching characteristics and dynamic characteristics to understand the differences in more detail.

As can be seen from the DC parameters of the GaN transistor above, it has no reverse diode (0 Reverse Recovery) in the DC parameters, The main reason is that the GaN transistor does not have the parasitic PN junction of the SJ MOSFET. In addition, there is a considerable difference amongst DC parameters, Vth, etc. Under the same specification, GaN transistors have a smaller saturation current and a higher BV value than SJ MOS, which is also limited by its special characteristics of chip size and avalanche-free capability. Meanwhile, the lower drive voltage and gate charge Qg contribute to the excellent switching characteristics of high frequency and low loss.

As seen in the capacitance of the device, the SJ MOSFET has a significant nonlinear characteristic within 50V, while the overall capacitance value is much larger than that of the GaN device (the junction capacitance is three times higher than that of GaN). This is because, although the two-dimensional electrically coupled SJ device has a smaller device area than the planar MOS, it relies on the transverse depletion of the PN junction to realize voltage resistance, so the contact area of the PN junction is much larger. When the voltage between the D-S of the device is low, the contact surface formed by the built-in electric field of the PN junction causes the initial Coss&Crss and other parameters to be several orders of magnitude larger than that of the D-S high voltage state. At the same time, the space charge region of the device is widened from an incomplete depletion state to a full depletion state, resulting in the abrupt change of CGD and CDS on the capacitance curve. The sudden change of this electric field in a very narrow voltage range also precisely affects the EMI problem that engineers pay attention to. How to optimize and slow its curve has become the characteristic technology of many design companies.

However, the emergence of GaN solves this problem easily. The capacitance curve of GaN changes in a relatively small range, and there is no mutation. Therefore, in the EMI debugging process of the power supply application, the effect is better than SJ MOSFET, and the Coss is close to linear, which makes the waveform of DV/DT in the application switching process closer to a non-radian oblique line, making it elegant.

The low junction capacitance also makes the energy equivalent capacitance (Coer) and Eoss of the device much smaller than SJ MOS devices of the same specification, which greatly reduces the capacitive loss of the power supply in the process of hard switching and thus significantly reduces the heating. At the same time, in the soft switching process of the power supply, less junction capacitor charge is extracted to achieve ZVS, which enables the system with higher switching frequency and smaller dead time and further reduces the system volume.

As the high efficiency of GaN is verified in practice, the market confidence in GaN is gradually increasing. The advantages are becoming more and more significant and the usage is growing. The future power GaN technology will become the new standard for high-efficiency power conversion. The following are the new E-Mode GaN devices launched by WAYON, welcome to request samples and discuss their characteristics with WAYON 's experts.

+1 Like
Add to Favorites

Recommend

Technical Resources

More>

New Products & Solutions

More>

This document is provided by Sekorm Platform for VIP exclusive service. The copyright is owned by Sekorm. Without authorization, any medias, websites or individual are not allowed to reprint. When authorizing the reprint, the link of www.sekorm.com must be indicated.

Integrated Circuits