Efficiency-Related Issues in GaN and SiC Power Systems

When it comes to an efficient and dependable replacement for Silicon-based devices, Silicon Carbide (SiC) and Gallium Nitride (GaN) are the ideal substitutes in power-related applications. SiC and GaN possess extraordinary switching characteristics in high-power circuits making them more efficient and highly compact. This is achieved due to the low on-state resistance of the transistors together with the high breakdown voltage in switching devices.

Fig 1: Future applications of SiC and GaN.

Until now, transistors made of these wide bandgap materials have working prototypes that have shown much better outputs, indicating great potential for future high-power applications. However, there are still several challenges regarding the architectural design of SiC and GaN that engineers have to face when it comes to the understanding of switching behavior, and thermal management to overcome high power density. Reliability is a factor that must be well established in power devices in terms of circuit operation and the future market scope of the system.

Reliability in GaN Power Devices

1. Architectural design and Problems of GaN transistors: During their operation, GaN transistors make use of the AlGaN/GaN hetero structures with 2D electron gas (2DEG) with high carrier density and high electron mobility. During this phase, there can be multiple technical complications, particularly the high drain voltage current collapse wherein the drain current is reduced once high drain voltages are applied. To eliminate this trapping Drain collapse has been resolved using electric field relief to stop this entrapment, making GaN samples available for the analysis of the switching systems. Also, GaN transistors have found applications in the commercialization of high-frequency power transistors in cellular base stations. Despite the fact that the voltage has increased to 50V, the systems can demonstrate a smooth functioning of transistors at higher voltages. Figure 2 shows the current collapse in GaN transistors

Fig 2: Current collapse in GaN transistors.

2. Reliability of Gate Injected Transistors: High carrier concentration in the AlGaN/GaN has made the normally-off operation of the GaN transistor slightly problematic. Holes have to be injected from the p-gate to enhance the formation of electrons at the channel. This process is called conductivity modulation. This leads to low on-state resistance and high drain current even in the usually-off transistors. Gate Injection Transistors (GITs) can be used as a replacement to remove this issue by placing a p-type AlGaN gate over the heterojunction. By doing so the channel gets fully depleted under the gate. The low RonQg (Ron: on-state resistance, Qg: gate charge) of 700mnC, which is one-third smaller than the most advanced super junction Si MOS, stands out as having a significantly higher potential for high-speed switching. Pulsed current-voltage (I-V) measurements are used to describe the current collapse of the GIT on a Si substrate. Gate voltages from both the off-state and the zero-bias state are used, along with very brief drain pulses. Figure 3 shows the schematic cross-section of a GIT.

Fig 3: A schematic cross-section of a GIT with the working/operating principle.

Reliability in SiC Power Devices

1. Architectural design and Problems of SiC transistors: SiC power devices are established on SiC substrates by making use of upgraded quality of the bulk crystal and the epitaxially grown SiC film on the off-axis substrates. The configuration of the system made for mass production has been double-diffused MOSFET (DMOS). This is commonly used for currently existing Si power MOSFETs. One major disadvantage SiC transistors have is the suppression of shifts in the threshold gates once high positive or negative voltages are applied. This can compromise the reliability of SiC transistors, which eventually disrupts the operation of the device. When a high gate voltage is applied, the trapped electrons/holes depend on the poles of the gate voltages. This results in a shift in threshold voltage.  Another reliability issue can be the degradation of the body diode based on the PN junction. This gets initiated once the basal plane starts to form, which is why better epitaxial quality would be mandatory.

Fig 3: Schematic of a conventional SiC DMOS with reliability issues.

2. Diode-integrated MOSFET (DioMOS) and Reliability: A diode-integrated metal oxide semiconductor (DioMos) was proposed in order to ultimately decrease the chip cost in SiC switching devices.

Fig. 4: A schematic cross-section of a SiC DioMOS device

As shown in fig 4, the DioMOS has an additional channel epitaxial layer which gets developed under the gate insulator. By using the channel layer as a path for the reverse current, the integrated body diode is not subjected to the reverse current. Under the gate of the DioMOS, the thin channel epitaxial layer with heavy n-type doping lowers the electron's potential barrier down to 0.8eV. This is essential in making the device suitable for reverse conduction. When the I-V characteristics of the DioMOS were observed, it was found the reverse diode had a low built-in voltage. Apart from this, there was also no shift in the threshold voltages for positive gate voltage bias till 15V and a negative value to -15V.  A DioMOS demonstrated with sufficient reliability can be used in practical switching applications due to its stable characteristics for up to 1000 hours.

Conclusion

Following a summary of conventional devices and their reliability issues, state-of-the-art GaN and SiC power switching devices developed at Panasonic are discussed. In practical switching applications, normally-off GaN GITs on Si substrates are free from current collapse up to 600V. It is possible to reduce the total chip cost of the system by using SiC DioMOS, which integrates the reverse diode. Once the application of high voltage gate bias and degradation of body diodes is complete, there is a stable operation free from the shift of the threshold voltages.

The demonstrated power devices comprised of SiC and GaN have achieved the high-temperature reverse bias tests. These tests conducted were over 1000 hours and therefore are very promising for increased efficiency in power switching systems in the future power electronics industry.