The improvements offered by the GaN devices are yet to be realized in deployed subsystems. Several groups of researchers are experimenting and reporting upon GaN transistors that are aimed at replacing Si IGBTs. The results achieved by GaN Systems are presented and these are compared to other producers of GaN devices.
The chart shown in Figure 1 describes the relationship between required motor power and power source voltage for some of the HV/EVs 2010/12 . The dashed lines show the average current flow, while the power source voltage indicates the voltage limits we have to minimally reach. Blocking Voltage ratings of 650V and 1200V for high current large area GaN devices have been difficult to achieve. For that reason ARPA-E projects involving IR/Delphi and DOE projects of GaN Systems/APEI have been directly aimed to meet the needs of the automotive industry , . Similarly at HRL, development work was partially supported by ARPA-E and the General Motors Company .
Device Technology and Performance
The vehicles shown use Si-IGBT devices in the inverters. However, the material limits of Silicon prevent further substantial improvements of the performance of these devices. As shown in Figure 2, both GaN and SiC devices potentially offer alternative solutions. The chart compares the on-resistance achieved per unit of device area at a given blocking (rated) operating voltage. GaN and SiC devices can achieve substantially lower losses per unit area than Si-IGBT devices. In addition, conventional IGBT devices have an offset voltage of 2-2.5 volts that is present over the entire rated operating current range. This leads to inefficient operation at low currents.
However, the voltage offset is almost temperature invariant. The on-resistance of GaN devices doubles if they are allowed a temperature rise of 150°C. When higher speed operation is required the GaN and SiC devices provide substantially lower losses because they have lower capacitances and near zero charge storage. A complex tradeoff between chip size,