Thermal modeling and measurement of AlGaN/GaN HFETs built on sapphire and SiC substrates

We present thermal modeling and measurement results of AlGaN/GaN Heterojunction Field Effect Transistors (HFETs) fabricated on sapphire and SiC substrates. The temperature profiles on the device surface were calculated using in-house codes PAMICE and UNITHERM. These 3-D codes allow us to compute temperature at any location on and inside the device. Thermal measurement was performed using nematic liquid crystal thermography. This technique is nondestructive and can be applied while the device is in operation condition. It has submicron spatial resolution and ±1°C temperature accuracy. In addition to temperature measurement, the technique can also be employed to detect hot spots that are possibly caused by defects. For the devices studied, current versus voltage was first measured. The results indicate that these devices behave nicely. Measured peak temperatures agree well with the calculated ones. Since sapphire has much lower thermal conductivity than SiC, AlGaN/GaN/sapphire device has higher thermal resistance than AlGaN/GaN/SiC device. A linear relationship between temperature and power was observed for both types of devices. AlGaN/GaN devices are known to have superior high power capability at high frequency. Thus, thermal modeling and measurement are valuable in determining the junction temperature and subsequently to modify device and package design so as to keep the junction temperature within acceptable range.