Impact of SiC Switch–MOS for Thermal Impedance Evaluation of Power Module
Authors: Kato Fumiki , Yao Atsushi , Sato Shinji , Hozoji Hiroshi , Harada Shinsuke , Sakai Aki , Watanabe Kinuyo , Yamaguchi Hiroshi , Sato Hiroshi 
Published: December 2020

In this paper, thermal impedances of Silicon carbide (SiC) power module assembled with a Schottky Barrier Diode (SBD)–wall–integrated trench MOSFET (SWITCH–MOS) and a conventional trench gate MOSFET (UMOS) are measured. Then they are compared to show the advantage of SWITCH–MOS for thermal impedance measurement. SiC power device is attracting attention as a candidate for future power device, since it can be operated at higher current density and higher switching speed than conventional silicon (Si) power device. The power module packaging technology for SiC power device should have low thermal impedance (both transient and steady state thermal resistance) to bring out a potential ability of the device. Thermal design often has difference from measurement due to various modeling errors such as power device heat generation, module structure, interfacial thermal resistance, and cooling environment. Therefore, accurate thermal impedance measurement is an essential technology. However, the thermal impedance measurement of SiC power modules has the significant issue due to the instability of the body diode built into the SiC–MOSFET. The thermal impedance is the junction temperature rise per heat generation amount of the device. The temperature dependence of the knee voltage of the body diode is used to measure the junction temperature of the MOSFET. Since the knee voltage of SiC–MOSFET changes depending on the gate voltage, a high negative gate bias was necessary to avoid this effect. However, the negative gate bias stress may cause serious device damage such as threshold shift. The SWITCH–MOS which has SBD built in near the MOSFET channel is expected to stabilize the junction temperature measurement and avoid the risk of device deterioration. Figure 1 shows the cross–sectional schematic of the SWITCH–MOS and the conventional UMOS. The SBD are formed on the trench sidewall in SWITCH-MOS, and the device structure is almost same to the UMOS. The temperature is detected by the PiN diode for the UMOS and by the SBD for the SWITCH–MOS. Both of them measure the temperature inside of the MOSFET cell. The appearance of the test module is shown in Fig. 2. Each type of the test module is composed of the SiC die (SWITCH–MOS, UMOS, and SBD), an SiN–AMC insulating circuit substrate and an Al baseplate. The components are attached each other using AuGe solder. The junction temperature is measured from the built–in voltage of the body diode in response to a constant current flowing from the source to the drain. In the case of the UMOS, the gate bias of −18 V is applied. The gate bias is zero for the SWITCH–MOS. The thermal impedance of each test module was measured by static mode (Fig. 3). Then, the thermal structure function was calculated by transient thermal analysis (Fig. 4). Both the thermal impedance and the structure function for the SWITCH–MOS and UMOS were in good agreement. Since the SWITCH–MOS does not need negative gate bias, the threshold shift issue does not occur in principle. In addition, it is possible to perform a transient thermal testing with a large current for a long time. Because the forward degradation does not occur even if a large current is applied to the SBD in the SWITCH–MOS. The SBD built into the SWITCH–MOS makes it easier and more accurate to measure the junction temperature of the SiC–MOSFET. The thermal impedance measurement using SWITCH-MOS not only allows the precise thermal modeling of SiC power modules to be performed, but also has the potential to be applied to the calibration of the thermal design and the evaluation of thermal properties inside of the package structure.

Figure 1