Thermal Reliability Of Normally-on Silicon Carbide Junction Field Effect Transistor

SiC JFETs are considered to be the most promising high-temperature power devices by virtue of the excellent material properties and the structural advantages. However, the thermal stability problems still limit the operation temperature of the devices. In this case, the thermal reliability of SiC JFET in static operation and dynamic avalanche mode operation are studied in detail in this thesis.Through the analysis of the thermal stability of SiC JFET, the device reach a thermal equilibrium state when the conduction power is equal to the power can be dissipated. The device can only reliably operate in the stable equilibrium point and is likely to occur thermal runaway once it jumps to the instable equilibrium point. When the SiC JFET actual works, the internal temperature distribution is non-uniform and the high temperature region is mainly the channel region. The simulation results illustrate that the junction temperature rises due to the self-heating effect of the device, which may cause thermal reliability problems. The carrier mobility decreases with the junction temperature rising and may result in more than 40% degradation of the current density. In addition, the current density will reach a maximum value at a certain condition called critical condition/point. The forward voltage in conduction mode should not exceed the critical voltage above which the device cannot get any improvement in performance and reliability.With the increase of the device gate voltage, the current capacity is improved and both the critical voltage and the critical junction temperature are reduced, which means the thermal reliability of the device is improved. The rise of ambient temperature and the deterioration of cooling conditions can lead to the performance and thermal reliability degrading at the same bias. For a given device, the device performance and stability degradation at elevated temperature can be somewhat compensated by reasonably increasing the gate bias or improving the cooling conditions. The influence of structural parameters on the static characteristics and junction temperature of SiC JFET are analyzed in detail. It is concluded that a reasonable increase in the channel width and a reasonable reduce in the drift layer width can significantly increase the current density of the device even at a small bias. Thus the device area can be reduced and the cost can be much lower. In other words, the forward voltage and the junction temperature of the device can be reduced at the same current density, which means the device power consumption can be reduced and the device thermal reliability can be improved. The simulations of the temperature impact on the threshold voltage of SiC JFET indicate that the threshold voltage linearly decreases with the increase of temperature by the rate of about 1.8~2m V/K. But the effect of temperature on threshold voltage can be neglected by reasonable design, which means the undesirable conduction caused by the rise of temperature can be avoided.Several avalanche mode simulations of SiC JFET in different conditions are conducted in this thesis. The results show that an increase in the load inductance and the initial avalanche current significantly increase the duration of the avalanche mode. The avalanche voltage of the device also increase somewhat. For these reasons the instantaneous power and avalanche energy increases seriously and result in grievous rise of junction temperature which may cause reliability problems. The avalanche curve is distorted when the inductor or the initial avalanche current increases to a certain extent. The reason is the junction temperature reaches about 1500 K, where the intrinsic carrier concentration can be comparable with the doping concentration. In this case, the device is most likely to occur thermal failure if the junction temperature cannot lower down timely. Therefore, the maximum limit of inductive load and initial avalanche current should be noticed in order to ensure the safety and reliability of the device during the avalanche mode.These results are valuable and can provide important guidance to the use of a given device and the design of a new device.