Catastrophic Burn Out in Power VDMOS Field-Effect Transistors

Power vertical double-diffused MOS field-effect transistors (VDMOS) are finding wide use in satellite applications. In particular, IRF-l5O transistors (HEXFETs) have been designed into an electronic power conditioner (EPC) for a commercial C-band communications satellite with a ten-year mission. In this application, these devices are switched at a 50 kHz rate between the conducting and blocking state. During testing of prototype electronic power conditioners, catastrophic failures were encountered in these VDIMOS transistors. The failures were electrically characterized as low values of the drain-source resistance in the "Off" or blocking state. Analysis of the failed devices showed minimal physical evidence of damage, with the damage generally confined to three or four hexagonal cells (HEX) located adjacent to one another. In some cases, there was no visual evidence of damage to the device. Failure sites, when present, were always located near a gate stripe. A review of the application use conditions of the device within the EPC indicated that the drain-source was driven negative as the device was switching from the conducting to the blocking state. This results in the body-drain diode of the device (with a rated current of 28 amperes) being turned "On" and conducting a maximum of 3 amperes. The negative drain-source voltage is followed by a rapidly rising positive voltage as the device turns "Off". These conditions were reproduced in the laboratory. Failures identical to those encountered in the EPC were observed when the following three conditions were met: 1. The body-drain diode must be turned "On".