TIME-RESOLVED ANALYSIS OF SELF-HEATING IN POWER VDMOSFETS USING BACKSIDE LASERPROBING

A laserprobe technique for characterization of transient self-heating in power devices is proposed and its applicability is demonstrated on VDMOSFETs. The change in the silicon lattice temperature in the device active region under pulsed operation is sensed by an infrared laser b eam focused on a single VDMOS cell from the substrate side. The temper ature-induced change in the silicon refractive index causes an optical phase change which is detected interferometrically. The transient pha se measurements are carried out under various VDMOSFET bias conditions and for heating pulse widths in the mu s range. The laser beam modula tion is numerically modeled using a solution of the transient heat tra nsport equation and a transmission line approach to model the beam pro pagation. Calculated optical phase signals show a very good agreement with the experiments. The phase change calculated by using a simple ge ometric optic approximation was compared with the phase variation obta ined by the rigorous transmission line approach. The results show that the geometric optic approach can accurately be used in the analysis o f transient self-heating of VDMOSFETs. (C) 1997 Elsevier Science Ltd.