Wide-channel MOSFETs have typically 10 to 30% lower breakdown voltages than narrow-width (W approximately=L) transistors and are less likely to exhibit clear snapback characteristics. These observations can be explained using a simplified model to determine the width dependence of the MOSFET substrate resistance. The normalized substrate current I/sub sub//W required for source turn-on predicted by this model is found to decrease by an order of magnitude for wide-channel-width transistors in agreement with measured data. This results in the observed decrease in breakdown voltage for wide-channel MOSFETs. >