avoid second breakdown through positive temperature coefficient

# Avoid Second Breakdown Through Positive Temperature Coefficient

## 6. Avoid Second Breakdown Through Positive Temperature Coefficient

Unlike the bipolar transistor's current-crowding failure mode described in step 1, a power MOSFET's on-resistance rises, rather than falls, as the device heats up — a positive temperature coefficient that makes current crowding self-correcting instead of self-reinforcing, and eliminates second breakdown as a failure mode entirely. In a bipolar transistor, a hot spot conducts more current, which heats that spot further, which lets it conduct still more — a runaway loop. In a power MOSFET, a hot spot's local resistance rises as it heats, which diverts current away toward cooler, lower-resistance parts of the same device, cooling the hot spot back down.

This single physical difference is why power MOSFETs can be paralleled directly — multiple devices sharing a load with no external current-balancing circuitry — something bipolar power transistors could not safely do without added ballasting components. It is also why power MOSFET datasheets specify a continuous safe operating area covering their full voltage and current rating, rather than the sharply reduced pulsed-only ratings common on bipolar power transistor datasheets.