3-D Properties of the Switching Transient in a High-Speed Avalanche Transistor Require Optimal Chip Design

We have recently shown that only a small part of a Si bipolar junction transistor (BJT) conducts the current in a short-pulsing mode (≤ 2 ns), and a complicated temporal variation takes place in the size of operating emitter-base perimeter. Namely, the switched-on region in the corner of an emitter finger first shrinks down to just a few micrometers and only then spreads to ~ 100 μm by the end of the transient. Additionally important is the demonstrated ability of a tiny filament (≤ 10 μm) to quench the switching in the entire perimeter (1.6 mm). This creates the impression that an initial triggering inhomogeneity of the smallest size will always win the switching competition. It has been shown experimentally, however, that the sharpest corners (in size) “lose out” to the ~ 100 μm corners, a fact that has not been explained so far. It is shown here using quasi-3-D modeling that an optimal curvature for the corner of an emitter finger exists that provides minimal switching delay, resulting in the shortest current pulses of the highest amplitude. This finding is especially important when designing unique subnanosecond avalanche BJTs, the 3-D transient properties of which are of major importance.