4.3.1 Introduction

The phenomenon of avalanche injection in bipolar junction transistors (BJTs) is extremely important in practice and very interesting from a physical point of view. It is this that is responsible for the destructive second (thermal) breakdown in all types of BJT, and it has also been shown to be responsible for very effective and very fast switching of avalanche bipolar transistors (ABTs).

Although avalanche injection can be highly essential in any type of BJT, this phenomenon is especially important in high voltage ones. Indeed, to block the bias of several hundreds (or even a couple of thousand) volts, it is necessary to have a depleted space-charge region W ₀ of several dozens microns in the structure (see Fig. 3.9 and Eqs. (3.4) and (3.5)). The base layer, which is the lowest-doped layer in a conventional BJT, should be thin enough to maintain the current gain ? at an acceptable magnitude. The maximum possible width of the base is usually close to 1 ?m. On the other hand, to block the bias V = 1000 V, the width of the appropriate depleted layer W in a Si structure must be W > W ₀ = 2 V/ F _i ? 70 ?m (with F _i ? 3 10 ⁵ V/cm). It is for this reason that the collector layer is the lowest-doped region in a high-voltage BJT (Figure...