5.1 Introduction

As mentioned in Section 2.1, the breakdown scenario depends critically on the ramp dV ₀/ dt. Over a very wide range of magnitudes of dV ₀/ dt the conventional (static or quasi-static) breakdown scenario discussed in Chapter 3 is followed, but if dV ₀/ dt ramp becomes extremely large the picture changes dramatically.

To establish the criteria for static and dynamic breakdown, let us consider the situation in which a relatively small voltage V ₀ << V _i (where V _i is the static breakdown voltage) is applied to a reverse-biased p - n junction (Figure 5.1, curve 1).

Figure 5.1: Qualitative field distribution across the base of a reverse-biased p ⁺ - n junction when a large reverse current I ₀ is flowing through the diode. At every point in the space-charge region (SCR) the field increases in time according to the law dF/ dt = I ₀/ ? ? ₀ S = j ₀/ ? ? ₀. Point x ₀ moves to the right with a velocity , where j _N = eN _dv _s.

The left-hand part of the base region of W length is completely depleted (space-charge (SCR) or depleted region). The electric field in this region decreases linearly with x according to the Poisson equation dF/ dx = eN _d/ ? ? ₀.