3.1 Introduction

Systematic studies of breakdown phenomena in solids began more than 80 years ago, in the early 1920s. Approximately at the same time it was found that the scenario for breakdown depends critically on the magnitude of the ratio dV ₀/ dt, where V ₀ is the bias applied to the structure and t is time. Over a very wide range of magnitudes of dV ₀/ dt from very small (quasi-static) to fairly large, just the same "conventional" scenario is followed, that usually known simply as "breakdown", but if dV ₀/ dt becomes extremely large the picture changes dramatically to that termed "dynamic breakdown".

The critical value of dV ₀/ dt depends on the parameters of the object in which the breakdown is observed. We will establish the related criteria in Chapter 5, where the phenomenon of dynamic breakdown will be considered. For the moment we will remark only that "conventional" (quasi-static or static) breakdown takes place, roughly speaking, if the bias is applied to the sample in a time t which exceeds the carrier transit time through the sample at saturated velocity: t ? t _s = L/ v _s. With 0.1 ?m < L < 100 ?m and v _s ? 10 ⁷ cm/s, we have t ? 10 ^-12 10 ^-9 s. Taking the characteristic breakdown field F _i ~ 10 ⁵