The Avalanche Diode

Abstract

In the preceding chapter we analyzed the behavior of a p-n junction (diode) under forward bias. Let us now reconsider Fig. 3–5b under reverse bias conditions (Φ _v < 0). Here, the tunneling current increases when |Φ _v | increases because the electric field F _max increases in proportion to (Φ_bi — Φ _v )^1/2 [Eqs. (3–18) and (3–19)], resulting in an enhanced tunneling probability [Eqs. (3–30) and (3–32)]. However, this is only one consequence of the increased F _max. In addition, after tunneling to the n-side of the barrier, the electron finds itself in a high electric field region (see Fig. 3–2). When a sufficiently large reverse bias [e(Φ_bi — Φ _v ) > E _G ] is applied to the diode, the energy received by the electron from the electric field becomes so large that it can reach an energy higher than E _G . Now the electron can cause impact ionization and create an electron-hole pair. The primary and secondary electrons are accelerated by the field, but they are now leaving the high-field region (Figs. 3–5b and 3–2c). However, the secondary hole moves further into the high-field region, where it can reach sufficiently high energy to produce another electron-hole pair. An avalanche “breakdown” develops, which results in junction breakdown. The current then increases abruptly at the avalanche voltage Φ _B , as shown in Fig. 4–1. Under certain conditions it is sometimes possible for the avalanche process to be self-supporting at a lower bias |Φ _v | than the breakdown voltage Φ _B .