A1 Tunnel and Breakdown Effect

Topic

When this p-n junction is heavily doped on both sides and subjected to a very high electric field under reverse bias, it ceases to block the reverse current it would normally impede; this occurs because the intense electric field narrows the junction's depletion region to such an extent that—via the Fowler-Nordheim mechanism—electrons tunnel directly from the valence band to the conduction band without requiring the energy needed for a classical transition between the two. It is this band-to-band tunneling electron current that constitutes the Zener breakdown current in this heavily doped p-n junction. If the electric field across this heavily doped junction increases further, the depletion region narrows even more, causing the Zener breakdown current to rise rapidly.

Conversely, when this p-n junction is lightly doped on both sides and subjected to a very high electric field under reverse bias, breakdown does not occur via this Zener tunneling current, but rather through avalanche multiplication: in this lightly doped junction, the depletion region is too wide for band-to-band tunneling to be significant; however, the intense electric field accelerates the few free charge carriers crossing the depletion region until, through impact ionization, each accelerated carrier releases an additional electron-hole pair upon collision. These additional electron-hole pairs are, in turn, accelerated by the same intense electric field, releasing yet more electron-hole pairs. The fact that the electric field is higher across this same lightly doped p-n junction causes impact ionization to generate a greater number of additional electron-hole pairs during each successive collision in the avalanche multiplication process.

Consequently, this model—incorporating both Zener breakdown current and avalanche multiplication—shows that these two mechanisms (each dominant depending on the junction's doping level) jointly determine the p-n junction's breakdown voltage: the reverse-bias electric field at which the junction can no longer block current. A heavily doped p-n junction reaches this breakdown voltage via Zener breakdown (tunneling), whereas a lightly doped junction reaches it via avalanche multiplication (impact ionization). If the p-n junction is moderately doped—being neither clearly heavily doped nor clearly lightly doped—the breakdown voltage results from a combination of both mechanisms (Zener breakdown and avalanche multiplication) acting together rather than in isolation.

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