Physics · Ch 14 — Electronic Devices
Zener Diode: Structure and Breakdown
Zener Diode: Structure and Breakdown
Structure. A Zener diode is, physically, a p-n junction diode -- but doped much more heavily than an ordinary rectifier diode, which makes its depletion region considerably THINNER for the same applied voltage. Its circuit symbol is drawn as an ordinary diode symbol with the cathode bar bent into a small Z-shaped kink, distinguishing it visually from a plain rectifier diode even though, physically, it is used quite differently.
Why heavy doping matters: breakdown at a LOW, precise voltage. Because its depletion region is so thin, even a comparatively MODEST reverse voltage is enough to set up an extremely strong electric field across it -- strong enough to pull valence electrons directly out of their covalent bonds within the depletion region itself (rather than relying on carriers gaining enough kinetic energy to knock others loose, as in ordinary avalanche breakdown). This direct field-induced rupturing of covalent bonds, occurring at a comparatively low and very precisely reproducible reverse voltage, is called Zener breakdown, and it is what a Zener diode is purpose-built to undergo, safely and repeatedly.
Zener breakdown versus avalanche breakdown. An ORDINARY (lightly doped) junction diode, pushed to reverse breakdown, instead breaks down by a different mechanism called AVALANCHE breakdown: the small population of existing minority carriers is accelerated by the (wider) depletion region's field to very high kinetic energies, and, on colliding with lattice atoms, knock loose further electron-hole pairs, which are themselves accelerated and knock loose still more pairs -- a rapidly multiplying chain reaction ('avalanche') that suddenly floods the junction with current. Avalanche breakdown typically occurs at a HIGHER reverse voltage than Zener breakdown and is generally destructive if sustained, since an ordinary diode is not designed to survive the resulting current and heat. A Zener diode, by contrast, is specifically designed and rated to operate INDEFINITELY in its own (lower-voltage) breakdown region, provided the current through it is kept below its rated maximum -- exceeding that rated current, even in a genuine Zener diode, causes excessive power dissipation and permanent damage. …
What this figure shows. The standard circuit symbol for a Zener diode is drawn: an ordinary diode triangle-and-bar symbol, but with the bar (cathode) end bent into a small Z-like or S-like kink at both corners, distinguishing it from a plain rectifier-diode symbol. Beside the symbol, a V-I characteristic graph is drawn similar in overall shape to Section 9.11's diode curve, but with the REVERSE (third-quadrant) breakdown drawn at a much SMALLER, more sharply defined reverse voltage, labelled the ZENER VOLTAGE , where the curve turns and drops almost perfectly VERTICALLY -- indicating that, once breakdown begins, the voltage across the diode stays very nearly constant at even as the current through it varies over a wide range. A caption notes that the diode must never be allowed to exceed its maximum rated (Zener) cur …