Editing Zener diode (section)

===Surface Zeners===
The emitter–base junction of a bipolar [[NPN transistor]] behaves as a Zener diode, with breakdown voltage at about 6.8&nbsp;V for common bipolar processes and about 10&nbsp;V for lightly doped base regions in [[BiCMOS]] processes. Older processes with poor control of doping characteristics had the variation of Zener voltage up to ±1&nbsp;V, newer processes using ion implantation can achieve no more than ±0.25&nbsp;V. The NPN transistor structure can be employed as a ''surface Zener diode'', with collector and emitter connected together as its cathode and base region as anode. In this approach the base doping profile usually narrows towards the surface, creating a region with intensified electric field where the avalanche breakdown occurs. [[Hot carrier]]s produced by acceleration in the intense field can inject into the oxide layer above the junction and become trapped there. The accumulation of trapped charges can then cause 'Zener walkout', a corresponding change of the Zener voltage of the junction. The same effect can be achieved by [[radiation damage]].

The emitter–base Zener diodes can handle only low currents as the energy is dissipated in the base depletion region which is very small. Higher amounts of dissipated energy (higher current for longer time, or a short very high current spike) causes thermal damage to the junction and/or its contacts. Partial damage of the junction can shift its Zener voltage. Total destruction of the Zener junction by overheating it and causing migration of metallization across the junction ("spiking") can be used intentionally as a 'Zener zap' [[antifuse]].<ref>{{cite journal|doi=10.1155/1996/23706|title=Zener Zap Anti-Fuse Trim in VLSI Circuits |year=1996 |last1=Comer |first1=Donald T.|journal=VLSI Design|volume=5|page=89|doi-access=free}}</ref>