sentences:  Older processes with poor control of doping characteristics had the variation of Zener voltage up to ±1 V, newer processes using ion implantation can achieve no more than ±0.25 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.The hot carriers produced by acceleration in the intense field sometime shoot into the oxide layer above the junction and become trapped there.