Zeners, avalanche

Constant voltage devices in everyday use are avalanche and zener diodes and varistors, or voltage dependent resistors. 

Avalanche diodes exhibit a sharp turn at the knee, but zener diodes go through this transition more gradually. This implies that the avalanche diode is a better suppressor for transients than zener diodes. 

Breakdown of a semiconductor electrode occurs when the limiting current at reverse bias sharply increases with increasing potential. At breakdown the electrode loses its insulating character and becomes conductive. Two types of breakdown may occur in a semiconductor at high field Zener breakdown and avalanche breakdovra." " ... 

FIGURE 1.17. Illustration of the breakdown of the potential barrier at the semiconductor/electrolyte interface, (a) Zener breakdown (b) avalanche breakdown (c) interface tunneling. 

The field required for breakdown to occur and the mode of breakdown depend on doping level. As the dopant concentration increases, the width of the space charge layer decreases and the probability of tunneling increases rapidly so that Z mr breakdown becomes more likely than avalanche breakdown. Zener breakdown is, in general, involved in the electrode processes on p and n materials under a reverse bias. 

Zener diode - A control device utilizing a p-n junction with a well defined reverse-bias avalanche breakdown voltage. 

Single-crystal silicon diodes (avalanche or Zener) are used only for low-voltage applications. 

In addition to the Poole-Frenkel effect and the field-induced tunneling from traps to conduction band states, the Zener effect (field-induced transitions from valence band to conduction band) and various forms of avalanche breakdown effects, can give a bulk conductivity rising sharply with field. These effects are difficult to assess in the present systems, because little is known about the electronic states in amorphous oxides, the electronic transport process, or the lattice vibration spectrum. 

The Zener effect will require very large fields with wide band semiconductors. In p-n junctions in silicon, avalanche rather than Zener breakdown normally occurs except in very thin junctions. ... 

The diac can be made with either three "layers," PNP, or with five, NPNPN. The former type is simply forced into avalanche like a Zener diode, and its characteristic curve was shown on page 153. (The PNPN diode s curve looks like half of that, with the other half being just a vertical line of high voltage but no current.) The triac requires an extra "layer" of N-type silicon that only covers part of the underlying P-type material, as shown at the top of Fig. 21.4. It operates something like an SCR or the two-transistor circuit that we made in the previous experimental section, on page 228, but it works in both directions. 

Zener diode A diode is a solid-state device when it is forward biased it allows current flow in one direction only. A Zener diode is a special diode that blocks current flow in reverse bias until a critical voltage is reached. At critical voltage on account of avalanche it is able to conduct current in this reverse bias mode without damaging itself. (This current cannot be controlled until the source is controlled.) Any standard book on solid-state devices may be referred to recapitulate Zener diode. 

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