Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Junction Properties
Zener Breakdown
When a heavily doped junction is reverse biased, the energy bands become crossed at relatively low voltages (i.e., the n-side conduction band appears opposite the p-side valence band).
Zener Breakdown:
- As Fig. indicates, the crossing of the bands aligns the large number of empty states in the n-side conduction band opposite the many filled states of the p-side valence band.
- If the barrier separating these two bands is narrow, tunneling of electrons can occur.
- Tunneling of electrons from the p-side valence band to the n-side conduction band constitutes a reverse current from n to p, this is the Zener effect.
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The basic requirements for tunneling current are a large number of electrons separated from a large number of empty states by a narrow barrier
of finite height.
- Since the tunneling probability depends upon the width of the barrier (d in Fig.), it is important that the metallurgical junction be sharp and the doping high, so that the transition region W extends only a very short distance from each side of the junction.
- If the junction is not abrupt, or if either side of the junction is lightly doped, the transition region W will be too wide for tunneling.
- As the bands are crossed (at a few tenths of a volt for a heavily doped junction), the tunneling distance d may be too large for appreciable tunneling.
- However, d becomes smaller as the reverse bias is increased, because the higher electric fields result in steeper slopes for the band edges.
- This assumes that the transition region width W does not increase appreciably with reverse bias.
- For low voltages and heavy doping on each side of the junction, this is a good assumption.
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However, if Zener breakdown does not occur with reverse bias of a few volts, avalanche breakdown will become dominant.
- In the simple covalent bonding model, the Zener effect can be thought of as field ionization of the host atoms at the junction.
- That is, the reverse bias of a heavily doped junction causes a large electric field within W; at a critical field strength, electrons participating in covalent bonds may be torn from the bonds by the field and accelerated to the n side of the junction.
- The electric field required for this type of ionization is on the order of 106 V/cm.