6. Deviations from the ideal (6-1) Reverse-bias breakdown
: The absolute value of the reverse voltage where the current goes off to infinity is known as the breakdown voltage and is given the symbol VBR.
(6-2) Avalanching
(a) A reverse-biased diode where VA is relatively small and far below the breakdown voltage:
Ideally, the reverse current flowing in the diode is due to minority carriers randomly entering the depletion region and being accelerated by the electric field in the region to the other side of the junction.
(b) In crossing the depletion region the carrier acceleration is not continuous but is interrupted by energy-losing collisions with the semiconductor lattice. Sine the mean free path between
collisions is ~ 10-6 cm, and a median depletion width is ~ 10-4cm, a carrier can undergo tens to thousands of collisions in crossing the depletion region. Thus at small applied reverse biases the energy lost by the carriers per collision is relatively small. The energy transferred to the lattice simply causes lattice vibrations-there is just localized heating that is readily dissipated.
(c) With increasing reverse bias the amount of energy transferred to the semiconductor lattice per collision systematically increases: Approaching the breakdown voltage, the energy transferred per collision frees a valence electron from the atom, or causes an electron from the valence band to jump into the conduction band, thereby creating an electron-hole pair.
(d) The added carriers created by this phenomenon, called impact ionization, are immediately accelerated by the electric field in the depletion region. At the breakdown voltage the carrier creation and reverse current effectively go off to infinity.
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(6-3) Zenor process
1) The name is given to the occurrence of “tunneling” in a reverse-biased diode.
2) The particles doing the tunneling are valence band electrons on the p-side of the junction. The potential energy barrier classically restricts these electrons to the p-side of the junction.
3) Tunneling takes place when the electrons pass through the barrier to empty states at the same energy in the conduction band on the n-side of the junction.
4) The greater the reverse bias, the larger the number of filled valence electron states on the p-side placed opposite empty conduction-band states on the n-side, and hence the greater the reverse- bias tunneling current.
5) There are two major requirements for tunneling to occur and be significant:
① There must be filled states on one side of the barrier and empty states on the other side of the barrier at the same energy. Tunneling cannot take place into a region void of allowed states.
② The width of the potential energy barrier, d, must be very thin. Quantum-mechanical tunneling becomes significant only if d<100Å =10-6cm.
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(6-4) The R-G current
1) The observed “extra” current arises from thermal carrier recombination-generation in the
depletion region that was assumed to be negligible in the derivation of the ideal diode equation.
2) For the reverse-bias case, we associated the reverse current with the minority carriers wandering into the depletion region from the two sides of the junction.
① When the diode is reverse biased, the carrier concentrations in the depletion region are reduced below their equilibrium values, leading to the thermal generation of electrons and holes
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② The large electric field in the depletion region rapidly sweeps the generated carriers in the quasineutral regions, thereby adding to the reverse current.
3) Forward biasing increases the carrier concentrations in the depletion region above their equilibrium values giving rise to carrier recombination in the region.
4) The resulting added forward current as arising from the carriers that cannot make it over the potential hill is partially eliminated via recombination at R-G centers in the depletion region.
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