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Chapter IVChapter IV

April 18, 2023April 18, 2023

P-n Junction

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Fabrication of a p-n JunctionFabrication of a p-n Junction

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I-V curve of p-n JunctionI-V curve of p-n Junction

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Equilibrium Fermi LevelEquilibrium Fermi Level

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Should be positive

with DiFi NkTEEnn ]/)exp[(

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Space Charge or Depletion RegionSpace Charge or Depletion Region

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Abruption JunctionAbruption Junction

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Linearly Graded JunctionLinearly Graded Junction

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Depletion CapacitanceDepletion Capacitance

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Depletion CapacitanceDepletion Capacitance

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Evaluation of Impurity Evaluation of Impurity DistributionDistribution

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Current Voltage Current Voltage Characteristics for Ideal Characteristics for Ideal

DiodeDiode

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Current Voltage Current Voltage Characteristics Characteristics

Electron current flow

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Fig. 17 Injected minority carrier distribution and electron and hole currents. (a) Forward bias. (b) Reverse bias. The figureillustrates idealized currents. For practical devices, the currents are not constant across the space charge layer.

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Generation and Generation and Recombination and High Recombination and High

Injection EffectInjection Effect

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=ni/τg

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From continuity equation

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η=1, diffusion current dominates, η=2, recombination current dominates.

Series resistance or high injection condition, pn(xn)=nn.

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Temperature EffectTemperature Effect

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Generation current dominates

Diffusion current dominates

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Charge Storage and Charge Storage and Transient BehaviorTransient Behavior

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Junction Breakdown under Junction Breakdown under reverse biasreverse bias

1. Tunneling Effect: it occurs only when the elec1. Tunneling Effect: it occurs only when the electric field is very high and is above 10tric field is very high and is above 1066V/cm for Si V/cm for Si or GaAs. The doping concentration for both p- aor GaAs. The doping concentration for both p- and n-regions must be greater than 5x10nd n-regions must be greater than 5x101717cmcm-3-3. T. The breakdown voltage is less than 4Ehe breakdown voltage is less than 4Egg/q./q.

2. Avalanche Multiplication: for p2. Avalanche Multiplication: for p++-n one sided a-n one sided abrupt junction, the doping concentration of Nbrupt junction, the doping concentration of ND D iis less than 10s less than 101717 cm cm-3-3 and the breakdown voltage and the breakdown voltage is greater than 6Eis greater than 6Egg/q./q.

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no

nn I

WIM

)(

A multiplication factor Mn is defined as

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)(1

ppnnA JJq

G

The number of electron-hole pairs generated by an electron per unit distance traveled is called the ionization rate of the electron, αn. The electron-hole pair generation rate GA is given by:

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Using Eqs 19 and 21

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For large a and low NB

For small a and high NB

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HeterojunctionHeterojunction

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Assume that there is a negligible number of traps or generation-Assume that there is a negligible number of traps or generation-recombination centers at the interface. This assumption is valid recombination centers at the interface. This assumption is valid

only for two semiconductors with closely matched lattice only for two semiconductors with closely matched lattice constantsconstants

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