Mobile Carrier ActionReading Assignment

Pierret : Chap 2 and Chap 3

InstructorInstructor : Prof. Dr. Ir. Djoko Hartanto, M.Sc.: Prof. Dr. Ir. Djoko Hartanto, M.Sc.: Arief Udhiarto, S.T, M.T: Arief Udhiarto, S.T, M.T

SourceSource :: Professor Nathan Cheung, U.C. Berkeley

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Carrier Concentration vs Carrier Concentration vs TemperatureTemperature

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Dependence of EDependence of Eff on on TemperatureTemperature

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Degenerate Semiconductors

If dopant concentrations are very high such that EF is < 3kT from EC or EV, we have to to use the full Fermi-Dirac probability function

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Electron as Moving ParticleElectron as Moving Particle

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Semiconductor Carriers Effective Semiconductor Carriers Effective MassMass

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Carrier ScatteringCarrier Scattering

Because of scattering, mobile cariers in a semiconductor do not achieve constant acceleration. However, they can be viewed as classical particles moving at a constant average drift velocity

1) “Lattice Vibration (phonons) scattering”-No dopant dependence-Increases with increasing temperature

2) Ionized impurity scattering-Increases with NA +ND (total dopant conc)-Decreases with increasing temperature

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Carrier Drift

With an electric field, mobile charge-carriers will be accelerated by the electrostatic force. This force superimposes on the random thermal motion of electrons:

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The average current in any direction is zero, if no electric field is applied

Electrons drift in the direction opposite to the E – field Current flows

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Electron MomentumElectron Momentum

With every collision, the electron loses With every collision, the electron loses momentum momentum

Between collision, the electron gains Between collision, the electron gains momentum momentum

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Balancing momentum gain and momentum lost

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Mobility Dependence on Mobility Dependence on DopingDoping

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Electrical Conductivity σ

When an electric field is applied, current flows due to drift of mobile electrons and holes:

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Electrical Resistivity ρ

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Electrical Resistance

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Example

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Example (continued)

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Temperature Effect on Mobility

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Example: Temperature Example: Temperature Dependence of Dependence of ρρ

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Diffusion CurrentDiffusion Current

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Diffusion Current Density ( Fick’s First Law)

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Total Current Density