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Carrier Motion - Electric FieldsPowerPoint Presentation

Carrier Motion - Electric Fields

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### Carrier Motion - Electric Fields

ECE 2204

Movement of Electrons and Holes

- Nearly free electrons can easily move in a semiconductor since they are not part of a chemical bond between atoms.
- Valence electrons are shared between atoms. It turns out that a valence electron can also exchange places with another valence electron that is being shared with a different atom.

Carrier Mobility and Velocity

- Mobility - the ease at which a carrier (electron or hole) moves in a semiconductor
- Symbol: mn for electrons and mp for holes

- Drift velocity – the speed at which a carrier moves in a crystal when an electric field is present. The electric field is the force applied to the carrier.
- For electrons: vd= mnE
- For holes: vd = mpE

Carrier mobility

- The ease at which electrons and holes can move depends on the semiconductor material.
- Nearly free electrons in direct semiconductors are faster than nearly free electrons in indirect semiconductors. Extremely high speed electronic devices are usually made from these materials.

Direction of Carrier Motion

- Suppose we consider a piece of intrinsic semiconductor to be a resistor (which it is) and attach a dc voltage source to it.
- Let say that the length of the semiconductor is L, its width is W, and the height is Z.
- The magnitude of the voltage source is Va.

Resistance L is the distance that the current has to flow as it enters and leaves the resistor. WZ is the cross-sectional area A of the material.

- The equation for resistance that we used in ECE 2004 is shown below.
- R is resistance in W.
- is resistivity with units of W-cm.

Resistivity and Conductivity

- Fundamental material properties

Questions

- Since the resistance of the semiconductor depends on its geometry
- What do you expect to happen to the resistance of the Si bar if L increases?
- How about as either W and H increases?

Current

Current that is a result of an applied electric field is called a drift current.

EC

Energy Diagrame

eVa

EF

EV

Slopes on the energy diagram indicate that an electric field is present at that location.

h

e

In

h

Ip

Va

Questions

- Assume that the electron and hole mobilities are constant.
- What happens to the resistance of the Si bar as the temperature increases?

- Suppose there were bars of Si, Ge, and GaAs that had exactly the same dimensions.
- At a particular temperature (say 300K), which bar has the lowest resistance?

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