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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.
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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.
L W Z Va Va
Resistance • 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. • 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.
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 Diagram e 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?