1. Carrier Motion - Electric Fields ECE 2204

2. 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.

3. Since valence electrons can move, holes can move also.

4. Carrier Mobility and Velocity Mobility - the ease at which a carrier (electron or hole) moves in a semiconductor ▫Symbol:  n for electrons and  p 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: v d =  n E ▫For holes: v d =  p E

5. 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. Semiconductor  n (cm 2 -V -1 -s -1 )  p (cm 2 -V -1 -s -1 ) Si1350450 Ge2000 GaAs8500400

6. 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.

7. VaVa Z L W VaVa

8. Resistance The equation for resistance that we used in ECE 2004 is shown below. R is resistance in .  is resistivity with units of  -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.

9. Resistivity and Conductivity Fundamental material properties

10. 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?

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

12. Drift Currents

13. eV a ECEC EVEV EFEF e h h e VaVa IpIp InIn Energy Diagram Slopes on the energy diagram indicate that an electric field is present at that location.

14. 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?