1. Conduction processes in semiconductors

2. Two form of charge carrier transport (1) Drift (due to E-field) (2) Diffusion (due to density gradient) for two types of charge carriers (a) n-type (electrons) (b) p-type (holes)

3. Drift current – due to E-field → Ohm’s law: J =  E where  = conductivity  ≡ mobility Drift velocity: v =  E

4. Conductivity of semiconductors  =  n +  p  n = q  n n electrons  p = q  p p holes

5. Mobility: scattering by (a) lattice (b) ionized impurities

6. Mobilities for silicon [7.3-2] Where T n = T/300 N I = total density of ionized impurities

7. Diffusion current holes: electrons: where D n, D p are defined as the diffusion coefficients for electrons and holes, respectively

8. Relationship between mobility and diffusion coeffcients D n =  n V T D p =  p V T V T = kT/q ≡ thermal voltage

9. Electrical current in semiconductors J = J(drift) + J(diffusion) Two transport mechanisms Two types of charge-carriers

10. For n = n(x) and equilibrium conditions Equilibrium → E F = same everywhere. So for n=n(x) then E C = E C (x) (and likewise for E i and E V )

11. Induced E-field Equilibrium: Current = 0 = J n (drift) + J n (diffusion)