Chapter 3, Current in
Homogeneous Semiconductors
• Carrier Motion
• Current Flow
– Drift
– Diffusion
• Recombination/Generation
• Continuity Equations
• Use of Continuity Equations
•Drift: Motion due to the electric field.
•Diffusion: Net motion from high to low concentration.
•Both very important in devices.
•Apply electric field, what happens to an electron.
•F=qE, so the (quasi) free electron accelerates.
•The velocity increases
•Velocity is limited by collisions with imperfections like:
•phonons
•impurities
•Eventually the effective mass approximation may not be valid.
•Optical Phonons
•Ionization
•Intra-valley scattering
•Inter-valley scattering
•Optical Phonons
•Ionization
•Intra-valley scattering
•Inter-valley scattering
•Notation Reminder
•no, po: equilibrium
•n, p: general carrier
concentrations
Reference: Pierret, Section 5.2
Electrons removed.
Holes removed.
3
Electrons added to condution band.
Holes added to valence band.
(Definitions)
From nT/NT
no
Note: R corresponds to generation here!!
Reference: Pierret, Section 5.3.
Next: Continuity Equations
(Fn – flux of electrons)
particles
F  flux 
m2  s
Equilibrium
Excess carriers
Gop is from light shining on the
semiconductor
Normal recombination.
Depend on details of situation.
-
-
+
(Error in eq. 3.66, Text, p. 141)
For normal, low-level injection, p<<ND
Minority carrier
diffusion length
for holes.
For direct bandgap semiconductors, R=βnp for direct band to band recombination.