1. Quasi-Fermi Levels
The equilibrium EF is split into the quasi-Fermi
levels, Fn, Fp
Fn-Fp=qVf
In forward bias in the depletion region
Quasi-Fermi levels are more or less flat within the depletion region, why?

2. Figure 5—15
Forward-biased junction: (a) minority carrier distributions on the two sides of the transition region and definitions of distances xn and xp measured from the transition region edges; (b) variation of the quasi-Fermi levels with position.

3. Another Way of Calculating the Total Current
consider the injected current as supplying the carriers for the excess distributions
For example, Ip (xn=0) must supply enough holes per
second to maintain the steady state exponential distribution
p(xn) as the holes recombine
The total positive charge stored in the excess carrier
distribution at any instant of time is
The average lifetime of a hole in the n-type material is p. Thus, on the average, the entire
charge distribution recombines and must be replenished every p seconds.
 charge control approximation

4. In Summary
We can calculate the current at a p-n junction in two ways:
From the slopes of the excess minority carrier
distribution at the two edges of the transition region
b) From the steady state charge stored in each distribution
Assumptions: no recombination within W
Signs of different current components !

5. Figure 5—16
Two methods for calculating junction current from the excess minority carrier distributions: (a) diffusion currents at the edges of the transition region; (b) charge in the distributions divided by the minority carrier lifetimes; (c) the diode equation.

6. Majority Carrier Currents in the Two Neutral Regions
Since the total current I must be constant throughout the device, the majority carrier
component of the current at any point is just the difference between I and the minority
component
The nature of the majority current at any
point?
(The drift of minority carriers can be neglected in the neutral regions outside W)

7. Figure 5—17
Electron and hole components of current in a forward-biased p-n junction. In this example, we have a higher injected minority hole current on the n-side than electron current on the p side because we have a lower n doping than p doping.

8. 5.3.3 Reverse Bias
V=-Vr (p negatively biased with respect to n)
minority carrier extraction (the reverse-bias
depletion of minority carriers near the edge
of the transition region
Quasi-Fermi levels in reverse bias

9. Figure 5—18
Reverse-biased p-n junction: (a) minority carrier distributions near the reverse-biased junction; (b) variation of the quasi-Fermi levels.