1. Current Flow
ECE 2204

2. n+-n and p+-p Junctions
In the previous set of slides on diffusion currents, we showed that a diffusion current flowed when an n+ layer was in contact with an n layer
But, current can’t flow continuously without some source of energy
Violation of the conservation of energy and momentum

3. Continuity of Fermi Energies
The slope in the conduction band energy and the valence band energy indicate that an electric field has been induced at the n+-n junction as the electrons

4. Induced Electric Field
There are charged donors, ND+, left behind in the n+ region as electrons diffuse into the n region.
Another source of positive charge in the n+ region is a result of the diffusion of holes from the n region into the n+ region
There is an increase in negative charges in the n region because of the inflow of electrons from the n+ region and the loss of holes as the holes move into the n+ region.
Seen as the sloped region between the n+ and n regions
The electric field causes a drift current in the opposite direction that counters the diffusion current.

5. Total Carrier Flow diffusion drift e e e e e e e e e e
e e e e e e e e e e e e
h h h h h h
h h h h
drift
diffusion
Drift currents flows because of the electric field that balance the diffusion currents

6. Total Current Flow is the sum of the drift and diffusion currents
J is the current density or current I divided by the cross-sectional area A through which the carriers move.

7. When no voltage is applied

8. A more subtle balance
All electron flow must sum to zero and all hole flow must sum to zero.

9. p+-p junction
You should be able to determine the direction of carrier flow and determine whether the currents are diffusion or drift.