Current Flow

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 eeeeeeeee e eeeeeeeeeee h hhhhh h hhh 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.