P-N JUNK

by Dr. James D. Livingston

Two kinds of charges, electrons and holes
And each moves by drift and diffusion
Four kinds of currents can flow in the thing
Which can easily cause some confusion

With no applied volts, diffusion and drift
Are opposite, equal, and small
Electrons and holes ping-pong back and forth
Added up, nothing happens at all

If bias is forward (plus upon p)
This lowers the potential barrier
Increasing diffusion now dominates drift
In the junction for both kinds of carrier

Net current increases exponentially
And holes into n are injected
And meanwhile electrons are pushed into p
But of course oppositely directed

With bias reversed (that's plus upon n)
The barrier's very much higher
Diffusion poops out, leaving just a small drift
So our junction is a rectifier

All life is diffusion, if you get my drift
Random motion can lead to rejection
To lower the barrier and put things across
You have to pick the right direction!

The pn junction is the key element for transistors, diodes, LEDs, solid state lasers, and photodetectors.

The physics of p-n junctions