Minority Carrier Diffusion Equations
Carrier Lifetimes & Diffusion Lengths
Minority carrier lifetimes, denoted tn
for electrons in a p-type material and tp
for holes in an n-type material, are the average time a typical
minority carrier survives in a sea of majority carriers. Lifetimes
vary from carrier to carrier and from sample to sample. An electron
is considered to be "alive" when it is in the conduction band. It
"recombines" after some amount of time (on average, that time is tn)
by returning to the valence band where it is no longer able to conduct/move
because it is participating in a covalent bond to hold the crystal together.
An electron can also be captured by a trap or recombination center.
A hole is considered to be "alive" when it is in the valence band (remember
the hole is simply the lack of an electron). It recombines when an
electron annihilates it by falling from the conduction band.
Minority carrier diffusion lengths, denoted LN for electrons
in a p-type material, and LP for holes in an n-type
material, vary with the minority carrier lifetimes. They represent
the average distance the excess minority carrier is from where it
was created to when it is annihilated.