More Semiconductors Flashcards
What do donors and acceptors do?
Donors donate 1 electron into conduction band, acceptors remove one electron from valence band.
What happens when, if we model the substitution of, say P, by adding a fixed positive charge of +e at the site along with an additional electron?
This introduces additional electronic levels close to the conduction band at ε(d) just below ε(c).
What is a similar model we can use to model acceptor impurities?
Model is a superposition of a fixed charge, -e, on top of the host atom with the absence of 1 electron in the crystal. Missing electron represented by a bound “hole” creating additional electronic energy level at ε(a) just above the valence bands.
What is the energy needed to excite an electron from top of valence band into an acceptor level?
ε(a)-ε(v) - excited electron into acceptor level, filling a hole near an acceptor impurity, leaving a free hole in the valence band. Produces p-type semiconductors.
What is the difference between n-type and p-type semiconductors?
The position, μ, where the new energy level is.
When does the p-n junction occur?
At the boundary of one region of semiconductor doped with p-type impurities and another with n-type.
What is the I-V property of a p-n junction?
Graph is negative then goes to zero and increases a lot from there - one way device.
How do we explain the p-n junction behaviour?
Put the energy level diagrams of each together and have minus on left and plus on right with currents running through middle. ipr going into ipg on bottom and ing going into inr on top.
What does the model tell us about the p-n junction for diffusion from p to n?
- In equilibrium there is no net current across junction, but carriers cross back and forth.
- Many holes in p-region act as hole gas which diffuse across junction where they can recombine with some of the free electrons
What does the model tell us about the p-n junction for diffusion from n to p?
-Sets up an electric field Eelec and the potential energy associated with this field raises the enrgy levels in p-region relative to those in n region
What does the diffusion process lead to?
Recombination of currents of holes and electrons, ipr, inr.
What does Eelec generate?
Eelec sweeps out of the junction, with corresponding ipg and ing.
What is ing proportional to? What does it represent?
ing proportional to exp(-Δ/k(B)T). Is the probability of exciting minority carrier, e-, in the p-type region.
What is inr proportional to? What does it represent?
inr proportional to exp(-Δ/k(B)T). Is the probability that it has enough energy to get up the potential barrier.
What happens if we apply a positive potential difference, V, across the junction?
Eelec is decreased and the difference between energy levels in the p- and n-type regions gets redued by ΔE = -e*V
