Lecture 5 Flashcards
What is a p-n junction?
A semiconductor that has been selectively doped to produce an interface between p-type and n-type materials.
Give 2 examples of semiconductor devices that use p-n junctions
- Diodes
- Transistors
Describe the carrier concentrations of a p-n junction
Where is the chemical potential in an n-type semiconductor?
Above the middle of the bandgap
Where is the chemical potential in an p-type semiconductor?
Below the middle of the bandgap
What happens when electrons move from an n-type to a p-type semiconductor?
They fill empty valence band states, causing the p-type to become negatively charged and the n-type to become positively charged.
What happens when a p-n junction reaches dynamic equilibrium?
- The p-type electron energy levels rise with respect to the n-type material (band bending).
- A large electric field is produced close to the p-n interface to offset the movement of electrons.
- The chemical potential is constant throughout the device.
What is the depletion region?
The absence of electrons and holes close to the interface of a p-n junction
Give the equation for the electrostatic voltage drop across a p-n junction
∆φ = electrostatic voltage drop
n_i = intrinsic electron density if the semiconductor was undoped
Describe the graphs for the carrier densities and the net charge densities through the depletion region of a p-n junction
What is the charge density in the n-type half of the depletion region?
What is the charge density in the p-type half of the depletion region?
The total charge of the depletion region must be ____ as the number of electrons removed from the left ______ the number of holes filled on the right.
Zero
Equals
State Poisson’s equation
φ = electrostatic voltage drop
x = position
ρ = charge density
Give the equation for the depletion width of a p-n junction
d_n = n-type depletion width
d_p = p-type depletion width
How does a p-n junction produce a ‘generation current’?
The electron-hole pairs created in the depletion region move parts in the strong electron field. This produces a generation current in the negative x direction.
Give the equation for the generation current density
J_gen = generation current
A = positive constant
E_g = bandgap energy
How goes the p-n junction produce a ‘recombination current’?
Electrons in the conduction band with energies grater than e∆φ can move into the p-type material where they recombine with holes. This produces a recombination current in the positive x direction.
Give the equation for the recombination current density
J_rec = recombination current
B = constant
e∆φ = electrostatic voltage difference
What is a forwards bias p-n junction?
A p-n junction where the p-type region is biased positive with respect to the n-type region.
The bias voltage is dropped across the depletion region since the carrier density is low, hence, resistivity is high.
Give the equation for the total potential difference across the depletion layer of a forwards bias p-n junction
Give the equation for the total potential difference across the depletion layer of a reverse bias p-n junction
Give the equation for the relationship between current density and voltage for a p-n junction
J = current
V = voltage
Describe the graph for the current-voltage characteristics through a p-n junction
How do solar cells work?
Photons of energy greater than the bandgap produce electron-hole pairs in the depletion region of a p-n junction. The strong electric field generated causes the electrons to move into the n-type material and holes into the p-type material. This generates a net current (aka electrical power).
