p-n junction (p-n diode)

• Two semiconductor regions of opposite type
• Can be used as
• Rectifier
• Isolation structure
• Voltage-dependent capacitor
• Other use: solar cells, photodiodes, light emitting diodes, laser diodes
• Essential part of
• Metal-Oxide-Silicon Field-Effects Transistors (MOSFETs)
• Bipolar Junction Transistors (BJTs)

Structure

• Two semiconductor regions with opposite doping type
• Region on the left is p-type with an acceptor density N_a
• Region on the right is n-type with a donor density N_d
• Electron (hole) density in the n-type (p-type) region is approximately equal to the donor (acceptor) density (shallow dopants)
• Transition between the two regions is abrupt - abrupt p-n junction
• Anode - contact to the p-type region
• Cathode - contact to the n-type region
• Junction is biased with a voltage V_a
• Forward-biased junction - a positive voltage is applied to the p-doped region
• Reversed-biased junction if a negative voltage is applied to the p-doped region.

Principle of operation

Figure a)

Energy band diagram of a p-n junction before merging both semiconductor regions

Figure b)

Flatband diagram - obtained when both semiconductor regions merged together
and conduction and valence band energies of each region aligned

Fermi energy - strictly associated with a particle, which is in thermal equilibrium with the system of interest
(does not consist even in part of heat or work)
often called the electro-chemical potential

E_Fn, E_Fp ... Fermi energies - not automatically aligned

Flatband diagram is not an equilibrium diagram

both electrons and holes can lower their energy by crossing the junction