PHT.301 Physics of Semiconductor Devices




Electrons in crystals

Intrinsic Semiconductors

Extrinsic Semiconductors


pn junctions




Bipolar transistors




Exam questions

Html basics

TUG students

Student projects


pn junctions

Reading: Sze chapter 4 or Singh sections 5.1-5.9 or Thuselt 3.1 - 3.4

    For the exam:
  • Know the depletion approximation and how the the charge distribution, electric field, the potential, and the width of the depletion layer are calculated in this approximation. Known what the built-in voltage $V_{bi}$is and why it appears. See: Abrupt junctions.
  • Be able to draw a band diagram (conduction band, valence band, Fermi energy) of a pn-junction in forward or reverse bias.
  • Know how the width of the depletion layer and the doping can be measured using a Capacitance - Voltage (CV) measurement.
  • Be able to explain how a pn junction can be used as a variable capacitor.
  • Be able to explain carrier generation and recombination in a pn-diode. Know the equations for drift ($\vec{j}_{n,drift}= ne\mu_n\vec{E}$, $\vec{j}_{p,drift}= pe\mu_p\vec{E}$) and diffusion ($\vec{j}_{n,diff}= eD_n\nabla n$, $\vec{j}_{p,diff}= -eD_p\nabla p$) of electrons and holes.
  • The diode equation $I=I_0(\exp(eV/k_BT)-1)$ describes the diffusion current in a pn-junction. Know how to determine the electron and hole diffusion currents that lead to the diode euquation. Know why a nonideality factor sometimes has to be included in this equation.
  • Be able to explain what happens to the depletion widths, the drift and diffusion currents, and the minority carrier concentrations for forward and reverse bias.
  • Be able to describe avalanche breakdown and Zener tunneling.

Self-assessment questions on pn-junctions


1. The depletion approximation for a pn junction

2. p-n junction

3. Depletion width in reverse bias