Set of supplementary notes.

8.2. P-N JUNCTION 107
Figure 8.2: . Schematic picture of a Schottky diode. In the upper panel, applying a positive
bias across the junction lowers the barrier for electrons to enter the metal, and can eventually
tilt the electron bands so much that the barrier disappears. The current grows rapidly with
positive bias. However, if the bias is negative, the depletion width grows and the current is
little changed.
• Deep inside the n-doped (p-doped) regimes, the chemical potential must lie close to the
donor (acceptor) levels, and thereby also close just below the edge of the conduction band
(just above the edge of the valence band).
• If we were instantaneously to place the n-type and p-type regions in contact, charge would
flow because of the different chemical potentials.
• In doing so, the interface region becomes depleted of carriers, and the ionised donors
(acceptors) now have positive (negative) charge (see Fig. 8.4.
• The electrostatic potential so generated shifts the energy levels of the donors (acceptors)
down (up) and the chemical potential is equalised
The typical extent of the depletion region is between 10 nm and 1 µm. See Fig. 8.5 for a
summary of the physics of a p − n junction in equilibrium.
8.2.1 Rectification by a p-n junction
A p-n junction behaves as a diode, allowing current to flow much more readily in one direction
from the opposite. A simple picture can be given as follows, with reference to the diagram in
Fig. 8.6. Our sign convention is to apply an electrical bias where positive voltage V is applied
to the p-type side of the junction.