4.2 Structure and Principle of Operation
Table of Contents -
In this Section:
- Principle of Operation
4.2.1 Structure of a p-n junction
A p-n junction consists of two semiconductor regions with opposite
doping type as shown in the figure below. The region on the left
is p-type with an acceptor density NA,
while the region on
the right is n-type with a donor density
ND. The electron (hole)
density in the n-type (p-type) region is approximately equal to the donor
Fig.4.2.1 Crosssection of a p-n junction
We will assume, unless stated otherwise, that the doped
regions are uniformly doped and that the transition between
the two regions is abrupt. We will refer to this structure as being
an abrupt p-n junction.
Frequently we will deal with p-n junctions in which one side is distinctly
higher doped than the other. We will find that in such a
case only the low doped region needs to be considered, since it
primarily determines the device characteristics. We will refer to
such a structure as a one-sided abrupt p-n junction.
The junction is biased with a voltage Va
as shown in the figure.
We will call the junction
forward-biased if a positive voltage is applied to the
p-doped region and reversed-biased if a negative voltage is applied
to the p-doped region. The contact to the p-type region is also called the
anode, while the contact to the n-type region is called the cathode,
in reference to the anions or positive carriers and cathions
or negative carriers in
each of these regions.
4.2.2 Principle of operation
The principle of operation will be explained using a gedanken experiment, were we
imagine that one can bring both regions simply together, aligning both the
conduction and valence band
energies of each region as shown in the figure below.
Fig.4.2.2 Energy band diagram of a p-n junction just after merging the
n-type and p-type regions
Note that this does not automatically align the fermi energies.
The electrons (holes) close to the metallurgical junction diffuse across the junction
into the p-type (n-type) region where hardly any electrons (holes) are present. This process leaves the ionized
donors (acceptors) behind, creating a region around the junction which is depleted of mobile
carriers. We call this region the depletion region, indicated by the
symbol w as shown on the above figure.
The charge due to the ionized donors and acceptors causes an electric
field which in turn causes a drift of carriers in the opposite direction.
The diffusion of carriers continues until the drift current balances the
diffusion current, thereby reaching equilibrium. This situation is shown below:
Fig.4.2.3 Energy band diagram of a p-n junction in thermal
While in thermal equilibrium no voltage difference exists between the
n-type and p-type material, there is an internal potential which is
caused by the fermi energy difference between the n-type and p-type
semiconductors. This built-in potential is discussed in the
© Bart J. Van Zeghbroeck, 1996, 1997