5.0 $p-n$ junction

  Semi-conductor Devices and Electronics

When donor impurities are introduced into one side and acceptor impurities into the other side of a single crystal of an intrinsic semiconductor a $p-n$ junction is formed.

It is also known as junction diode.
It is symbolically represented as shown in the figure.

The most important characteristics of $p-n$ junction is its ability to conduct current in one direction only. This is because, it offers very high resistance in the reverse direction.

The current in the junction diode is given by, $$I = {I_0}\left( {{e^{\frac{{eV}}{{kT}}}} - 1} \right)$$ where,
$V$: Potential difference across its end
$k$: Boltzmann constant
$I_0$: Reverse saturation current

In forward biasing, $V$ is positive and low. So, $\left( {{e^{\frac{{eV}}{{kT}}}} \gg 1} \right)$, then forward current is,
$${I_f} = {I_0}\left( {{e^{\frac{{eV}}{{kT}}}}} \right)$$
In reverse biasing, $V$ is negative and high. So, $\left( {{e^{\frac{{eV}}{{kT}}}} \ll 1} \right)$, then reverse current is,
$${I_r} = - {I_0}$$