2.1 Intrinsic semiconductor

  Semi-conductor Devices and Electronics

A pure semiconductor which is free from every impurity is known as an intrinsic semiconductor.
Germanium $(Ge)$ and silicon $(Si)$ are the examples of an intrinsic semiconductor.
In intrinsic semiconductor, ${n_e} = {n_h} = {n_i}$

where,
$n_e$: Density of electron in conduction band
$n_h$: Density of holes in valence band
$n_i$: Intrinsic carrier concentration

When an electric field is applied across an intrinsic semiconductor, electrons and holes move in opposite directions, so that total current $(I)$ through the pure conductor is given by,
$$I = {I_e} + {I_h}$$
where,
$I_e$: Free electron current
$I_h$: Hole current

2.1.1 Hole

It is a seat of positive charge which is produced when an electron breaks away from a covalent bond in a semiconductor.

A hole has a positive charge equal to that of an electron.
The mobility of hole is smaller than that of an electron.

2.1.2 Intrinsic concentration

The intrinsic concentration $\left( {{n_i}} \right)$ varies with temperature $(T)$ as,
$$n_i^2 = {A_0}{T^3}{e^{ - \frac{{{E_g}}}{{kT}}}}$$
where,
$E_g$: Energy gap at $0\ K$ in electron volt $(eV)$
$k = 8.62 \times {10^{ - 5}}\;eV{K^{ - 1}}$: Boltzmann constant
$A_0$: Constant independent of temperature $(T)$

2.1.3 Effect of temperature on conductivity of intrinsic semiconductor

An intrinsic semiconductor will behave as a perfect insulator at absolute zero.
With increasing temperature, the density of hole-electron pairs increases and hence the conductivity of an intrinsic semiconductor increases with increase in temperature.
Similarly, the resistivity decreases as the temperature increases.
The semiconductors have a negative temperature coefficient of resistance.