carriers is mainly responsible for the current transport. The dominant transport mechanism
is thermionic emission of the majority carriers over the barrier into the metal. As we have
seen in Figure 6.5, electrons in the conduction band have a droplet-shaped distribution
with respect to the energy n(E). The fraction of electrons with energies above a certain
barrier energy E B is given by
For a junction between a metal and an n-type semiconductor we find
Figure 8.15 depicts the energy-dependent electron distribution and the resulting
currents for different applied voltages. If no voltage is applied, as shown in Figure 8.15
(a), the currents due to thermionic emission are equivalent in both directions, so no net
current is flowing. However, when a forward bias is applied, the conduction band is
shifted upwards with respect to the metal and the barrier decreases. As the barrier
decreases, the density of the thermionic electrons n th is increasing. Therefore, the
thermionic current from the semiconductor into the metal is larger than that flowing the
other way. In contrast, if a reverse bias is applied, n th in the semiconductor decreases with
respect to the value in the metal. As a consequence, a net current is flowing from the metal
into the semiconductor.
Figure 8.15: The energy distributions of the electrons, the thermionic electrons, and the resulting currents in a metalsemiconductor junction with n-type substrate under (a) no external bias; (b) f orwardbias; and (c) reverse bias.
The general current–voltage characteristic of a metal-semiconductor contact under
thermionic emission is given by
where the saturation current density J s is given by
The constant A
∗
is the Richardson constant for thermionic emission, which is given by
is thermionic emission of the majority carriers over the barrier into the metal. As we have
seen in Figure 6.5, electrons in the conduction band have a droplet-shaped distribution
with respect to the energy n(E). The fraction of electrons with energies above a certain
barrier energy E B is given by
For a junction between a metal and an n-type semiconductor we find
Figure 8.15 depicts the energy-dependent electron distribution and the resulting
currents for different applied voltages. If no voltage is applied, as shown in Figure 8.15
(a), the currents due to thermionic emission are equivalent in both directions, so no net
current is flowing. However, when a forward bias is applied, the conduction band is
shifted upwards with respect to the metal and the barrier decreases. As the barrier
decreases, the density of the thermionic electrons n th is increasing. Therefore, the
thermionic current from the semiconductor into the metal is larger than that flowing the
other way. In contrast, if a reverse bias is applied, n th in the semiconductor decreases with
respect to the value in the metal. As a consequence, a net current is flowing from the metal
into the semiconductor.
Figure 8.15: The energy distributions of the electrons, the thermionic electrons, and the resulting currents in a metalsemiconductor junction with n-type substrate under (a) no external bias; (b) f orwardbias; and (c) reverse bias.
The general current–voltage characteristic of a metal-semiconductor contact under
thermionic emission is given by
where the saturation current density J s is given by
The constant A
∗
is the Richardson constant for thermionic emission, which is given by