8.1.3
The p-n junction under applied voltage
When an external voltage, V a , is applied to a p-n junction the potential difference between
the n- and p-type regions will change and the electrostatic potential across the spacecharge region will become (V bi − V a ). Remember that under equilibrium the built-in
potential is negative in the p-type region with respect to the n-type region. When the
applied external voltage is negative with respect to the potential of the p-type region, the
applied voltage will increase the potential difference across the p-n junction. We refer to
this situation as p-n junction under reverse-bias voltage. The potential barrier across the
junction is increased under reverse-bias voltage, which results in a wider space-charge
region.
Figure 8.7 (a) shows the band diagram of the p-n junction under reverse-biased
voltage. Under external voltage the p-n junction is no longer under equilibrium any more
and the concentrations of electrons and holes are described by the quasi-Fermi energy for
electrons, E Fn , and the quasi-Fermi energy for holes, E Fp , respectively. When the applied
external voltage is positive with respect to the potential of the p-type region, the applied
voltage will decrease the potential difference across the p-n junction. We refer to this
situation as p-n junction under forward-bias voltage. The band diagram of the p-n junction
under forwardbiased voltage is presented in Figure 8.7 (b). The potential barrier across the
junction is decreased under forward-bias voltage and the space–charge region becomes
narrower. The balance between the forces responsible for diffusion (concentration
gradient) and drift (electric field) is disturbed. Lowering the electrostatic–potential barrier
leads to a higher concentration of minority carriers at the edges of the space-charge region
compared to the situation in equilibrium. This process is referred to as minority-carrier
injection. This gradient in concentration causes diffusion of the minority carriers from the
edge into the bulk of the quasi-neutral region.
The p-n junction under applied voltage
When an external voltage, V a , is applied to a p-n junction the potential difference between
the n- and p-type regions will change and the electrostatic potential across the spacecharge region will become (V bi − V a ). Remember that under equilibrium the built-in
potential is negative in the p-type region with respect to the n-type region. When the
applied external voltage is negative with respect to the potential of the p-type region, the
applied voltage will increase the potential difference across the p-n junction. We refer to
this situation as p-n junction under reverse-bias voltage. The potential barrier across the
junction is increased under reverse-bias voltage, which results in a wider space-charge
region.
Figure 8.7 (a) shows the band diagram of the p-n junction under reverse-biased
voltage. Under external voltage the p-n junction is no longer under equilibrium any more
and the concentrations of electrons and holes are described by the quasi-Fermi energy for
electrons, E Fn , and the quasi-Fermi energy for holes, E Fp , respectively. When the applied
external voltage is positive with respect to the potential of the p-type region, the applied
voltage will decrease the potential difference across the p-n junction. We refer to this
situation as p-n junction under forward-bias voltage. The band diagram of the p-n junction
under forwardbiased voltage is presented in Figure 8.7 (b). The potential barrier across the
junction is decreased under forward-bias voltage and the space–charge region becomes
narrower. The balance between the forces responsible for diffusion (concentration
gradient) and drift (electric field) is disturbed. Lowering the electrostatic–potential barrier
leads to a higher concentration of minority carriers at the edges of the space-charge region
compared to the situation in equilibrium. This process is referred to as minority-carrier
injection. This gradient in concentration causes diffusion of the minority carriers from the
edge into the bulk of the quasi-neutral region.