8.1.4
The p-n junction under illumination
When a p-n junction is illuminated, additional electron-hole pairs are generated in the
semiconductor. The concentration of minority carriers (electrons in the p-type region and
holes in the n-type region) strongly increases, leading to flow of the minority carriers
across the depletion region into the quasi-neutral regions. Electrons flow from the p-type
into the n-type region and holes from the n-type into the p-type region. The flow of the
photogenerated carriers causes the so-called photogeneration current density, J ph , which
adds to the thermal-generation current, J gen . When no external electrical contact between
the n-type and the p-type regions is established, the junction is in open circuit condition.
Hence, the current resulting from the flux of photogenerated and thermally-generated
carriers has to be balanced by the opposite recombination current. The recombination
current will increase through lowering of the electrostatic potential barrier across the
depletion region.
The band diagram of the illuminated p-n junction under open circuit condition is
presented in Figure 8.9 (a). As we have seen in Section 7.6, under non-equilibrium the
Fermi level is replaced by quasi-Fermi levels that are different for electrons and holes and
denote their electrochemical potential. In the open circuit condition, the quasi-Fermi level
of electrons, denoted by E Fn , is higher than the quasi-Fermi level of holes (denoted by E Fp )
by an amount of qV oc . This means that a voltmeter will measure a voltage difference of V oc
between the contacts of the p-n junction. We refer to V oc as the open circuit voltage.
The bands in the quasi-neutral regions (i.e. outside the depletion region) are flat.
Under the assumption that the charge density in each of the regions is homogeneous, the
bands follow a parabolic shape in the depletion region. In this case there is generation
anywhere in the device. In the open circuit condition, the external current density is zero.
So we have:
The carrier densities are given by:
So we find for the derivatives
We then find for each current component
The p-n junction under illumination
When a p-n junction is illuminated, additional electron-hole pairs are generated in the
semiconductor. The concentration of minority carriers (electrons in the p-type region and
holes in the n-type region) strongly increases, leading to flow of the minority carriers
across the depletion region into the quasi-neutral regions. Electrons flow from the p-type
into the n-type region and holes from the n-type into the p-type region. The flow of the
photogenerated carriers causes the so-called photogeneration current density, J ph , which
adds to the thermal-generation current, J gen . When no external electrical contact between
the n-type and the p-type regions is established, the junction is in open circuit condition.
Hence, the current resulting from the flux of photogenerated and thermally-generated
carriers has to be balanced by the opposite recombination current. The recombination
current will increase through lowering of the electrostatic potential barrier across the
depletion region.
The band diagram of the illuminated p-n junction under open circuit condition is
presented in Figure 8.9 (a). As we have seen in Section 7.6, under non-equilibrium the
Fermi level is replaced by quasi-Fermi levels that are different for electrons and holes and
denote their electrochemical potential. In the open circuit condition, the quasi-Fermi level
of electrons, denoted by E Fn , is higher than the quasi-Fermi level of holes (denoted by E Fp )
by an amount of qV oc . This means that a voltmeter will measure a voltage difference of V oc
between the contacts of the p-n junction. We refer to V oc as the open circuit voltage.
The bands in the quasi-neutral regions (i.e. outside the depletion region) are flat.
Under the assumption that the charge density in each of the regions is homogeneous, the
bands follow a parabolic shape in the depletion region. In this case there is generation
anywhere in the device. In the open circuit condition, the external current density is zero.
So we have:
The carrier densities are given by:
So we find for the derivatives
We then find for each current component