Question

10. Sketch a silicon p-n junction, showing the depletion region, band bending, and the Fermi level in the absence of light or applied potential. In the dark, the p-n junction acts as a rectifier. (a) Which way do electrons and holes flow most easily in the dark? (b) Does the built in electric field increase or decrease under forward bias? (c) In the light, the junction acts as a photodiode. In this case, under short circuit conditions, do electrons flow in the same direction or in the opposite direction as in (a)? Explain.

Answer 1

(c) In light, when light falls on the depletion region, electron-hole pair creates (one electron absorb the energy from the light and jump from valance band to conduction band). In the depletion region, electric field is opposite direction that of the total p-n junction diode. The electric field in the depletion region pushes the hole to the p-type region and electron to the n-type region. Thus electron density in the n-type and hole density in p-type increases, as a result, we get current. The current flow is same as (a).