How the Photoelectric Effect Influences Semiconductors

A photoelectric effect is any effect in which light energy is converted to electricity. When light strikes certain light-sensitive materials, it may cause them to give electrons or change their ability to conduct electricity or may cause them to develop an electrical potential or voltage across two surfaces.

When a surface is exposed to electromagnetic radiation above a certain threshold frequency, the radiation is absorbed and electrons are emitted. This is termed to as photoelectric effect. The required photon energy must be equal or exceed the energy of a single photon, which is given by:

E = hv

Where v is the frequency of light and h is the Plank’s constant equal to 6.63 x 10^-34 Js

We can rewrite the above equation as the function of wavelength (λ), since we know that, frequency v = c/λ

Where c is the light speed in materials and λ is the wavelength of light.

For Si, the value of E at room temperature is 1.12 eV. But, in a real semiconductor crystal other excitation mechanisms are possible. These include absorption through transitions between the permitted bands and absorption through high levels of distortion in the forbidden band. However, the greatest excitation effect is still provided by band-to-band absorption.

When light is absorbed by a semiconductor, a current can be induced and hence cause the change of resistance of the material. This is illustrated in the figure below:

From the figure above, the semiconductor in thermal equilibrium contains free electrons and holes. The optical field to be detected is exposed to electromagnetic radiation and which is absorbed in the crystal, thereby exciting electrons into the conduction band or in p-type semiconductor, holes into the valence band. Therefore, the electronic deficiency that is created is acted upon by the electric field, and its drift along the field direction gives rise to the signal current.

Related: Einstein Contribution to Planck Formula on the Quantum Nature of Radiation