Here is a list of some special pn junction diodes used in electrical appliances.
1. The Photodiode:
We know that a built-in-voltage exists across a p-n junction, but this voltage cannot deliver current in an external circuit. However, if light is shown on the junction, there is current in the circuit. This effect is called the photovoltaic effect, and the junction is known as the photodiode.
To understand the principle underlying the effect, let the junction be illuminated. Under this condition, many excess electron-hole pairs are generated in the regions on the either side of the junction via absorption of photons.
As the field within the junction is from n side to p side, the excess minority carriers thus generated diffuse to the junction where they are carried across and become majority carriers, the holes generated on n side move towards p side and the electrons generated on p side move towards n side.
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If the junctions now open-circuited, the majority-carrier excess charge will build up on both sides of the junction (+ve charge on p-side and -ve charge on n-side) tending to lower the built-in-voltage from VB to some value VB – V0 as shown in Fig. 7.24. This change in built-in voltage appears as a measurable potential difference across the junction, which thereby behaves as if a source of voltage V0.
If the external circuit is closed, the current will therefore, flow therein. This current will continue so long as there is diffusion of excess electrons from n side and of excess holes from p-side. This in turn means that the current will flow so long as the semiconductor regions are illuminated. This explains how the incident light sets up the current flow in the external circuit.
Photodiodes can be used as useful electrical generators employing solar energy that strikes the earth, i.e., as solar cells. Such a cell has indeed been built and operated, but its efficiency is not as much as one would wish. The problem is mainly connected with difficulties in fabricating high-quality junctions of large areas necessary to intercept the maximum amount of incident light.
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Another major use of photodiodes is in radiation detection. The radiation to be detected is allowed to fall on a suitable p-n junction and the voltage produced across the junction is measured. This voltage is proportional to the intensity of incident radiation. A typical photo detector is CdS. A photo detector has several advantages over thermal detectors such as bolometer or thermopile.
2. Light-Emitting Diode (LED):
Light-emitting diodes are pn junction diodes which emit electromagnetic radiation mostly in the infrared or visible regions of em spectrum when an electric current flows through these. LED’s are generally made up of III-V compound semiconductors, such as GaAs, which have a direct band gap.
They worked on the principle that the flow of electric current through a forward biased p-n diode causes the injection of minority carriers into the regions where they can recombine with majority carriers thus producing electromagnetic radiation. The effect is known as injection electroluminescence.
In indirect band gap semiconductor, such as Si or Ge, where the electrons in the conduction band have a momentum different from holes in the valence band, this type of radiative recombination is unlikely to occur, generally, the recombination releases heat to the lattice in such cases.
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The wavelength of emitted radiation is given by-
ʎ = hc/Eg
where h in Planck’s constant, c the velocity of light and Eg, the band gap energy. The transition of electrons and then emission of photons is shown in Fig. 7.25. Most of the radiations are emitted within a distance comparable to the diffusion length of the carriers from the junction. Substituting the values of h and c, and expressing Eg in eV in the above expression, we get-
ʎ = 1.24/Eg m
Diodes built with GaAs1-x Px are most commercially available because of lower cost and case of fabrication. The brightness of such diodes is a maximum at x = 0.4 when Eg = 1.9 eV. The corresponding radiation is red and the wavelength is about 6500 Å. By changing x it is possible to obtain orange, yellow and green emissions but reduced brightness and efficiency. GaAs1-x Px light-emitting diode are used in the manufacture of signal lamps and displays. The infrared LED is a potential source for optical-fibre communication.