Photodiode is a light detector semiconductor device that converts light energy into electric current or voltage which depends upon the mode of operation.
The upper cut-off wavelength of a photodiode is given by;
where, šš is the cut-off wavelength in nm and šøš is the band gap energy in eV.
A normal p-n junction diode allows a small amount of electric current, under reverse bias, due to minority charge carriers. To increase the electric current under reverse bias condition, we need to generate more minority carriers. The external reverse voltage applied to the p-n junction diode will supply energy to the minority carriers, but it will not increase the population of minority charge carriers.
A small number of minority carriers are generated due to external reverse bias voltage.
The minority carriers generated at n-side or p-side will recombine in the same material, before they cross the junction.Ā As a result, no electric current flows due to these charge carriers. For example, the minority carriers generated in the p-type material experience a repulsive force from the external voltage and try to move towards n-side. However, before crossing the junction, the free electrons recombine with the holes withinthe same material. As a result, no electric current flows.
To overcome this problem, we need to apply external energy directly to the depletion region to generate more charge carriers. A special type of diode called photodiode is designed to generate more number of charge carriers in depletion region. In photodiodes, we use light or photons as the external energy to generate charge carriers in depletion region.
Construction
The typical construction of a photodiode is illustrated in the following Figure. This example uses construction technique called ion implantation where the surface of a layer of N-type is bombarded with P- type silicon ions to produce a P-type layer of about 1 Āµm (micrometre) thick. During the formation of the diode, excess electrons move from N-type towards P-type and excess holes move from P-type towards N-type; this process is calledĀ diffusion, resulting in the removal of free charge carriers close to the PN junction, so creating a depletion layer as shown in the following Figure.
The (light facing) top of the diode is protected by a layer of Silicon Dioxide (SiO2) in which there is a window for light to shine on the semiconductor. This window is coated with a thin anti-reflective layer of Silicon Nitride (SiN) to allow maximum absorption of light and an anode connection of aluminium (AI) is provided to the P-type layer. Beneath the N-type layer, there is a more heavily doped N+ layer to provide a low resistance connection to the cathode.
Working Principle:
When the conventional diode is reverse biased, the depletion region starts expanding and the current starts flowing due to minority charge carriers. With the increase of reverse voltage, the reverse current also starts increasing. The same condition can be obtained in photodiode without applying reverse voltage.
The following Figure shows photo diode bias symbol. The junction of Photodiode is illuminated by the light source; the photons strike the junction surface. The photons impart their energy in the form of light to the junction. Due to which electrons from valence band get the energy to jump into the conduction band. This leaves positively charged holes in the valence band, so producing āelectron-hole pairsā in the depletion layer. Some electron-hole pairs are also produced in P and N layers, but apart from those produced in the diffusion region N layers, most will be re-absorbed within the P and N materials as heat. The electrons in the depletion layer are then swept towards the positive potential on the cathode, and the holes swept towards the negative potential on the anode, so creating a photo current. In this way, the photodiode converts light energy into electrical energy.
V-I Characteristics of Photodiode:
The characteristics curve of the photodiode can be understood with the help of the following Figure. The characteristics are shown in the negative region because the photodiode can be operated in reverse
mode only
The reverse saturation current in the photodiode is denoted by I0, It varies linearly with the intensity of photons striking the diode surface. The current under large reverse bias is the summation of reverse saturation current and short circuit current.
Where Isc is the short circuit current, V is positive for forward voltage and negative for reverse bias, Vt is volt equivalent for temperature, ā is unity for germanium and, 2 for silicon.
Applications
- Photodiodes are used in consumer electronics devices like smoke detectors, compact disc players,and televisions and remote controls in VCRs.
- Ā In other consumer devices like clock radios, camera light meters, and street lights, photoconductors are more frequently used rather than photodiodes.
- Photodiodes are frequently used for exact measurement of the intensity of light in science and industry. Generally, they have an enhanced, more linear response than photoconductors.
