When light radiation falls on the crystal, electrical conductivity of the insulating crystal increases. This phenomenon is called photoconductivity.
When the energy of incident photon, is greater than the energy gap Eg, free electron-hole pairs are produced in the crystal by the absorption of the incident photons and these electrons and holes serves as the carriers of electrical current.
Impurities and imperfections in the crystal also contribute towards photoconductivity. If these are present in the crystal, then even the photons having energy below the threshold for the production of electron-hole pairs may be able to produce mobile electrons or holes. Imperfections and impurities introduce discrete energy levels in the forbidden energy levels in the forbidden energy gap called traps.
11.7.1 Variation of photoconductivity will illuminations (Simple model of Photoconductor)
To study the variation of photoconductivity with illumination, consider a simple model of photoconductor.
When light radiations falls on the crystal, electron-hole pairs are produced uniformly throughout the crystal volume.
In the recombination process, electrons directly recombine with holes.
Mobility of holes is smaller than the mobility of electrons and therefore mobility of holes can be neglected.
Using these assumptions, rate of change of electron concentration can be written as
The expression of G is given in equation (11) is quite general, however, this theoretical value of t0 do not agree and in some instances the discrepancy = 108 times. The result then indicates.
Model in failure
We should look for some missing phenomenon not considered in the expression.
11.7.2 Effect of Traps on Photoconductivity
A trap is an impurity atom or imperfection in the crystal. Capable of capturing an electron or hole. In simple words, it is an energy level, that can capture either electrons or holes.
There are two types of traps.
One type of traps helps electrons and holes to recombine and thereby assists restoration of thermal equilibrium. These type of traps are called recombination centres. In the presence of traps, recombination proceeds at much higher rate.
Second type of traps does not contribute directly in recombination process, but restricts the freedom of motion of holes or electrons. This is shown in Figure 11.19.
The photoelectric Current: If potential difference is applied across the semiconductor bar and light is allowed to fall on it, the charge are generated and those move under the influence of applied potential difference. The chage carriers, which, do not undergo recombination, reach the conducting at the ends of the bar and hence constitute electric current. This current is given by
The devices with the help of which light energy is converted to electrical energy are called photoelectric cells. Three types of photoelectric cells are:
photo-voltaic cells, and
Principle of Photoconductive Cells: It is based upon the principle that electrical resistance of semiconductors materials like selenium, lead sulphide etc. decreases when they are exposed to light radiations. The amount of this decrease in resistance (or increase in photoconductivity) depend on the light intensity and frequency of the incident light. So, if such a substance is inserted in the circuit and light is allowed to fall on it, its electric resistance (or photoconductivity) will change consequently, there will be a change of current in the current in steps accordingly to the variation of incident light intensity. The decrease in resistance (or increase in photoconductivity) of semiconductors on exposing to light may be explained as below:
If a photon striking the surface of such photosensitive material has energy E(=h v) greater than the energy gap between the valence and conduction bands of the material, sufficient energy will imparted to an electron to raise it to the conduction level. Consequently, a hole is left in the valence band. This electron-hole pair is free to serve as current carriers and hence, the conductivity of the material increases or the electrical resistance is reduced.
Construction and Working: The material generally used in photoconductive cells are cadmium sulphode (CdS), Cadmium salenide (CdSe) or lead sulphide (PbS). Commonly used material in CdS. The construction of one form of CdS cell and its circuit symbol is shown in Figure 11.22.
In this cell, a thin film of CdS is deposited on one side of an iron plate and placed below a transparent film of metal. When light radiation of sufficient energy falls on transparent metal film, resistance of CdS layer get reduces and hence its conductance is increased. Consequently, a current starts flowing the battery circuit connected circuit to generate a direction and provide a path for the current flow. Voltages varying from a few volts to several hundred volts and applied depending upon the application of photoconducting cells.
Characteristics and Spectral Response: The following Figure 11.23 shows the illumination characteristics of a CdS cell. It depicts the relationship between illumination and resistance. It may be seen that when non-illuminated (i.e., in absolute darkness), cells has a illuminated with strong light, the cell resistance falls to only a few hundred ohms. So, the ratio of dark to light resistance of cell is about 1000:1.
The spectral response of CdS cell is shown in Figure 11.23 (b). It matches response of human eye. Like human eye, response is sensitive is visible.
Applications: As photocurrent increases linearity with light intensity and spectral response of CdS cell is similar to that the human eye, it is used for following purposes:
To measure light intensity i.e., to work as light meters
To work as ON-Off switch
In street light control
In camera exposure setting
In counting applications
For relay control
In burglar alarm
As voltage regulator
In aircraft and missile tracking system.