Light Emitting Diode (LED) is a heavily doped p-n junction used in forward bias. When current flows in forward-bias, electrons from n-side reach to p-side where they combines with the holes. This recombination is associated with the release of a photon and hence light is released. Note: 1. Intensity of the emitted light depends on the magnitude of current. 2. Colour of the emitted light depends on the bandgap of the semiconductor. 3. Reverse breakdown voltage is low and device must be protected against it. 4. Only compound semiconductors (like GaAs) are capable of emitting light. Elemental semiconductors emit heat and no light.
Describe the working and uses of a LDR(Light Dependent Resistor)
Working: The snake like track shown in the figure is the Cadmium Sulphide (CdS) film which also passes through the sides. On the top and bottom are metal films which are connected to the terminal leads. It is designed in such a way as to provide maximum possible contact area with the two metal films. The structure is housed in a clear plastic or resin case, to provide free access to external light. As explained above, the main component for the construction of LDR is cadmium sulphide (CdS), which is used as the photoconductor and contains no or very few electrons when not illuminated. In the absence of light it is designed to have a high resistance inthe range of megaohms. As soon as light falls on the sensor, the electrons are liberated and the conductivity of the material increases. When the light intensity exceeds a certain frequency, the photons absorbed by the semiconductor give band electrons the energy required to jump into the conduction band. This causes the free electrons or holes to conduct electricity and thus dropping the resistance dramatically (< 1 Kiloohm).
Uses: 1. Automatic Street Light Circuit 2. Simple Fire Alarm Circuit 3. Light Activated Switch Circuit 4. Automatic LED Emergency Light 5. Night Security Light