LIGHT EMITTING DIODE (LED)

Beside tunnel diode, varactor diode, schottky diode, step recovery diode, pin diode, there is a gun diode which is used for controlling electronic signals in the GHz range.

A P-N junction diode, which emits light when forward biased, is knows as a lights emitting diode. The emitted light may be visible or invisible. The amount of light output is directly proportional to the forward current. Thus higher the forward current, higher is the light output. The arrows, pointing away from the diode symbol represent the light, which being transmitted away from the junction.

Here and type layer is grown on P type substrate by a diffusion process. Then a thin P type layer is grown on the N-type layer. The metal connections to both the layers make anode and cathode terminals as indicated. The light energy is released at the junction, when the recombination of electrons with holes takes place. After passing through the P-region, the light is emitted through the window provided at the top of surface.

It will be inserting to know that when LED is forward bias, the electrons and holes move towards the junction and the recombination take place. After recombination, the electrons, laying in the conduction bands of N-region, fall into the holes lying in the valence band is radiated in the form of light energy. In ordinary diodes, this energy is radiated in the form of heat.

The semiconducting materials used for manufacturing light emitting diodes are gallium phosphate and gallium arsenide phosphate. The silicon and germanium is not used for manufacturing light emitting diodes because these are heat producing materials. Moreover, these materials are very poor in emitting light radiations.

The LED, radiate light in different colors such as red, green, yellow, blue, orange etc. some of the LED’s emits  infrared light also. The colors, of the emitted light, depend upon the type of the semiconductor used. Thus gallium arsenide emits infrared radiations, gallium arsenide phosphate produces either red or yellow light gallium phosphate emits red or green light and gallium nitrite produces blue light.

Zener Diode And Tunnel Diode

Zener Diode:

It is also a p-n junction diode. Its safe inverse voltage or break down voltage is kept lower than that of an ordinary diode and each diode is designed to have a specific brake down voltage. The amount of leakage current increases suddenly by increasing the reverse bias. The current flowing through the zener diode on break down voltage is called avalanche current or zener current.

A zener diode is used in voltage regulator circuits. A zener diode is connected across the voltage to be regulated with a series registers in opposite polarity. The load register is connected in parallel to the zener diode.

When the input voltage rises beyond the Break down voltage of the zener diode, the conduction of current is started through the series resistor Rs. The magnetic of current conducting through the resistor Rs is equal to the sum of zener plus load current. The magnitude of Rs resistors current will increase further for increase in input voltage. 

But, due to decreased zener resistance, only the magnitude of zener current will increase and not the magnitude of load current. Now, the will be more voltage drop across Rs and the output voltage will remain unchanged. In this way, the circuit can effectively work for small changes of even less than one volt.

The break down voltage of zener diodes in printed on them eg.12 v, 27v etc.

TUNNEL DIODE:

In this diode a P-N junction is made in such a way that the electrons passing through the junction have to cross a very narrow junction region and they have to managing any how to cross the junction.

In forward bias state, the magnitude of current reaches at its maximum value at a very low voltage, whereas the flow of current doesn’t start in an ordinary diode at a voltage. On increasing the forward bias, the current will decrease and it will be reduced to a minimum value at a certain high forward bias. The property of tunnel diode is termed as its negative resistance characteristic. If the forward bias is increased further, the magnitude of current will start to increase again.

A tunnel diode is used in amplifiers and oscillators of gigahertz frequency and range and in flip-flop gates.