Special Purpose Diodes:
PN Junction diode cannot be used for all type of applications .Depending upon the type of application the PN junction diode is modified to obtain a new kind of diode.These diodes are known as special purpose diode.Following are the different types of special purpose diodes:
ii.Light Emitting Diode(LED)
Zener diode is a special purpose diode which is designed to operate in the breakdown region.Unlike the other ordinary PN junction diodes, the Zener diode can operate in breakdown region without damage to the junction.For this Zener diode is heavily doped.In the forward biased condition,it’s characterstics is same as that of ordinary PN junction diodes.It is always reverse connected in the circuit.Due to the very steep I-V characterstics in the breakdown region(ideally a straight vertical line) the Zener voltage remains constant over the large change in current.This property of Zener diode is utilized in the design of voltage regulator circuits so as to get stable output voltage.The operation of zener diode as a voltage regulator will be studied in the later section.The voltage at which Zener Breakdown occurs is known as reverse breakdown voltage or in specific Zener Breakdown voltage.It is designated as VZThe symbol of the Zener diode is:
ii.Light Emitting Diode:
The special purpose semiconductor diode which emits visible light when forward biased is known as Light Emitthing Diode(LED).The symbol of LED is
In the light emitting diode the PN junction is not made from Si or Ge.Instead it is made from the materials like gallium,phosphorous and arsenic.During the forward biased condition the recombination of holes and electrons cause the electron to radiate energy in the form of heat and light which lies in the visible region.
The special purpose semiconductor diode in which the reverse current increase when the junction is exposed to light is called as photodiode. The reverse current in the photo diode is directly proportional to the intensity of light falling on the PN junction of the Photo diode.This means greater the intensity of light falling on the junction larger will be the magnitude of the reverse current.The symbol of the photo diode is:
Characterstics of Photo Diode:
a.Reverse Current illumination curve:(IRVs E)
It is the graph plotted between reverse current IR illumination E of the photo diode.
b.Reverse voltage-reverse current curve:
It is the graph between reverse voltage VR and reverse current IR of the photo diode.
IV. Varactor Diode:
The special purpose semiconductor diode which is designed to exhibit the variable capacitance under the reversed biased condition is known as varicap(Vari for variable and cap for capacitance) or Varactor diode.The symbol of varactor diode is:
When reversed biased,the width of the depletion layer is increased.Here,the positive and negative ions on the either side of depletion region forms a parallel plate capacitor in which capacitance c is the function of width of depletion layer(Wd) which is again the function of reverse voltage VR.
. i.e C=f(Wd)
Thus,the capacitance of the varactor diode is controlled by the reverse voltage across the varactor diode.
The capacitance is given by:
ℰ=Permitivity of semiconductor material
A=Cross sectional area of semiconductor material
Wd=Width of the depletion layer.
C=Capacitance of the Varactor diode
It is important to note that the variation of capacitance for ordinary PN junction diode is not so appreciable because they are not designed for capacitance.So,we use varactor diode for this purpose in which the variation of capacitance is appreciable as shown in the figure.
The Special purpose semiconductor diode which exhibits negative resistance between two values of forward resistance (i.e between peak-point voltage and valley point voltage) is called as Tunnel diode.The symbol of the Tunnel diode is:
The Tunnel diode is doped approximately 1000 times the ordinary PN diode.The operation of the Tunnel diode depends upon the tunneling phenomenon.So the diode is called Tunnel diode.
Vi.Schottky Barrier Diode(SBD) or Schottky Diode or Hot carrier diode:
Schottky Barrier Diode(SBD) or Hot carrier diode is an special purpose semiconductor diode which is formed by bringing the metal in contact with the moderately doped n-type semiconductor material.Here the metal is used instead of p-type material.The resulting metal-semiconductor junction acts like a diode which conduct the current in one direction and acts as an open circuit in opposite direction.The characterstics of Schottky diode is same as that of ordinary PN junction diode with two major exceptions.
a.In the schottky diode the current is conducted entirely by majority carriers(electrons).Thus there is no effect of minority charge stored therby giving the fastest possible switching speed.
b.The forward voltage drop is less than the ordinary PN junction diode voltage drop.It is around
0.3V-0.5V whereas for ordinary diode forward voltage drop is around 0.6-0.8V.The symbol of Schottky diode is shown below:
The Schottky diode has been extensively used in design of Schottky TTL*.Where the function of Schottky diode is to prevent the transistors to go into the saturation so as to provide the fastest possible switching speed. *TTL stands for Transistor-Transistor Logic which is one of the important logic family of digital circuits.