The most common use of diodes is rectification. Rectifier diodes are use in power supplies to convert a. c. voltage to d. c. voltage. But the diode can’t do only rectification.
Now a days in this fast developing world many specific types of diodes are manufactured for specific application. So some special purpose diodes are as follows :
- Zener diode
- Tunnel diode
- Photo diode
- Light emitting diode (LED)
- Varactor diode
So now let us discuss about special purpose diodes one by one.
What is a Zener diode explain?
A properly doped crystal diode which has a sharp breakdown voltage is call a Zener diode. Zener diode uses the reverse (VR /IR) characteristic for its operation. Hence it is always reverse connected in the circuit that is Zener diode is always reverse biased.
Let us look at the reverse characteristic of a Zener diode (given below see figure-1). As we increase the reverse voltage from 0 V, there is a very small reverse current IR (a few µA). Which is essentially constant until the breakdown voltage is reach.
Once the breakdown voltage ( = VZ, Zener voltage ) is reach, the Zener diode conducts large amount of current.
In figure-2 you can see the symbol of Zener diode. It can be seen that it is just like a simple diode except that the bar is changed to z-shape. The following points may be noted about the Zener diode.
- A Zener diode is like a simple diode except it is properly dope so that it has a faster breakdown voltage.
- A Zener diode will always be connect i.e. it will always be reverse bias.
- A Zener diode has a sharp breakdown voltage, called the Zener voltage VZ.
- When forward biased, it has the same characteristics as a simple diode.
- The Zener diode doesn’t burn out just because it has entered the breakdown region. As long as the external circuit connected to the diode limits the diode current to less than the burn out value. Till then the diode will not burn out. In fact, by reducing the reverse voltage below Zener voltage (VZ), the Zener can be brought out of its breakdown level. And can be restore to the pre-breakdown condition.
See also this : Four bit ripple counter – What is 4-bit ripple counter?
What is tunnel diode ?
Tunnel diode is a PN junction which exhibits negative resistance between two values of forward voltage (i.e. between peak point voltage and valley point voltage). A conventional diode exhibits positive resistance when forward biased or reverse biased.
However, if a semiconductor junction diode is heavily dope with impurities, so it exhibits negative resistance in forward direction in some regions (i.e. current decreases as voltage increases). Such diode is called tunnel diode.
Theory :
Tunnel diode is basically a PN junction with heavy doping of p-type and n-type semiconductor material. In fact, a tunnel diode is about 1000 times more doped than a conventional diode.
So this heavy doping results in a large number of majority carriers. Due to the large number of carriers, most are not used during the initial recombination that produces the depletion layer.
As a result, the depletion layer is very narrow. Compared to conventional diodes, the depletion layer of a tunnel diode is 100 times narrower. The operation of tunnel diode depends on the tunneling effect and the name.
What is tunneling effect in tunnel diode ?
A heavy doping provides a large number of majority carriers. Due to the large number of carriers, there is a lot of drift activity in the p and n sections.
This causes many valence electrons to raise their energy levels closer to the conduction region. Therefore, only a very small applied forward voltage is need to generate conduction.
The movement of valence electrons from the valence energy band to the conduction band with little or no applied voltage is call tunneling. The valence electrons appear to tunnel through the forbidden energy band.
As the forward voltage is first increased, the diode current increases rapidly due to the tunneling effect. Soon the tunneling effect wears off and the current starts to flow less as the forward voltage across the diode increases.
As the voltage is increase further, the tunneling effect plays less and less role until the valley-point is reach. From now on, the tunnel diode behaves as a simple diode, i.e. diode current increases with increase in forward voltage.
See also this : What is semiconductor and its types.
What is a photo diode ?
A Photo-diode is a reverse-biased silicon or germanium PN junction in which reverse current increases when the junction is expose to light.
The reverse current in a photo-diode is proportional to the intensity of light falling on its p-n junction. It means that higher the intensity of light falling on the PN junction of the photo-diode, higher will be the reverse current.
What is the basic principle of photodiode?
When a rectifier diode is reverse bias, it has very little reverse leakage current. The reverse current is produced by thermally generated electron-hole pairs being swept across the junction by the electric field created by the reverse voltage.
In a rectifier diode, the reverse current increases with temperature due to the increase in the number of electron-hole pairs. Photo diode differs from a rectifier diode in that when its PN junction is exposed to light. The reverse current increases with increase in light intensity and vice versa.
When light (photon) falls on the PN junction, so energy is impart to the atoms in the junction by the photons. This will create more free electrons and more holes.
These extra free electrons will increase the reverse current. As the light intensity increases across the PN junction, the reverse current also increases.
Photodiode package
See Figure-1 below which shows a typical photo-diode package. It consists of a PN junction mounted on an insulated substrate and sealed inside a metal case.
A glass window is place on the top of the case to allow light to enter and strike the PN junction. The two leads coming out of the case are labeled as anode and cathode.
The cathode is usually identify by a tab extending from the side of the case. See Figure 2. It shows the schematic symbol of a photo-diode. Inward arrows represent incoming light.
See also this : Feedback in electronics
What is light emitting diode explain?
A light-emitting diode (LED) is a diode that emits visible light when forward biased. Light-emitting diodes are not made from silicon or germanium, but using elements such as gallium, phosphorus, and arsenic.
By varying the amounts of these elements, it is possible to produce light of different wavelengths, with the colors red, green, yellow and blue.
For example, when an LED is manufacture using gallium arsenide, it will produce red light. And if the LED is made from gallium phosphide, it will produce a green light.
Theory :
When the light-emitting diode (LED) is forward bias, electrons from the n-type material cross the p-n junction. And recombine with holes in the p-type material. See picture 1 which is below.
Remember that these free electrons are in the conduction band. And are at a higher energy level than the holes in the valence band. When recombination occurs, the recombining electrons release energy in the form of heat and light.
In germanium and silicon diodes, almost the energy is given off as heat and the light emitted is negligible. However, in a material such as gallium arsenide, the number of photons of light energy is sufficient to produce very intense visible light.
See Figure 2 below which shows the schematic symbol for an LED. Arrows are shown pointing away from the diode, indicating that light is being emitted by the device when forward biased.
Although LEDs are available in many colors the most common of which are red, green, yellow and orange. And the schematic symbol is the same for all LEDs. There is nothing in the symbol to indicate the color of a particular LED.
See also this : What is JK flip-flop explain? – Working of JK flip flop
What is varactor diode ?
The junction diode which acts as a variable capacitor under varying reverse bias is know as a varactor diode. When a PN junction is formed, a depletion layer is created in the junction region. Since there are no charge carries within the depletion zone, the zone acts as an insulator.
The p-type material with holes (considered positive) as majority carries and n-type material with electrons (negative charge) as majority carries act as charged plates.
Thus the diode may be consider as a capacitor with N-region and the P-region forming oppositely charged plates and with depletion zone between them. You can see this in Figure 1 above.
A varactor diode is specially construct to have high capacitance under reverse bias. You can see the symbol of varactor diode in figure 2. The capacitance values of varactor diodes are in the picofarad (10-12 F) range.
See also this : Explain push pull amplifier
Theory :
A varactor diode is always reverse bias for normal operation. Thus the capacitance of a varactor diode is found :
Where, CT = Total capacitance of the junction
ε = Permittivity of the semiconductor material
A = Cross-section area of the junction
Wd = Width of the depletion layer
When the reverse voltage across the varactor diode is increase. So the width Wd of the depletion layer increases. Hence, the total junction capacitance CT of the junction decreases.
On the other hand, if the reverse voltage across the diode is reduce. So the width Wd of the depletion layer reduce. As a result, the total junction capacitance CT increases.
Figure-(a) shows the curve between reverse bias voltage VR across varactor diode and total junction capacitance CT. Note that CT can be easily changed by changing the voltage VR. For this region, a varactor diode is sometimes called a voltage-controlled capacitor.
See also this : What is rectifier in electronics