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@Prajakta Kelapure • 07 Aug, 2014 • 1 like

Two Terminal Devices

1. There are many two terminal devices which has a single P-N junction such as zener diode, varactor diode, schottky diode, tunnel diode etc. Let’s discuss them all.

2. Zener Diode : 

a. Zener diode also known as the breakdown diode is designed to work in the breakdown region in the reverse bias condition.

b. It should be properly doped . this means that the characteristics of the zener are all related to its doping. If it is heavily doped then thin depletion layer and low zener voltage.

c. Silicon is preferref for zener diode over germanium because of higher operating point and current capability.

d. Works in reverse bias.


e. Applications of zener diode:

i. Zener is used for voltage regulation(later discuss in detail)

ii. Meter protection

iii. Zener as a reference element.

3. Tunnel diode: 

a. It’s a high conductivity P-N junction device which is heavily doped about 1000 times that of the conventional diode.

b. Depletion region is very small of the order of 10^-5mm because of which the carriers move through with the speed of light even if they do not have the energy to overcome its potential barrier.

c. Thus a large forward current is produced.

d. Such a mechanism where the charge carriers punch through the barrier directly is called as tunneling.(a route or a tunnel is produced for the flow of carriers). Thus called tunnel diode

e. Only works in forward bias. Also called as Esaki diode


f. Applications: It can be used as an amplifier, oscillator or switch.

4. Varactor diode:

a. Varactor diode is a semiconductor, voltage dependent, variable capacitor. Also called as varicap, VVC, voltcap etc.

b. Its works in reverse bias, i.e its mode of operation is based on transition capacitance.

c. The working is based on the same principle of transition capacitance. The depletion region serves as the dielectric.

d. Application: It is used as a tuning circuit.

@a.alandkar • 08 Aug, 2014 • 1 like Hello Prajakta, Its really nice that you are sharing basic electronics concepts. But if possible add some more explaination like (in this blog you may add how z-diode works i.e. the concept of zener breakdown), which would be really helpful to the beginners.
@Santhosh M • 08 Aug, 2014 Informative...👍
@Prajakta Kelapure • 08 Aug, 2014
Hello Prajakta, Its really nice that you are sharing basic electronics concepts. But if possible add some more explaination like (in this blog you may add how z-diode works i.e. the concept of zener breakdown), which would be really helpful to the beginners.
Yes i am trying to include as much as stuff I can. I appreciate you for letting me know a few more topics.
@Kavya Burramolla • 01 Feb, 2020 • 2 likes

Thanks @Prajakta Kelapure for sharing informative content.

Adding some more 2 terminal semiconductor devices with PN junction : -

1. Light Emitting diode(LED)

2. Laser diode(LD)

3. Photo Diode(PD)

LED diode - As the name suggests LED is a two lead(Anode and Cathode) semiconductor device that when forward biased emits the light. It is made with the special doping characteristics, and works with the basic principle of Quantum theory. 

The Current and voltage characteristics of LED diode are given below: VBuc-1580536336516.jpgApplications of LED : -

we can see the LED applications in most of our daily life - in traffic signals, used in projects to indicate different scenarios, street lighting, truck lighting, highways, indicators etc.,

2. Laser diode : - It is abbreviated as LASER is an acronym for Light Amplification by the Stimulated Emission of Radiation. Laser diode similar to LED is used to emit the light but the difference here is that laser diode consists only single frequency. Therefore, it can be focused by even a simple lens system to an extremely small point. 

The main diffrence between LED and LASER lies in the maximum level of emission of light which is high for LASER diode can be observed in the below diagram


Applications of LASER diode :-

In addition to the LED applications LASER diodes are used in scientific and instrumentation applications where keen focus and emission is required, Industrial cutting etc.,

3. Photo Diode : - LED & LASER is a device that when electrical charged converts it into light energy. On the other hand Photo diode consumes light energy to produce electrical energy. Photo diodes are designed to be operated in thereverse bias mode, that is when the Photo diode is reverse biased then the diode junction converts the photn energy and produces the electrical energy.

Applications of Photo diode : - Photo diodes are mostly used in medical applications instruments to analyze samples, detectors for computed tomography and also used in blood gas monitors.

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