What is Zener diode , Photo diode & Light Emitting diode ?

Zener diode : A properly doped crystal diode which has sharp Break down voltage is known as Zener diode.

Principle. It works on phenomenon of Zener breakdown at reverse voltage, for which large changes in diode current produce only a small change in diode voltage.

Arrangements. It is a junction diode specially made to work only in breakdown region. It is used in a circuit shown in figure

Working. As the input voltage (VI) is increased, reverse diode current (I) increases slowly. Output voltage (Vo) also increases.
Just when VI reaches a certain value, current I increases suddenly as shown in figure , but output voltage (Vo) becomes constant as shown in figure .

This constant output voltage, is called zener voltage (Vz). The current is called Zener current (Iz).

 Theory. The value of zener voltage depends upon circuit resistance R. The relation is Vo = VI − IR

Before breakdown, as VI is increased, I increases by lesser amount. Hence Vo increases. At breakdown and afterwards , I increases by large amount, keeping (VI − IR) constant . It makes Vo constant.

The breakdown phenomenon is reversible and harmless so long as the safe operating temperature is maintained.

Application. The zener diode is used as a constant voltage device. It provides a constant input voltage when input voltage exceeds zener voltage. Thus we can use zener diodes for making constant voltage power supplies.

Optoelectronic Devices

Semiconductor diodes in which carriers are generated by photons (photo excitation) are called optoelectronic devices. Example of optoelectronic devices are, photodiodes, Light Emitting Diodes (LED) and photovoltaic devices etc.

Photodiodes: Photodiodes are used as Photo detector to detect optical signals. They are operated in reverse biased connections. When light of energy greater than the energy gap falls on the depletion region of the diode, electron-hole pairs are generated. Due to the electric field of junction, electrons and holes are separated before they recombine. Electrons reach n-side and holes reach p-side giving rise to an emf. When an external load is connected, current flows. The magnitude of the photocurrent depends on the intensity of incident light.

Light Emitting Diode (LED) : It is heavily doped p-n junction diode which under forward bias emits spontaneous radiation. LEDs that can emit red, yellow, orange, green and blue light are commercially available. These LEDs find extensive use in remote controls, burglar alarm systems, optical communications etc.

Extensive research is being done developing white LEDs which can replace incandescent lamps. LED have the following advantages over conventional incandescent power lamps.
(i) long life
(ii) low operational voltage and less power
(iii) no warm up time is required. So fast on-off switching capability.

Solar Cell : It works on the same principle as the photodiode. It is basically a p-n junction which generates emf when solar radiation falls on the p-n junction. The difference between a photodiode and a solar cell is that no external bias is applied and the junction area is kept much larger for solar radiation to be incident because we require more power. The generation of emf by a solar cell (when light falls on it) is due to the following three processes.

(i) Generation of electron-hole pairs due to light (hf > Eg) falling on it.
(ii) Separation of electrons and holes due to electric field of the depletion region.
(iii) Collection Electrons are swept to n-side and holes to p-side. Thus p-side becomes positive and
n-side becomes negative giving rise to photovoltage.
Solar cells are used to power electronic devices in satellites and space vehicles and also as power supply to some calculators.

Also Read :

→ Energy Levels & Energy Bands
→ Intrinsic semiconductor & Extrinsic Semiconductor
→ P-N Junction & P-N Junction Diode
→ Junction Diode as Rectifier
→ Junction Triode (Transistor)
→ Transistor as Amplifier

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