What is special purpose diode

In today’s article what is special purpose diode, we are going to discuss special purpose diode in which we are going to discuss zener diode, zener Diode as a voltage regulator, photodiode, light emitting diode, solar cell etc. Along with that we will also discuss junction Transistor, emitter, base, collector, Transistor characteristics, input characteristics, output characteristics, Transistor parameters, Transistor as an amplifier (CE- configuration), Transistor as a switch, Transistor as an oscillator, logic gate etc in Detail.

Zener diode

It is heavily doped p-n junction and it is operated in reverse bias.

  1. Zener diode remains constant, even through current through the zener diode varies over a wide range.

  1. Zener diode as a voltage regulators: Any increase (decrease) in the input voltage results in, increase (decrease) of voltage drop across Rs without any change in voltage across the zener diode.

Photodiode

It is a special type of p-n junction diode fabricated with a transparent window to allow light to fall on the diode. It is operated under reverse bias.

  1.  When it is illuminated with light of photon energy greater than the energy gap of the semiconductor, electron-hole pairs are generated in near depletion region, hence reverse current increases.
  2. The symbol of a photodiode is shown in figure.

Light emitting diode

It is a heavily doped p-n junction which emits spontaneous radiation under forward bias.

1.It converts electrical energy into light energy.

2. The symbol of a LED is shown in the figure.

3. The I-V characteristics of a LED is similar to that of Si junction diode. But the threshold voltages are much higher than slightly different for each colour. The reverse breakdown voltages of LEDs are very low, typically around 5V.

4. The semiconductor used for fabrication of visible LEDs must have minimum band gap of 1.8 eV.

5. The compound semiconductor gallium arsenide phosphide (GaAsP) is used for making LEDs of different colours.

6. GaAs is used for making Infrared LED.

Solar cell

It is basically a p-n junction which generates emf when solar radiation falls on the p-n junction.

  1. It converts solar energy into electrical energy.
  2. It works on the same principle (photovoltaic effect) as the photodiode, except that no external bias is applied and the junction area is kept large.

Junction Transistor

  1. A semiconductor device which is obtained by growing either a very thin layer of n-type (p-type) crystal between two much thicker p-type (n-type) layers.
  2. It is a current operated device, i.e., action of Transistor is controlled by the motion of charge carriers,i.e, current.
    • Emitter: Supply a large number of majority charge carriers for the flow of current through the transistor. It is heavily doped and medium in size.
    • Base: Control the flow of majority charge carriers from emitter to collector. It is lightly doped and very thin in size.
    • Collector: Collects a major portion of the majority carriers supplied by emitters for the circuit operation. It is moderately doped and has large size.
    • Action: In general (active region), emitter base is forward biased and base collector is reverse biased.

Iemitter = Ibase + Icollector, (Ic >> Ibase)

Transistor characteristics

n-p-n transistor as CE mode .

1. Input characteristics

Input resistance, ri = (Δ Vbe/ Δ Ib) Vce

2. Output characteristics

Output resistance, ro = (ΔVce / ΔIc) Ib

Transistor parameters: α and β

  • Current amplification factor, βac = (ΔIc/Δ Ib)Vce and βdc = Ic/Ib; αdc = Ic/Ie
  • Relation between α and β, β = α/1-α and α= β/1+ β

Transistor as an amplifier (CE- configuration): To use Transistor as an amplifier it is being operated somewhere in the middle of its active region.

1. Voltage gain, Av = Vo/Vi = Ic Rc/ Ib Rb = β Rc/ Rb

2. Power gain = current gain× voltage gain

= βAv = β(β Rc/ Rb) = β² Rc/Rb

Transistor as a switch

Operated in cut off region or saturation region. Vcc = IcRc + Vce

Vce = Vcc – IcRc

When Vi= 0 or < 0.7 V, Ib =0 Hence Ic = 0.

∴ Vce = Vcc (open circuit (switch)) when Vi> 0.7 V, then this is similar to a closed circuit (switch).

Transistor as an oscillator

An oscillator works on positive feedback.

1. Voltage gain with feedback Aƒ = A/ 1- Aβ

A→ open voltage gain

β→ feedback factor

2. Barkhausen criteria for sustained oscillation, Aβ = 1

Logic Gates

  1. A logic gate is a digital circuit which is based on certain logical relationship between the input and the output voltage of the circuit.
  2. The operation of a logic gate is indicated in a tabel, known as truth tabel. This tabel contains all possible combination of inputs and corresponding outputs.

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