Test: Applications of P-N Diode - NEET MCQ

Knee voltage- it is that forward voltage beyond which current start increasing rapidly, but below knee voltage variation of forward current and applied voltage is linear.
The electric field in a region is given by, E=V/l.
where V is the potential and l is the length or distance of the region in which it has to be measured.
now, E=V/l
E=0.8/2×10^-6 m
E= 0.4 MV/m
 

A rectifier is a device which converts a.c (alternating current) to d.c( direct current), which flows in only one direction. This process is known as Rectification.
This is done using a p-n junction diode. 
A p-n junction diode allows electric current in only forward bias condition and blocks electric current in reverse bias conditions.
 In other words, a diode allows electric current in one direction. This unique property of diode allows it to act like a Rectifier.
 

In a half wave rectifier, the secondary coil S of the transformer is connected to​ both junction diode and a load resistance.

In full wave rectification the input frequency is doubled. Because all the negative components in the AC input signal are converted into positive components. Hence, the positive components are doubled.
Therefore in full wave rectification output frequency will be 100 Hz i.e. double that of input frequency 50 Hz.
 

Photoconductivity, as a well-known optical and electrical phenomenon in semiconductor, is an effect that the electrical conductivity increases due to the absorption of light radiation.

Zener Diodes can be used to produce a stabilised voltage output with low ripple under varying load current conditions. By passing a small current through the diode from a voltage source, via a suitable current limiting resistor (RS), the zener diode will conduct sufficient current to maintain a voltage drop of V_out.

In physics and in electronic engineering, dark current is the relatively small electric current that flows through photosensitive devices such as a photomultiplier tube, photodiode, or charge-coupled device even when no photons are entering the device; it consists of the charges generated in the detector when no outside radiation is entering the detector.

It is referred to as reverse bias leakage current in non-optical devices and is present in all diodes. Physically, dark current is due to the random generation of electrons and holes within the depletion region of the device.

Efficiency of half-wave and full wave rectifier is given by
η_h​=40.6% and η_f​=81.2%
So,  η_h/η_f​​​=40.6/81.2 ​=1/2​
∴f/n ​=2η_h​
Hence, the efficiency of a full wave rectifier is double of half wave-rectifier.

This energy is emitted in the form of heat and light. The electrons dissipate energy in the form of heat for silicon and germanium diodes but in gallium arsenide phosphide (GaAsP) and gallium phosphide (GaP) semiconductors, the electrons dissipate energy by emitting photons.

A diode as a rectifier is used for converting AC to DC.
Following are the applications of the diode as a rectifier:
It is used for the mixing of signals.
It is used for the detection of signals.
Used in lighting systems.
When a diode is used as a rectifier, it acts as a two-lead semiconductor which allows current to pass from only one direction.

Drift current:
Drift current is when electrons and holes respond to an applied electric field.  Holes move in the direction of the electric field while electrons move opposite the electric field.  This occurs as long as there are carriers available. 
Diffusion current:
Diffusion current is when holes and electrons move from areas of high concentration, where they are the majority carrier, to areas of low concentration, where they become minority carriers.  This occurs until they are uniformly distributed throughout the semiconductor. 

• When the diode is forward biased drift current is present, but because diffusion current grows exponentially, it dominates.
• The forward-biased diode current is mostly made up of majority carrier diffusion. 

Here is the table to summarise:

Reverse Saturation Current:

• The reverse saturation current (I₀) of a p-n junction diode is highly temperature dependent and follows an exponential relation with temperature.
• For every 10°C increase in temperature, the reverse saturation current doubles, which is a well-known characteristic of diodes.

• Mathematically, if the reverse saturation current is I₀₁ at temperature T₁ and I₀₂ at T₂, then:

  I₀₂ = I₀₁ 2^{(T₂ - T₁)/10}

  
   

The dynamic resistance can be defined from the I-V characteristic of a diode in forward bias. It is defined as the ratio of a small change to voltage to a small change in current, i.e.

V_T = Thermal voltage
I = Bias current

∴ The dynamic resistance of the diode is directly proportional to the temperature.
The dynamic resistance is given by the inverse of the slope of i-v characteristics as shown: 

A PN-junction diode is formed when a p-type semiconductor is fused to an n-type semiconductor material creating a potential barrier voltage across the diode junction.

• In forward bias, the resistance of a PN junction is of the order of Ω, approximately 100 Ω
• Reverse bias PN junction has a resistance of the order of MΩ 

Concept:

• The dynamic resistance can be defined from the I-V characteristic of a diode in forward biased
• It is defined as the ratio of small change to voltage to a small change in current,
  
• It is the inverse of the slope of I-V characteristics curve

The dynamic resistance is given by the inverse of the slope of i-v characteristics

Calculation:
Given that, current (I) = 2 mA
We know that voltage (V_T) = 25 mV
Dynamic resistance 

The electric field in a PN junction without an external voltage is known as the barrier field. This field arises due to the following:

• The diffusion of electrons and holes across the junction, which leaves behind charged ions.
• These ions create an electric field that opposes further charge carrier movement.
• This field is crucial for maintaining the equilibrium state of the junction.

Explanation:

• In forward biasing, the applied voltage V of the battery mostly drops across the depletion region and the voltage drops across the p-side and n-side of the p-n junction is negligibly small.
• It is due to the fact that the resistance of the depletion region is very high as it has no free charge carriers.
• In forward biasing the forward voltage opposes the potential barrier Vb. As a result of it, the potential barrier height is reduced and the width of the depletion layer decreases. Therefore option (2) is the incorrect Statement.
• As forward voltage is increased, at a particular value the depletion region becomes very much narrow such that a large number of majority charge carriers can cross the junction.

A PN junction diode is typically made using an extrinsic semiconductor. It operates as a unidirectional switch. Here's why:

• Extrinsic semiconductors: These are semiconductors that have been doped with impurities to improve conductivity.
• Unidirectional switch: The diode allows current to flow in one direction only, which is why it's called unidirectional.

A P-N diode can experience breakdown due to several factors:

• Thermal instability: Excessive heat can cause the diode material to change, leading to failure.
• Tunneling effect: At high electric fields, electrons can tunnel through the energy barrier, causing breakdown.
• Avalanche multiplication: High-energy carriers collide with atoms, creating more carriers and resulting in a chain reaction.

All these factors can cause a diode to break down.

Applying voltage rule in loop ABCD.
E - IR - V_z = 0
20 - IR - 8 = 0
2.5 × 10^-3 × R = 12

Drift velocity, V_d = I/nAe
(v_d)_electron/(v_d)_hole = (I_e/I_h)(n_h/n_e) = (7/4) x (5/7) = 5/4 i.e., 5 : 4

J = neV_d
Resistivity, ρ = E/j = E/neV_d = 1/ne(v_d/E) = 1/neμ_e
Resistivity, 1/(10¹⁹ x 1.6 x 10^-19 x 1.6) = 0.39 Ω m = 0.4 Ω m

Band gap = Energy of photon of = 2480 nm
Energy = (h_c/λ) J = (h_c/λ_e) eV
Band gap = ([(6.63 x 10^-34) x (3 x 10⁸)]/[(2480 x 10^-9) x (1.6 x 10^-19)]) = 0.5 eV