What are intrinsic and extrinsic semiconductors?

An intrinsic semiconductor is a pure semiconductor material with no added impurities. Its conductivity depends on its temperature. An extrinsic semiconductor is a doped semiconductor, where impurities have been added to increase either electron or hole concentration.

What is the depletion region?

The depletion region is a region near the p-n junction where mobile charge carriers (electrons and holes) have been depleted, leaving behind immobile ions. This creates a potential barrier.

What are energy bands in solids?

Energy bands represent the allowed energy levels that electrons can occupy in a solid. The gaps between these bands are called band gaps.

Explain the Zener breakdown mechanism.

The Zener breakdown occurs in heavily doped p-n junctions under a strong reverse bias. The high electric field across the junction pulls electrons from the covalent bonds, creating a large number of charge carriers and allowing current to flow.

Why the I-V characteristics of a solar cell are plotted in the fourth quadrant.

The solar cell does not require current for its operation; instead, it supplies current to the load. Since the generated EMF is positive while the current is negative, the graph appears in the fourth quadrant. This is why the I-V characteristics of a solar cell are plotted in the fourth quadrant.

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JEE Physics

Electronic Devices

Electronic devices are powered by components like semiconductors, diodes, transistor, and integrated circuits. For instance, diodes function as rectifiers, transistors amplify signals, LEDs emit light, solar cells convert sunlight into energy, and logic gates process digital signals. These components are vital for driving the technology we use daily, including smartphones and computers.

1.0Classification Of Materials (Solids)

On the basis of Conductivity & Resistivity

Conductors

Insulators

Semiconductors

Abundance of free e−

Very few free e–

Few free e−

High conductivity

$(σ \sim 1 0^{2} - 1 0^{8} S m^{- 1})$

Low conductivity

$(σ \sim 1 0^{- 11} - 1 0^{- 19} S m^{- 1})$

Intermediate conductivity

$(σ \sim 1 0^{5} - 1 0^{- 6} S m^{- 1})$

Low resistivity

$(ρ \sim 1 0^{- 2} - 1 0^{- 8} Ω m)$

High resistivity

$(ρ \sim 1 0^{11} - 1 0^{19} Ω m)$

Intermediate resistivity

$(ρ \sim 1 0^{- 5} - 1 0^{6} Ω m)$

Effect of temperature

$(T ↑ R ↑ ρ ↑ σ ↓)$

(At very high temperature)

$(T ↑ R ↓ ρ ↓ σ ↑)$

(At room temperature)

$(T ↑ R ↓ ρ ↓ σ ↑)$

$α = + v e$

$α = - v e$

$(α = T e m p . C oe ff i c i e n t)$

E.g. : Metals

E.g.: Rubber, Wood, Plastic,Diamonds etc.

E.g. : Si, Ge, GaAs, CdS, anthracene,

Polypyrole, Polyaniline etc

2.0Energy Band Theory

1. Valence Band: This is a lower energy band, which contains valence electrons.This band is either partially or completely filled with electrons but never be empty.The electrons in this band are not capable of taking part in conduction of current.

2. Conduction Band: This is the higher band containing conduction electrons.It is either empty or partially filled with electrons.Electrons present in this band take part in the conduction of current.This band is completely empty. Electrons are forbidden to be in this energy gap.

3.Band Gap or Forbidden Energy GapEg:The minimum energy required to shift an electron from valence band to conduction band is called band gap.It depends on the nature of solid and on the interatomic separation. It also depends on temperature, but this dependence is very weak.Width of forbidden energy gap is represented in eV.As temperature increases forbidden energy gap decreases (very slightly).

Classification of solids According to Energy Band Theory

Conductor	Insulator	Semiconductor
Overlap of conduction and valence bands, high conduction	No conduction, large energy gap	Small band gap, some thermal excitation to conduction band
$Δ E g = 0$	$Δ E g > 3 e V$	$Δ E g < 3 e V$
Example:Gold, Silver, Copper	Example:Rubber, Glass	Example:Silicon, Germanium

3.0Types of Semiconductor

Intrinsic Semiconductor	N-type (Pentavalent impurity)	P-type(Trivalent impurity)
Current is due to both electrons and holes	Mainly due to electrons	Mainly due to holes
$n_{e} = n_{h} = n_{i}$	$n_{e} >> n_{h} (n_{D} ≅ n_{e})$	$n_{h} >> n_{e} (n_{A} ≅ n_{h})$
$I = I_{e} + I_{h}$	$I ⋍ I_{e}$	$I ⋍ I_{h}$
Entirely neutral	Entirely neutral	Entirely neutral
Quantity of electrons and holes are equal	Majority – Electrons Minority – Holes	Majority – Holes Minority - Electrons

4.0Mass Action Law

At room temperature, most acceptor atoms become ionized, creating holes in the valence band. Therefore, in an extrinsic semiconductor, the density of holes in the valence band is primarily determined by the impurities.
“Rate of generation of charge carriers is equal to rate of recombination of charge carriers.”

$n_{e} . n_{h} = n_{i}^{2}$

Note: ni depends only on the nature of semiconductor material and temperature, it does not depend on the doping level.
Semiconductor Diode(P-N Diode)& Diode Formation

When a P-type semiconductor is joined to a N-type semiconductor such that the crystal structure remains continuous at the boundary, the resulting arrangement is called a P-N junction diode.

Feature	Diffusion	Drift
Charge Carriers	Majority Carriers	Minority Carriers
Cause	Concentration Gradient	Electric Field in Depletion Region
Direction (Carriers)	Holes: p-side to n-side Electrons: n-side to p-side	Electrons: p-side to n-side Holes: n-side to p-side
Direction (Current)	p-side to n-side	n-side to p-side
Effect on Electric Field	Increases electric field strength	Influenced by electric field strength

Note:p-n junction under equilibrium there is no net current $.∣ I_{Dr i f t} ∣ = ∣ I_{D i ff u s i o n} ∣$

Depletion Width

Width of Depletion region ≈ 0.1μm
It depends on temperature, doping and type of material $Wi d t h α \frac{1}{Do p in g}$

Barrier Potential

The barrier potential is the electric field created across the p-n junction, with the n-side having a higher potential than the p-side. This potential difference creates a barrier that resists the movement of majority carriers but allows minority carriers to pass. It depends on temperature, doping, and semiconductor material.

Also Read : Transistor As Amplifier

5.0Biasing A Diode and I-V Characteristics

I-V Characteristics

Forward Biasing	Reverse Bias
p-side connected to positive terminal, n-side to negative	p-side connected to negative terminal, n-side to positive
Depletion layer width decreases, barrier height reduced	Depletion layer width increases, barrier height increase
Current mainly due to diffusion (mA)	Current mainly due to minority carriers (μA)
Current is almost zero until knee voltage (VTh), then increases exponentially	Current is very small and constant, limited by minority carriers

Reverse Breakdown

At high reverse voltage ( $V = V_{BR}$ )
Sharp increase in current
Large increase in reverse current
Current increases sharply with slight increase in reverse voltage

Diode As a Rectifier

Rectifier:It is a device which is used for converting alternating current into direct current. A diode can be used as a rectifier as it is a uni-directional device.

Half Wave Rectifier:

Positive Half of Input Signal:

$S_{1}$ is positive, $S_{2}$ is negative.
$D_{1}$ is forward biased, $D_{2}$ is reverse biased.
Only $D_{1}$ conducts, current flows through $R_{L}$ from A to B.

Negative Half of Input Signal:

$S_{1}$ is negative, $S_{2}$ is positive.
$D_{1}$ is reverse biased, $D_{2}$ is forward biased.
Only $D_{2}$ conducts, current flows through $R_{L}$
from A to B.

Full-wave Rectifier:

Positive Half of Input Signal:

$S_{1}$ is positive, $S_{2}$ is negative.
$D_{1}$ is forward biased, $D_{2}$ is reverse biased.
$D_{1}$ conducts, current flows from A to B through $R_{L}$

Negative Half of Input Signal:

$S_{1}$ is negative, $S_{2}$ is positive.
$D_{1}$ is reverse biased, $D_{2}$ is forward biased.
$D_{2}$ conducts, current flows from A to B through $R_{L}$
In both positive and negative halves, current always flows from A to B, making the output a full-wave rectified signal.

Zener Diode And Used as a Voltage Regulator

Zener Diode:Invented by C. Zener, heavily doped P-N junction.Operates mainly in reverse bias breakdown region.

6.0I-V Characteristics

Handles large current variations without changing Zener voltage, useful in regulation.
An increase in reverse bias voltage generates a strong electric field in the depletion region, causing a breakdown.

In reverse biasing, breakdown can occur in two ways:

1. Avalanche Breakdown :It occurs in p-n junctions having low doping. Breakdown voltage is very high. Depletion layer width is also more. Charge carriers crossing the depletion region gain enough kinetic energy and make collisions with other atoms, and hence it starts the avalanche effect. Damage to the diode is permanent.

2. Zener Breakdown:Occurs in a heavily doped p-n junction. Breakdown voltage is very small. The depletion layer is very thin.Breaking of covalent bonds is mainly due to the electric field. Hence damage is not permanent.

Zener Diode as Voltage Regulator:In the breakdown region, the Zener voltage stays constant even as current change.Changes in input voltage cause corresponding changes in current through the series resistor (RS) and Zener diode, but the voltage across the load remains constant.

Input Current, $I_{S} = \frac{V _{in} - V _{Z}}{R _{S}}$

Zener current, $I_{Z} = I_{S} - I_{L}$

Power Dissipation,

$P_{Z} = V_{Z} \times I_{Z}$

Light Emitting Diode(L.E.D)

An LED (Light Emitting Diode) converts electrical current into light. It's a heavily doped p-n junction operated under forward bias. When forward biased, electrons move from N → P and holes from P → N, recombining at the junction and releasing energy as photons.
Light Intensity: Small forward current = low light intensity, large current = max intensity, then intensity decreases with further increase.
Material: LED semiconductors have a band gap of 1.8 eV to 3 eV (e.g., GaAs₁₋ₓPₓ).

Photodiode

A photo-diode converts light into electrical current or voltage. It’s a moderately doped p-n junction operated under reverse bias with a transparent outer material to allow photons to enter.
As light intensity increases, the photocurrent increases.Example: Used in video cameras to detect light intensity.

Solar Cell

A photovoltaic cell converts light energy into electrical energy, similar to a photo-diode but operates without bias. The thin p-region allows photons to easily reach the junction.
Generation: Light generates electron-hole pairs at the junction $(h > E_{g})$
Separation: The electric field of the depletion region separates the electrons (toward n-side) and holes (toward p-side).
Collection: This creates a photovoltage, with the n-side becoming negative and the p-side positive.

Logic Gates

It is a digital circuit that processes input signals to produce output based on logical operations, using semiconductors like diodes and transistors.

1. OR Gate:

Boolean expression Y=A+B

2. And Gate:

Boolean expression Y=A.B

3. NOT Gate (Inverter)

Boolean expression $Y = \overline{A}$

Universal Gates

1. NAND Gate

Boolean expression $Y = \overline{A . B}$

2. NOR Gate

Boolean expression $Y = \overline{A + B}$

Also Read : Exclusive OR Gate