CBSE Class 12 Physics Revision Notes Chapter 14: Semiconductor Electronics

Semiconductors have electrical conductivity between that of conductors and insulators, and their conductivity can be controlled using temperature, light, voltage or impurities. A p-n junction diode allows current mainly in one direction and can convert alternating voltage into rectified voltage.

Semiconductor Electronics: Materials, Devices and Simple Circuits explains the basic materials used in modern electronic devices. It covers energy bands, charge carriers, intrinsic and doped semiconductors, p-n junction formation and diode applications.

Use these CBSE Class 12 Physics Revision Notes Chapter 14 for quick revision. Begin with the classification of solids and energy bands. Then revise intrinsic and extrinsic semiconductors, p-n junction behaviour, diode characteristics and rectifier circuits.

Key Takeaways

• Energy gap: Conductors have overlapping bands, insulators have a large gap and semiconductors have a small gap.
• Intrinsic semiconductor: The number of free electrons equals the number of holes.
• Extrinsic semiconductor: Doping produces n-type or p-type material with one dominant carrier.
• Diode action: A p-n junction conducts strongly in forward bias and only weakly in reverse bias.

Access Class 12 Physics Chapter 14 Semiconductor Electronics Notes in 30 Minutes

Divide the chapter into three revision blocks:

• First 10 minutes: Energy bands, conductors, insulators and intrinsic semiconductors
• Next 10 minutes: Doping, n-type and p-type semiconductors, majority and minority carriers
• Final 10 minutes: p-n junction, diode characteristics, half-wave and full-wave rectifiers

While revising, do not treat a hole as a real positive particle inside the crystal. It is an effective description of a missing valence electron.

Need help revising semiconductor bands, diode graphs and rectifier circuits?
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Semiconductor Electronics in Class 12 Physics Chapter 14 Notes

Electronic circuits require devices in which the flow of charge can be controlled.

Before semiconductor devices were developed, electronic systems used vacuum tubes such as:

• Vacuum diodes
• Triodes
• Tetrodes
• Pentodes

Vacuum tubes required:

• A heated cathode
• An evacuated space
• High operating voltage
• Considerable electrical power

They were also bulky and had limited reliability.

Semiconductor devices improved electronic circuits because they:

• Are small in size
• Consume less power
• Operate at lower voltages
• Do not require a heated cathode
• Do not require a vacuum
• Have longer life
• Offer greater reliability

In semiconductor devices, charge carriers move within the solid material itself.

Classification of Conductors, Insulators and Semiconductors

Solids may be classified using their electrical conductivity or resistivity.

Since:

ρ = 1/σ

where:

• ρ is resistivity.
• σ is conductivity.

A substance with high conductivity has low resistivity.

Metals

Metals have very low resistivity and high conductivity.

Their approximate ranges are:

ρ ≈ 10⁻² to 10⁻⁸ Ω m

σ ≈ 10² to 10⁸ S m⁻¹

Metals contain a large number of mobile electrons.

Semiconductors

Semiconductors have resistivity and conductivity between those of metals and insulators.

Their approximate ranges are:

ρ ≈ 10⁻⁵ to 10⁶ Ω m

σ ≈ 10⁵ to 10⁻⁶ S m⁻¹

Examples include:

• Silicon
• Germanium
• Gallium arsenide
• Cadmium sulphide

Insulators

Insulators have high resistivity and low conductivity.

Their approximate ranges are:

ρ ≈ 10¹¹ to 10¹⁹ Ω m

σ ≈ 10⁻¹¹ to 10⁻¹⁹ S m⁻¹

They have very few mobile charge carriers under ordinary conditions.

Conductors, Semiconductors and Insulators Comparison

Types of Semiconductor Materials in Chapter 14 Physics Notes

Semiconductors may be elemental, compound or organic.

Elemental Semiconductors

Elemental semiconductors consist of a single element.

The most important examples are:

• Silicon
• Germanium

Both belong to Group 14 and have four valence electrons.

Compound Semiconductors

Compound semiconductors contain more than one element.

Examples include:

• Gallium arsenide
• Cadmium sulphide
• Cadmium selenide
• Indium phosphide

Organic Semiconductors

Some organic compounds and polymers also show semiconductor behaviour.

Examples include:

• Anthracene
• Polyaniline
• Polypyrrole
• Polythiophene

Most basic semiconductor devices are studied using silicon and germanium.

Energy Bands in Solids in Semiconductor Electronics Revision Notes

In an isolated atom, electrons occupy separate energy levels.

When a large number of atoms come together to form a solid:

• Their outer electron orbits come close.
• Interactions between neighbouring atoms alter the energy levels.
• A large number of closely spaced energy levels form energy bands.

The two important bands are:

• Valence band
• Conduction band

Valence Band

The valence band contains energy levels occupied by valence electrons.

At absolute zero, the valence band of an intrinsic semiconductor is completely filled.

Electrons in a completely filled band cannot easily contribute to electrical conduction.

Conduction Band

The conduction band lies above the valence band.

Electrons in the conduction band can move through the solid and contribute to current.

At absolute zero, the conduction band of an intrinsic semiconductor is empty.

Forbidden Energy Gap

The gap between the top of the valence band and the bottom of the conduction band is called the:

• Forbidden energy gap
• Band gap
• Energy gap

It is represented by:

Eg

No allowed electron energy states exist within this gap.

Energy-Band Classification of Materials

The electrical behaviour of a solid depends strongly on its energy-band arrangement.

Energy Bands in Conductors

In a conductor:

• The conduction band may be partially filled.
• The valence and conduction bands may overlap.
• Electrons can move into nearby available energy states easily.
• Electrical conductivity is high.

The effective energy gap is nearly zero.

Energy Bands in Insulators

In an insulator:

• The valence band is full.
• The conduction band is empty.
• The forbidden gap is large.
• Ordinary thermal energy cannot lift electrons across the gap.

For an insulator:

Eg > 3 eV

Energy Bands in Semiconductors

In a semiconductor:

• The valence band is almost full.
• The conduction band is almost empty.
• The energy gap is finite but small.
• Some electrons can cross the gap at room temperature.

For a semiconductor:

Eg < 3 eV

Typical values are:

• Germanium: approximately 0.7 eV
• Silicon: approximately 1.1 eV
• Diamond: approximately 5.4 eV

Energy-Gap Comparison

Carbon in the diamond form is an insulator because its energy gap is much larger than that of silicon and germanium.

Intrinsic Semiconductor in Class 12 Physics Chapter 14 Notes

A pure semiconductor without intentionally added impurities is called an intrinsic semiconductor.

Silicon and germanium are common intrinsic semiconductors.

Covalent Bonding

Each silicon or germanium atom has four valence electrons.

In the crystal:

• Every atom has four nearest neighbours.
• Each atom shares one electron with each neighbour.
• Four covalent bonds are formed.

At very low temperature, almost all valence electrons remain involved in covalent bonds.

Therefore, very few free charge carriers are available.

Intrinsic Semiconductor at Absolute Zero

At:

T = 0 K

an intrinsic semiconductor behaves like an insulator.

At absolute zero:

• All valence electrons remain bound.
• The valence band is full.
• The conduction band is empty.
• No free electrons or holes are available.

Electron-Hole Pair in Semiconductor Electronics Notes

At a temperature above absolute zero, thermal energy may break some covalent bonds.

When a bond breaks:

• One electron becomes free.
• The electron enters the conduction band.
• A vacancy is left in the valence bond.

This vacancy is called a hole.

The free electron and corresponding hole form an electron-hole pair.

Nature of a Hole

A hole:

• Represents the absence of a valence electron.
• Behaves as if it has a positive charge.
• Is not an independent material particle like a proton.
• Can appear to move through the crystal.

When a neighbouring valence electron fills a hole, it leaves another hole behind. The hole therefore appears to move in the direction opposite to electron movement.

Electron and Hole Current in Intrinsic Semiconductor Notes

When an electric field is applied:

• Conduction electrons drift opposite to the electric field.
• Holes appear to drift along the electric field.

The electron current is represented by:

Ie

The hole current is represented by:

Ih

The total current is:

I = Ie + Ih

Both electrons and holes contribute to semiconductor conduction.

Carrier Concentration

In an intrinsic semiconductor:

ne = nh = ni

Here:

• ne is electron concentration.
• nh is hole concentration.
• ni is intrinsic carrier concentration.

Every thermally generated free electron leaves one hole. Therefore, their numbers remain equal.

Generation and Recombination of Charge Carriers

Electron-hole pairs are continuously generated by thermal energy.

At the same time, free electrons may fall into holes.

This process is called recombination.

During recombination:

• A free electron fills a hole.
• Both mobile carriers disappear.
• A covalent bond is restored.
• Energy is released.

At thermal equilibrium:

Rate of generation = Rate of recombination

As temperature increases:

• More covalent bonds break.
• More electron-hole pairs form.
• Carrier concentration increases.
• Conductivity increases.
• Resistance decreases.

This is why semiconductors have a negative temperature coefficient of resistance.

Extrinsic Semiconductor in Class 12 Semiconductor Electronics Notes

An intrinsic semiconductor has relatively low conductivity at room temperature.

Its conductivity can be increased by adding a very small amount of a suitable impurity.

The resulting material is called an:

• Extrinsic semiconductor
• Impurity semiconductor
• Doped semiconductor

The process of adding an impurity is called doping.

The impurity atom is called a dopant.

The dopant concentration may be only a few parts per million.

Conditions for a Suitable Dopant

A suitable dopant should:

• Have an atomic size close to that of the host atom.
• Fit into the crystal lattice.
• Not seriously distort the crystal structure.
• Provide or accept charge carriers.

For silicon and germanium, common dopants are:

• Pentavalent elements
• Trivalent elements

N-Type Semiconductor in Physics Chapter 14 Revision Notes

An n-type semiconductor is formed by doping silicon or germanium with a pentavalent impurity.

Pentavalent dopants include:

• Phosphorus
• Arsenic
• Antimony
• Bismuth

Formation of N-Type Material

A pentavalent atom has five valence electrons.

When it replaces a silicon atom:

• Four electrons form covalent bonds.
• The fifth electron remains weakly bound.
• A small amount of energy frees this electron.
• The free electron enters the conduction band.

The energy required to free this fifth electron is much smaller than the normal band gap.

For example:

• About 0.01 eV in germanium
• About 0.05 eV in silicon

Donor Impurity

The pentavalent dopant supplies an extra electron.

It is therefore called a donor impurity.

After donating the electron, the impurity atom becomes a fixed positive ion.

The complete crystal remains electrically neutral.

Majority and Minority Carriers in N-Type Semiconductor

In n-type material:

• Electrons are majority carriers.
• Holes are minority carriers.

Therefore:

ne >> nh

The letter n refers to the dominance of negative charge carriers, not to a net negative charge of the crystal.

P-Type Semiconductor in Class 12 Physics Chapter 14 Notes

A p-type semiconductor is formed by doping silicon or germanium with a trivalent impurity.

Trivalent dopants include:

• Boron
• Aluminium
• Indium
• Gallium

Formation of P-Type Material

A trivalent atom has three valence electrons.

When it replaces a silicon atom:

• It forms only three complete covalent bonds.
• One neighbouring bond lacks an electron.
• A hole is produced.

A nearby valence electron may move into this hole, leaving a new hole at its previous position.

Acceptor Impurity

The trivalent dopant accepts an electron from a neighbouring bond.

It is therefore called an acceptor impurity.

After accepting the electron, the impurity becomes a fixed negative ion.

The crystal still remains electrically neutral.

Majority and Minority Carriers in P-Type Semiconductor

In p-type material:

• Holes are majority carriers.
• Electrons are minority carriers.

Therefore:

nh >> ne

The letter p refers to the dominance of positive holes.

N-Type and P-Type Semiconductor Comparison

Carrier-Concentration Relation in Extrinsic Semiconductors

At thermal equilibrium:

ne × nh = ni²

This relation applies to intrinsic and extrinsic semiconductors.

For an n-type semiconductor:

• ne is large.
• nh becomes small.

For a p-type semiconductor:

• nh is large.
• ne becomes small.

Doping increases one type of carrier while reducing the equilibrium concentration of the other through recombination.

Donor and Acceptor Energy Levels

Doping introduces additional energy levels within the forbidden gap.

Donor Level in N-Type Material

The donor energy level ED lies slightly below the conduction-band edge EC.

A very small amount of energy can move a donor electron into the conduction band.

Acceptor Level in P-Type Material

The acceptor energy level EA lies slightly above the valence-band edge EV.

A valence electron can easily move into this acceptor level, leaving a hole in the valence band.

P-N Junction Formation in CBSE Class 12 Physics Chapter 14 Notes

A p-n junction is formed when p-type and n-type regions are created within a single semiconductor crystal.

It is not made by simply pressing separate p-type and n-type slabs together.

A mechanical contact would contain surface irregularities much larger than atomic spacing and would not create a continuous crystal junction.

Carrier Concentration Difference

Before the junction forms:

• The p-region contains many holes.
• The n-region contains many electrons.

Because of this concentration difference:

• Holes diffuse from p-side to n-side.
• Electrons diffuse from n-side to p-side.

This movement produces diffusion current.

Depletion Region in P-N Junction Notes

When electrons diffuse from n-side to p-side:

• They leave behind fixed positive donor ions.

When holes diffuse from p-side to n-side:

• They leave behind fixed negative acceptor ions.

These immobile ions form a charged region around the junction.

This region is called the depletion region because it is depleted of mobile electrons and holes.

Its thickness is approximately of the order of one-tenth of a micrometre.

Space-Charge Region

The depletion region contains:

• Positive fixed ions on the n-side
• Negative fixed ions on the p-side

These ions create an electric field directed from the n-side towards the p-side.

Diffusion and Drift Current in P-N Junction

Two processes occur during junction formation.

Diffusion Current

Diffusion current is produced by the movement of majority carriers due to a concentration gradient.

• Electrons diffuse from n to p.
• Holes diffuse from p to n.

Drift Current

The electric field of the depletion region moves minority carriers in the opposite direction.

• Electrons on the p-side are swept towards the n-side.
• Holes on the n-side are swept towards the p-side.

This produces drift current.

Initially:

• Diffusion current is large.
• Drift current is small.

As the depletion region grows:

• The electric field increases.
• Drift current increases.
• Diffusion becomes more difficult.

At equilibrium:

Diffusion current = Drift current

Therefore, the net current across an unbiased junction is zero.

Barrier Potential in P-N Junction Revision Notes

The fixed charges in the depletion region produce a potential difference across the junction.

This is called the:

• Junction potential
• Barrier potential
• Built-in potential

The barrier opposes further diffusion of majority carriers.

The n-side is at a higher potential than the p-side because:

• The n-side contains fixed positive donor ions.
• The p-side contains fixed negative acceptor ions.

The exact barrier potential depends on:

• Semiconductor material
• Doping concentration
• Temperature

Semiconductor Diode in Chapter 14 Physics Notes

A semiconductor diode is a p-n junction provided with metallic contacts at both ends.

It is a two-terminal device.

Its terminals are:

• P-side terminal
• N-side terminal

The diode allows current easily in one direction but offers very high resistance in the opposite direction.

This one-way behaviour is used in rectifier circuits.

P-N Junction Diode Under Forward Bias

A diode is forward biased when:

• The p-side is connected to the positive battery terminal.
• The n-side is connected to the negative battery terminal.

Effect on the Junction

In forward bias:

• The applied field opposes the built-in field.
• Barrier potential decreases.
• Depletion-region width decreases.
• Majority carriers move towards the junction.
• A large current may flow.

The effective barrier becomes approximately:

V0 − V

where:

• V0 is the built-in barrier.
• V is the applied forward voltage.

Minority-Carrier Injection

Under forward bias:

• Electrons cross from n-side into p-side.
• Holes cross from p-side into n-side.

After crossing:

• Electrons become minority carriers in the p-region.
• Holes become minority carriers in the n-region.

This process is called minority-carrier injection.

The forward current is mainly a diffusion current.

Its magnitude is usually measured in milliamperes.

P-N Junction Diode Under Reverse Bias

A diode is reverse biased when:

• The p-side is connected to the negative battery terminal.
• The n-side is connected to the positive battery terminal.

Effect on the Junction

In reverse bias:

• The applied field supports the built-in field.
• Barrier potential increases.
• Depletion-region width increases.
• Majority-carrier diffusion is strongly suppressed.

The effective barrier becomes approximately:

V0 + V

Reverse Current

A very small current still flows because of thermally generated minority carriers.

This current is called:

• Reverse current
• Reverse saturation current

It is generally measured in microamperes.

The reverse current remains almost constant over a wide range of reverse voltage because it is limited mainly by minority-carrier concentration.

Forward and Reverse Bias Comparison

V–I Characteristics of P-N Junction Diode

The V–I characteristic shows how diode current changes with applied voltage.

Forward-Bias Characteristic

At small forward voltage:

• Current remains very small.
• The barrier is only slightly reduced.

After a particular voltage:

• Current rises rapidly.
• A small voltage increase produces a large current increase.

This voltage is called:

• Threshold voltage
• Cut-in voltage
• Knee voltage

Approximate threshold values are:

• Germanium diode: 0.2 V
• Silicon diode: 0.7 V

Reverse-Bias Characteristic

In reverse bias:

• Current is extremely small.
• It remains almost constant initially.
• It is called reverse saturation current.

At a sufficiently large reverse voltage:

• Current increases sharply.
• The junction enters breakdown.

The corresponding voltage is called the breakdown voltage.

Breakdown Voltage in Semiconductor Diode Notes

The reverse current remains small until the reverse voltage reaches a critical value.

This value is the breakdown voltage, represented by:

Vbr

At breakdown:

• Reverse current increases suddenly.
• A small voltage change may produce a very large current.
• Excess current may overheat the junction.
• An ordinary diode may be destroyed.

A current-limiting resistance is required whenever a diode is operated near breakdown.

General-purpose diodes are normally not used in the breakdown region.

Dynamic Resistance of a Diode

The diode current-voltage relation is not linear.

Therefore, its resistance at a point is described using dynamic resistance.

Dynamic resistance is:

rd = ΔV/ΔI

Here:

• ΔV is a small change in diode voltage.
• ΔI is the corresponding change in diode current.

In forward bias, dynamic resistance is relatively low.

In reverse bias before breakdown, resistance is extremely high.

Junction Diode as a Rectifier in Class 12 Physics Chapter 14 Notes

A rectifier converts alternating voltage or current into a unidirectional output.

A p-n junction diode is useful for rectification because:

• It conducts during forward bias.
• It blocks current during reverse bias.

When an alternating voltage is applied, the diode conducts only during the half-cycle in which it is forward biased.

The output is therefore unidirectional but usually pulsating.

Half-Wave Rectifier in Semiconductor Electronics Notes

A half-wave rectifier uses one diode.

Its circuit generally contains:

• An alternating-voltage source
• A transformer
• One diode
• A load resistance RL

Working During Positive Half-Cycle

Suppose terminal A of the transformer secondary becomes positive.

Then:

• The diode is forward biased.
• Current flows through the diode.
• Current flows through the load.
• Output voltage appears across RL.

Working During Negative Half-Cycle

When terminal A becomes negative:

• The diode is reverse biased.
• Current is almost zero.
• No significant output appears across the load.

Thus, only one half of the alternating input appears in the output.

Half-Wave Output

The output:

• Is unidirectional
• Is pulsating
• Uses only one half of each input cycle
• Has the same frequency as the input

For an input frequency of 50 Hz:

Half-wave output frequency = 50 Hz

Full-Wave Rectifier in Class 12 Semiconductor Revision Notes

A full-wave rectifier produces output during both halves of the alternating-input cycle.

The centre-tapped full-wave rectifier uses:

• A centre-tapped transformer
• Two diodes
• A load resistance

The two ends of the secondary winding provide voltages that are out of phase.

Positive Half-Cycle

During one half-cycle:

• One end of the transformer is positive.
• Diode D1 becomes forward biased.
• Diode D2 becomes reverse biased.
• Current flows through the load in one direction.

Negative Half-Cycle

During the next half-cycle:

• The other end becomes positive.
• Diode D2 becomes forward biased.
• Diode D1 becomes reverse biased.
• Current again flows through the load in the same direction.

Therefore, both halves of the input are used.

Full-Wave Output

The output:

• Is unidirectional
• Contains pulses in every half-cycle
• Is more efficient than half-wave rectification
• Has twice the input frequency

For an input frequency of 50 Hz:

Full-wave output frequency = 100 Hz

Half-Wave and Full-Wave Rectifier Comparison

A bridge rectifier can also provide full-wave rectification using four diodes without a centre-tapped transformer.

Capacitor Filter in Rectifier Circuits

The output of a rectifier is pulsating rather than perfectly steady.

A filter is used to reduce the alternating ripple.

A capacitor may be connected parallel to the load resistance.

Charging of the Capacitor

When the rectified voltage rises:

• The capacitor charges.
• Its voltage approaches the peak output voltage.

Discharging of the Capacitor

When the rectified voltage falls:

• The diode may stop conducting.
• The capacitor discharges slowly through the load.
• It maintains the output between successive peaks.

During the next rising pulse:

• The capacitor charges again.

The resulting output is smoother than the unfiltered rectified signal.

Time Constant

The discharge rate depends on:

RL C

This product is called the time constant.

A larger value of RL C gives:

• Slower capacitor discharge
• Smaller ripple
• Smoother output voltage

Large capacitors are therefore commonly used in power-supply filters.

Rectification and Filtering Process

The complete conversion occurs in stages:

1. An alternating input is applied.
2. The diode or diodes rectify the signal.
3. A pulsating unidirectional output is produced.
4. A capacitor reduces the ripple.
5. A smoother direct voltage is obtained.

The filtered voltage is not perfectly constant, but it is much closer to direct voltage than the raw rectifier output.

Semiconductor Electronics Formula Notes

Important Semiconductor Values

Important Terms in Class 12 Physics Chapter 14

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