On the basis of electrical resistivity ρ, objects are classified as metals, semiconductors and insulators, with metals being the most conductive or least resistant, and insulators being the least conductive or the most resistant. Metals have resistivity in the range of ρ ~ 10–2 – 10–8 Ωm. Semiconductors have resistivity in the range of ρ ~ 10–5 – 106 Ωm. Insulators have resistivity in the range of ρ ~ 1011 – 1019 Ωm.

Intrinsic semiconductors are chemically pure and contain almost zero impurities. As it has no impurities, the material of the semiconductor determines the number of holes and electrons. In intrinsic semiconductors, the number of free electrons ne is equal to the number of holes nh, i.e. ne = nh = ni. Silicon and germanium are two examples of intrinsic semiconductors. At temperature 0K, an intrinsic semiconductor acts like an insulator. At temperatures higher than 0K, the thermal energy excites some electrons, moving them from the valence band to the conduction band. These thermally excited electrons partially take up space in the conduction band.

Extrinsic semiconductors have specific impurities added to it, in a process known as doping, to modify the electrical properties of the semiconductor. Adding impurities increases the conductivity of the semiconductor. The material used as an impurity is called dopant. The dopant added to the semiconductor should have properties such that the original lattice of the pure semiconductor remains unaltered. Moreover, the size of the dopant atoms should be almost equal to the size of the semiconductor atoms.

Silicon and germanium belong to the fourth group in the Periodic table. Therefore, the dopant element should be taken from the fifth or third group to ensure the size of the dopant atom is almost the same as Si or Ge. This gives rise to two kinds of extrinsic semiconductors: n-type and p-type. An n-type semiconductor has a higher concentration of electrons than holes. A p-type semiconductor has a higher concentration of holes than electrons.

A p-n junction is the basis of many semiconductor devices. A p-n junction is a junction between the p-type and the n-type inside a semiconductor. In a semiconductor, a p-n junction is created by doping. Understanding the behaviour of the p-n junction is important to analyse the functioning of semiconductor devices. This section of the NCERT Solutions for Class 12 Chapter 14 Physics covers how a junction is formed and how the junction behaves when an external voltage is applied to the circuit.

a. Formation of P-N Junction

During the formation of p-n junction, holes diffuse from the p-side to the n-side (p → n) and electrons diffuse from the n-side to the p-side (n → p). This motion leads to a diffusion current across the junction. When an electron diffuses from n → p, an ionised donor on the n-side is left behind. This ionised donor or positive charge cannot move as it is attached to the surrounding atoms. As the electrons diffuse from n → p, the n-side of the junction develops a region of positive charge. Similarly, when a hole diffuses from p → n, an immobile ionised acceptor or negative charge is left behind.

As the holes continue to diffuse, the p-side of the junction develops a region of negative charge. This space-charge region on either side of the junction is called depletion region. The combination of the positive space-charge region on the n-side of the junction and the negative space-charge region on the p-side of the junction gives rise to an electric field. The electric field makes the electrons on the p-side travel to the n-side, and the holes on the n-side travel to the p-side. The movement of charge carriers caused by the electric field is known as drift.

A drift current is in the opposite direction of the diffusion current. Initially, the diffusion current is large and the drift current is small. Eventually, the space-charge regions on either side of the junction increase. The electric field strength also increases, and, in turn, the drift current increases. This process goes on until the diffusion current is equal to the drift current, thus forming a p-n junction.

Being a two terminal device, a semiconductor diode is a p-n junction that has metallic contacts at its ends so that an external voltage can be applied. This section of the NCERT Solutions for Class 12 Physics Chapter 14 covers what happens when a p-n junction diode is under forward and reverse biases.

a. P-N Junction Diode Under Forward Bias

A semiconductor diode is in forward bias when the p-side of the diode is connected to the positive terminal of the battery, the n-side to the negative terminal, and voltage is applied across the diode. Most of the applied voltage drops across the depletion region while the voltage drop across the p-side and n-side of the junction is negligible.

The direction of the applied voltage (V) is opposite to the built-in potential V0, causing a decrease in the depletion layer width and the barrier height. The barrier height under forward bias is given by (V0–V).

The applied voltage causes electrons from the n-side to cross the depletion region and reach the p-side, and holes from the p-side to reach the n-side. The concentration of minority carriers is higher at the junction boundary than at locations away from the junction. As a result, the electrons on p-side diffuse from the junction edge of p-side to the other end of p-side, and the holes on n-side diffuse from the junction edge of n-side to the other end of n-side. This motion of charged carriers generates current.

b. P-N Junction Diode Under Reverse Bias

A semiconductor diode in reverse bias is when the n-side is positive, the p-side is negative, and a voltage is applied across it. The applied voltage drops across the depletion region. The applied voltage and the barrier potential are in the range direction. This increases the barrier height and the change in electric friend widens the depletion region.

The effective barrier height in the case of reverse bias is (V0+V). This suppresses the flow of electrons from n → p and holes from p → n. Therefore, in the case of reverse bias, the diffusion current decreases enormously, unlike in the case of forward bias.

The electric field direction of the junction is such that if in their random motion the electrons on p-side or the holes on n-side move close to the junction, they will be swept to its majority zone. This carrier drift generates current. The diode reverse current does not depend on the applied voltage. Even a small voltage can sweep the minority carriers from one side of the junction to the other.

The V-I characteristics graph of a junction diode shows that current flows only when the diode is in forward bias. This means that if an ac voltage is applied across a diode, the current flows until the cycle finishes forward bias. This behaviour is used to rectify ac voltages by a rectifying circuit or a rectifier.

If an ac voltage is applied across a diode connected in series with a load, a pulsating voltage is seen across the load for only the half cycles of forward bias. A rectifying circuit in which the rectified output is only for half of the input ac wave is known as a half-wave rectifier.

A rectifying circuit using two diodes gives output voltage for both the positive and the negative halves of the ac cycle. This type of a rectifying circuit is known as a full-wave rectifier.

This section explores junction diodes that are manufactured for various applications.

a. Zener Diode

A Zener diode is a highly doped semiconductor that allows current to flow from the anode to the cathode, and has the ability to change the direction of the current once the Zener voltage is reached. In a forward-biased mode, a Zener diode behaves like a normal diode. In reverse-biased mode, there is a small amount of current that leaks through the Zener diode. As the reverse voltage increases to the breakdown voltage, the current starts flowing through the diode. The current increases to a maximum, and the potential reaches the Zener Voltage also called knee voltage, causing the junction to break down and the current to flow in the opposite direction. This is known as the Zener Effect. There are two types of Zener diode breakdowns: Avalanche Breakdown, and Zener Breakdown.

b. Optoelectronic Junction Devices

Optoelectronic devices are semiconductor diodes whose carriers are generated by the excitation of photons. Below are some examples of optoelectronic devices:

• Photodiodes are used for detecting optical signal.
• Light emitting diodes (LED) convert electrical energy into light.
• Photovoltaic cells or solar cells convert optical radiation into electricity.

a. Transistor Structure and Action

A transistor is made up of three doped regions forging two p-n junctions between them.

The two types of transistors are:

(i) n-p-n transistor: In this type of transistor, a segment of a p-type semiconductor (base) divides two segments of n-type semiconductor (emitter and collector).

(ii) p-n-p transistor: In this type of transistor, a segment of n-type (base) divides two segments of p-type semiconductor (emitter and collector).

The three segments of a transistor have varying levels of thickness and doping.

• Emitter: This segment is of moderate size and heavily doped, and is responsible for the supply of majority carriers required for the flow of current in a transistor.
• Base: This segment is the middle segment, and is very thin and lightly doped.
• Collector: This segment collects the majority carriers supplied by the emitter. The collector segment is moderately doped and bigger than the emitter.

b. Basic Transistor Circuit Configuration and Characteristics

A transistor has only three terminals: Emitter (E), Base (B) and Collector (C). The input and output connections in a transistor circuit are such that at least one of these terminals is common to both the input and the output. The transistor can be connected in any of the three configurations, i.e. Common Emitter (CE), Common Base (CB), Common Collector (CC).

c. Transistor as a Device

The configuration of a transistor, the biasing of the E-B and B-C junction, and the operation region are used to design a transistor in such a way that it can be used as a device. A transistor operating in the cutoff or saturation state behaves as a switch. A transistor operating in the active region is used as an amplifier.

d. Transistor as an Amplifier (CE Configuration)

For a transistor to be used as an amplifier, its operating point should be somewhere in the middle of its active region. If the value of VBB is fixed in relation to a point in the middle of the linear part of the transfer curve, then the dc base current IB and the collector current IC will be constant. The dc voltage VCE = VCC - ICRC also remains constant. The operating values of VCE and IB determine the operating point of the amplifier.

e. Feedback Amplifier and Transistor Oscillator

In an amplifier, a sinusoidal signal is given at the input which is then amplified at the output. An external input is required to deliver the ac signal in the output. Whereas for an oscillator, the ac output is obtained without any external input, meaning that the output for an oscillator is self-sustained. This is done using an amplifier. Part of the output power is sent back as feedback to the input in phase with the initial power. This process is called positive feedback. The feedback is obtained using inductive coupling networks. Different oscillators use different combinations of coupling the output to the input.

In circuits like amplifiers and oscillators, the signal is in the form of continuous voltage or current varying over time. These signals are analogue signals represented using binary numbers, which have only two digits 0 and 1. The signals obtained after converting analogue signals to binary form (for example, 0V=0, and 5V=1) are called digital signals. Logic Gates are used to translate the digital signals. They are used in household and industrial electronic appliances, and in telecommunications.

a. Logic Gates

A gate is a digital circuit that represents the logical relationship between the input and output voltages. They are used to control information flow. The five common logic gates used are listed below. Each logic gate has a specific function, whose truth table gives the combinations of possible output.

i. NOT: The NOT gate produces a binary output of ‘1’ when the input is ‘0’ and vice-versa. It can take only one input and give one output.

ii. OR: The OR gate takes two or more inputs and gives one output. The output is 1 when at least one of the input signals is 1.

iii. AND: The AND gate takes two or more inputs and gives one output. The output is 1 only when both the inputs are 1.

iv. NAND: The NAND gate is an AND gate followed by a NOT gate. The output is 0 if both the inputs are 1.

v. NOR: The NOR gate is an OR gate followed by a NOT gate. The output is 1 if both the inputs are 0.

Previously, components of a circuit such as resistors, inductors, capacitors, diodes and transistors used to be connected in the circuit by soldering wires in the required manner. Even with the miniaturisation provided by transistors, such circuits used to be bulky. In addition, these circuits used to be less reliable and less shock-proof. Thus, integrated circuits were created to integrate the massive components of a circuit onto a small semiconductor chip. This has made circuits more compact and reliable, revolutionising technology and enabling many technological advancements.

1. What is an impure or extrinsic semiconductor? What is the difference between extrinsic and intrinsic semiconductors?

Extrinsic semiconductors are doped with specific impurities to modify its electrical properties, making it more suitable for electronic devices. Intrinsic semiconductors are chemically pure, which means that they contain almost zero impurities.

2. What are the benefits of using NCERT Solutions for Class 12 Physics Chapter 14?

The NCERT Solutions for Class 12 Physics Chapter 14 are extremely beneficial to students as all the important topics and problems are provided in it. The questions follow the CBSE board pattern and are, therefore, a reliable source to study from. The chapter is explained in simple language, ensuring that students face minimal barriers while preparing for the exams.

3. What are the types of semiconductors?

There are two types of semiconductors: intrinsic and extrinsic.