Revision: Electronic Devices >> Semiconductor Electronics - Materials, Devices and Simple Circuits Physics Science (English Medium) Class 12 CBSE

A pure semiconductor in which no impurity is added intentionally.

The vacancy left in the valence band when an electron leaves it behaves like a positive charge carrier in semiconductor theory.

The concentration of charge carriers in an intrinsic semiconductor, where the number of electrons equals the number of holes.

The semiconductor to which the dopant is added is called the host.

Intrinsic semiconductors have very low conductivity at room temperature. Therefore, they are not useful for constructing electronic devices. Their electrical conductivity can be increased by adding a suitable impurity. This process is called doping.

A doped semiconductor is called an extrinsic semiconductor or impurity semiconductor.

The impurity added is called a dopant.

Since every pentavalent dopant atom donates one electron for conduction, it is called a donor impurity.

A p-type semiconductor is a semiconductor obtained by doping pure silicon or germanium with a trivalent impurity so that holes become the majority charge carriers. 

The process in which charge carriers move from a higher concentration to a lower concentration is called diffusion.

The motion of charge carriers under the influence of an electric field is called drift.

The minimum forward voltage after which the current rises sharply is called the knee voltage.

The small current flowing in reverse bias due to minority carriers is called the reverse saturation current.

The reverse voltage at which the current suddenly increases rapidly is called the breakdown voltage.

The ratio of voltage to current at any operating point of a diode is called static resistance.

The ratio of a small change in voltage to the corresponding small change in current is called dynamic resistance.

The boundary formed when p-type and n-type semiconductor regions are joined in a single crystal is called a p-n junction.

The potential difference developed across the depletion layer due to immobile ions is called the barrier potential.

The region around the junction that is free from mobile charge carriers is called the depletion region.

The two-terminal semiconductor device that allows current mainly in one direction is called a p-n junction diode.

A semiconductor diode's depletion zone is the area surrounding the p-n junction where there are no mobile charge carriers, this area generates an electric field that allows the diode to conduct in one direction while blocking in another.

The barrier that the repelling forces use to stop the mobile charge carriers (at the PN junction) is known as the potential barrier.

This results from the concentration of immobile charges close to the junction after electrons and holes diffuse across the function.

(i) Rectifier: It is a device which converts alternating current into direct current.

(ii) Amplifier: An amplifier is a device which increases the energy of a weak signal by supplying energy from an external source. An amplifier increases the amplitude of a input signal.

(iii) Oscillator: An oscillator is a device which produces electrical oscillations of adjustable frequency and constant amplitude. An oscillator is basically an amplifier. A part of the output energy is fed back into the L-C circuit to produce sustained oscillations.

A semiconductor diode is a two-terminal p-n junction device that allows current to pass easily in one direction and offers high resistance in the opposite direction.

A rectifier is an electronic circuit that converts alternating current (AC) into direct current (DC) by using one or more p-n junction diodes.

A half-wave rectifier allows only one half of the AC cycle to pass through the load, producing a pulsating DC output.

A full-wave rectifier uses both half cycles of the AC input to produce a more continuous pulsating DC output.

A filter is a circuit element or arrangement, often involving a capacitor or inductor, used to smooth the pulsating rectified output.

Pulsating DC is an output that flows in one direction but whose magnitude changes with time.

If hole concentration is high, then:

This is especially important for numerical problems in board examinations and entrance tests. 

R = \[\frac {V}{I}\]

\[r_d=\frac{\Delta V}{\Delta I}\]

Formation of Energy Bands

In a single isolated atom, electrons occupy discrete, well-defined energy levels (like rungs of a ladder).

When atoms are packed together in a crystal lattice, electrons are shared among neighbouring atoms. This causes:

• Each discrete energy level splits into closely spaced levels
• These closely spaced levels collectively form an energy band

Analogy: Think of a single tuning fork producing one frequency. When thousands of tuning forks are placed close together, they create a continuous range (band) of frequencies, not a single note.

Key Energy Bands

• Electronic circuits are built using devices that allow controlled flow of electrons.
• Before 1948, vacuum tubes or valves were commonly used.
• Electrons in vacuum tubes come from a heated cathode and move in the vacuum.
• Vacuum tube devices are bulky, high-power, high-voltage, with limited lifetime and low reliability.
• Semiconductor devices work within the solid itself.
• Semiconductor devices do not require external heating or a large evacuated space.
• Semiconductor devices are small, low-power, low-voltage, long-life, and highly reliable.

• Intrinsic semiconductors have very low conductivity at room temperature.
• Doping increases conductivity.
• A doped semiconductor is called an extrinsic semiconductor.
• The impurity added is called a dopant.
• The semiconductor receiving the impurity is called the host.
• The dopant size should be nearly the same as that of the host atom.
• Pentavalent and trivalent impurities are used as dopants.
• Extrinsic semiconductors are of two types: n-type and p-type.

• An n-type semiconductor is formed by doping silicon or germanium with a pentavalent impurity.
• Pentavalent impurities act as donor impurities.
• The fifth valence electron is weakly bound and can become free easily.
• Electrons are majority carriers and holes are minority carriers.
• For an n-type semiconductor, n_e >> n_h.
• Donor energy levels lie close to the conduction band.
• Extrinsic semiconductors are better conductors than intrinsic semiconductors.

• A p-n junction is formed by joining p-type and n-type semiconductor regions in a single crystal.
• Diffusion of carriers creates a depletion region and barrier potential.
• A p-n junction diode conducts mainly in one direction.
• In forward bias, the barrier potential decreases, and the current becomes large.
• In reverse bias, the barrier potential increases and only a small minority-carrier current flows.
• In zero bias, the diffusion and drift currents balance, so the net current is zero.
• The knee voltage is about 0.3 V for germanium and 0.7 V for silicon.
• Static resistance is given by R = V/I, and dynamic resistance is given by r_d = ΔV/ΔI.