Revision: Electronic Devices >> Semiconductor Electronics Physics (Theory) ISC (Science) ISC Class 12 CISCE

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.

A pure semiconductor in which no impurity is added intentionally.

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

The impurity added is called a dopant.

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 basic semiconductor device that controls the flow of electric current in a circuit, which when forward biased behaves as a closed circuit and when reverse biased behaves as an open circuit, is called a p-n Junction Diode.

• The conversion of AC voltage into a DC voltage is called Rectification.
• The process of converting an alternating current into a direct current is called rectification.

• The electronic circuit which rectifies AC voltage is called a Rectifier.
• The device used to convert an alternating current into a direct current is called a rectifier. 

A unique form of a bipolar device which permits the current flow in the reverse direction when the voltage applied is above a certain characteristic value called Zener voltage or breakdown voltage, most commonly used in voltage regulators to protect other semiconductor devices from fluctuations in voltage, is called a Zener Diode.

A special purpose junction diode that converts light energy into electrical current, works on the principle of the photoelectric effect, operates in reverse bias, and generates a current when exposed to light (proportional to the intensity of incident light), is called a Photodiode.

It is a semiconductor device used to convert photons of solar light into electricity. It generates emf when solar radiation falls on the p-n junction. A p-type silicon wafer of about 300 μm is taken over which a thin layer of n-type silicon is grown on one side by the diffusion process.

An energy band is the wide range of energies possessed by an electron in a solid.

A material with a small energy gap (about 1 eV) between valence and conduction bands, allowing limited conduction at room temperature.

The highest occupied energy band containing valence electrons.

OR

Valence band is the wide range of energies possessed by the valence electrons. Valence band is the highest energy band, occupied by the valence electrons. It is completely filled for inert gases, but partially filled for other materials. 

A material having a partially filled valence band (or overlapping valence and conduction bands), allowing electrons to move easily and conduct electricity.

An empty or partially filled band above the valence band in which electrons can move freely and conduct current.

OR

Conduction band is the wide range of energies possessed by the conduction band electrons. It is the lowest unfilled band, for insulators. But it is partially filled for conductors. Current conduction is due to the electrons in this band. 

A material in which the valence band is completely filled and the conduction band is empty, separated by a large energy gap (a few eV), so electrons cannot move freely.

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.

• A rectifier is a circuit which converts an AC supply into a unidirectional DC supply.
• A p-n junction diode acts as a rectifier because it allows current to flow in one direction only.
• The bridge rectifier circuit uses semiconductor diodes for converting AC, as it allows current to flow in one direction only.
• Input to the rectifier is AC \[(V_{IN})\]; output is DC \[(V_{OUT})\] — shown as a full-wave rectified signal.
• Rectification is the fundamental principle behind power supply circuits in electronic devices.

• Conductors have many free electrons, so electric current flows easily; insulators have almost no free electrons, so current does not flow easily.
• In conductors, resistance increases with temperature, whereas in semiconductors it decreases.
• Semiconductors have properties between conductors and insulators, and at absolute zero, they behave like insulators.

• An intrinsic semiconductor is a pure semiconductor (like silicon or germanium) without impurities.
• At low temperatures, it behaves like an insulator because all electrons are bound in covalent bonds.
• At room temperature, some bonds break and create electron–hole pairs.
• In an intrinsic semiconductor, the number of electrons equals the number of holes (ne = nh = ni).
• Its conductivity increases with temperature because more electron–hole pairs are produced.

• Semiconductors have a small energy gap (about 1 eV) between the valence band and conduction band.
• At absolute zero, they act like insulators because the valence band is full and the conduction band is empty.
• At room temperature, some electrons move into the conduction band, leaving holes, and both contribute to conduction; conductivity increases with temperature.