Semiconductor Devices | Intrinsic Semiconductor | Doping |Methods of Doping |PN type semiconductor|

⭐ Intrinsic Semiconductor
An intrinsic semiconductor is a pure semiconductor in which no impurity atoms are added.
It contains only its own atoms, and therefore the number of free electrons and holes is naturally generated inside it.


It is 100% pure — no doping / no added impurities.

Thermal energy (heat) breaks some covalent bonds and releases free electrons.

When an electron leaves the bond, it creates a hole (a vacant space).

So, in an intrinsic semiconductor:
👉 Number of electrons = Number of holes

Conductivity is very low, because only a few electrons are produced due to heat.

At absolute zero (0 K) temperature —
❗ It behaves like an insulator (no free electrons).

⭐ Doping

✔ Doping increases conductivity of intrinsic semiconductor
✔ Only a small amount of impurity is added
✔ Pentavalent impurity → N-type semiconductor
✔ Trivalent impurity → P-type semiconductor
✔ Impurity atoms are called dopants
✔ Doping does not change electrical neutrality of crystal

⭐ Methods of Doping in Semiconductors
Doping is the process of adding a small amount of impurity to a pure (intrinsic) semiconductor to increase its conductivity.
There are two main methods of doping:

🔹 1. Thermal Diffusion Method

The semiconductor wafer (silicon or germanium) is heated at high temperature.

Doping material (trivalent or pentavalent impurity) is placed near the semiconductor surface.

Due to heat, impurity atoms slowly diffuse into the semiconductor.

Suitable for precision doping in IC fabrication.

🔹 2. Ion Implantation Method

Impurity atoms are converted into ions (electrically charged).

These ions are accelerated at high speed using an electric field.

The high-speed ions are bombarded onto the semiconductor surface and enter deep inside the crystal.

This method gives accurate control over doping concentration and depth.

✔ Doping is carried out by two methods — Thermal Diffusion and Ion Implantation — to improve the conductivity of semiconductors by adding controlled impurities.

⭐ P-Type Semiconductor
A P-type semiconductor is formed when a pure (intrinsic) semiconductor like Silicon or Germanium is doped with a trivalent impurity (3 valence electrons), such as:
● Boron (B)
● Aluminium (Al)
● Gallium (Ga)
● Indium (In)

🔹 The trivalent atom forms three covalent bonds with the semiconductor atoms.
🔹 One bond remains incomplete → creating an electron vacancy / hole.
🔹 These holes act as the majority charge carriers.
🔹 Electrons become the minority charge carriers.

Current conduction in a P-type semiconductor is mainly due to holes (+ charge).

⭐ N-Type Semiconductor

An N-type semiconductor is formed when a pure (intrinsic) semiconductor like Silicon or Germanium is doped with a pentavalent impurity (5 valence electrons), such as:
● Phosphorus (P)
● Arsenic (As)
● Antimony (Sb)
● Bismuth (Bi)

🔹 The pentavalent atom forms four covalent bonds with surrounding atoms.
🔹 The fifth electron remains free → becomes a free electron.
🔹 These free electrons act as the majority charge carriers.
🔹 Holes become the minority charge carriers.

⭐ Formation of Electrons and Holes

In a semiconductor, atoms are arranged closely in a crystal structure. Each atom has valence electrons that take part in bonding.

🔹 At Absolute Zero (0 K)

All valence electrons remain bonded.
electrons are available for conduction.
Therefore, semiconductor behaves like an insulator.

🔹 At Room Temperature (or when heated)

Thermal energy breaks some covalent bonds.

When a bond breaks:
✔ One electron becomes free → moves to the conduction band
✔ The position left behind in the valence band becomes a hole

🔥 Formation Process

Electron gains energy → escapes from bond → becomes free electron

Empty space left → positive vacancy called a hole

⚡ Final Concept

Free Electrons → negatively charged carriers and move freely
Holes → positively charged carriers (absence of electron)
Both electrons and holes contribute to electric current in semiconductors

⭐ Forbidden Energy Gap

In a solid, electrons occupy different energy bands.
Two important bands are:

Valence Band – contains electrons that are tightly bound to atoms
Conduction Band – contains free electrons that can move and conduct electricity
Between these two bands, there is a region where no electron can exist.
This region is called the Forbidden Energy Gap.

🔹 Key Points

It is the energy difference between the top of the valence band and the bottom of the conduction band.
Electrons can never stay inside this gap because no allowed energy state exists there.

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