#semiconductors

#semiconductors #class12 #intrinsic semiconductor #extrinsic semiconductor
What is a Semiconductor?
A semiconductor is a material whose electrical conductivity lies between conductors and insulators. Its conductivity increases with temperature and can be controlled by adding impurities.
👉 Common examples: Silicon (Si) and Germanium (Ge)

1. Intrinsic Semiconductor
Definition
An intrinsic semiconductor is a pure semiconductor with no intentional impurities added.
Charge Carriers


Electrons (in conduction band)


Holes (in valence band)


Number of electrons = number of holes


Key Features


Very low conductivity at room temperature


Conductivity increases when temperature increases


Conduction occurs due to thermally generated electron–hole pairs


Energy Band Explanation


Small energy gap (~1 eV)


At higher temperature, electrons jump from valence band → conduction band


Leaves behind holes


Examples


Pure Silicon


Pure Germanium


📌 Important point:

In intrinsic semiconductors, electrons and holes are equally responsible for conduction.


2. Extrinsic Semiconductor
Definition
An extrinsic semiconductor is formed by adding a small amount of impurity to a pure semiconductor to increase its conductivity.
This process is called doping.
Types of Extrinsic Semiconductors


n-type semiconductor


p-type semiconductor



3. n-Type Semiconductor
Doping


Doped with pentavalent (Group V) impurities


Examples: Phosphorus (P), Arsenic (As), Antimony (Sb)


How It Works


Pentavalent atoms have 5 valence electrons


4 electrons form covalent bonds with silicon


1 extra electron becomes free


Charge Carriers


Majority carriers: Electrons


Minority carriers: Holes


Energy Band Diagram


Donor energy level lies just below the conduction band


Very little energy is required to free electrons


Key Characteristics


High conductivity due to free electrons


Electrical conduction mainly by electrons


Electrically neutral overall


📌 Memory tip:

n-type → negative charge carriers (electrons)


4. p-Type Semiconductor
Doping


Doped with trivalent (Group III) impurities


Examples: Boron (B), Aluminium (Al), Gallium (Ga)


How It Works
Trivalent atoms have 3 valence electrons
One bond remains incomplete → hole is created
Charge Carriers
Majority carriers: Holes
Minority carriers: Electrons
Energy Band Diagram
Acceptor energy level lies just above the valence band

Electrons easily move from valence band to fill holes


Key Characteristics


High conductivity due to holes


Electrical conduction mainly by holes


Electrically neutral overall


📌 Memory tip:

p-type → positive charge carriers (holes)


Comparison Table
PropertyIntrinsicn-Typep-TypePurityPureImpureImpureDopingNoPentavalentTrivalentMajority carriersElectrons & HolesElectronsHolesConductivityLowHighHighFermi levelMiddle of band gapNear conduction bandNear valence band

Conclusion


Intrinsic semiconductors are pure and have limited conductivity


Extrinsic semiconductors are doped to enhance conductivity


n-type: electrons dominate conduction


p-type: holes dominate conduction


These concepts form the foundation of diodes, transistors, ICs, and all modern electronics ⚡