Semiconductor Fundamentals

Semiconductor Fundamentals

Almost every electronic device around us, such as a mobile phone, a computer, a television, an LED bulb or a battery charger, works with the help of semiconductor materials. In this note we will learn what a semiconductor is, how it conducts current, and why adding a small amount of impurity changes its behaviour.

This is an introductory note for Unit I. Semiconductor materials are studied in more detail in the unit on Conducting and Semiconducting Materials.

1. What Is a Semiconductor?

Definition: A semiconductor is a material whose electrical conductivity is more than that of an insulator but less than that of a conductor. Its conductivity can be changed by temperature, light and by adding a small amount of impurity.

Think of a water tap. A conductor is like a tap that is fully open, an insulator is like a tap that is fully closed, and a semiconductor is like a tap that can be opened a little or more as we need. (This is only an analogy.)

Examples: Silicon and germanium.

In terms of energy bands, a semiconductor has a small forbidden energy gap (about 1 eV). At very low temperature its valence band is almost full and its conduction band is almost empty.

2. Characteristics of Semiconductors

1. Their conductivity is between that of conductors and insulators.
2. They have 4 valence electrons. Their atoms are joined to each other by covalent bonds in a crystal.
3. At absolute zero temperature (0 K) a pure semiconductor behaves like an insulator.
4. Their resistance decreases when the temperature increases. They have a negative temperature coefficient of resistance.
5. Their conductivity increases greatly when a small amount of impurity is added. This is called doping.
6. Their conductivity increases when light falls on them.
7. They have two types of charge carriers: free electrons and holes.

3. Commonly Used Semiconductor Materials

Silicon (Si) and germanium (Ge) are the two most commonly used semiconductor materials.

Point	Silicon	Germanium
Symbol	Si	Ge
Atomic number	14	32
Valence electrons	4	4
Forbidden energy gap	About 1.1 eV	About 0.67 eV
Working temperature	Can work at higher temperature	Works only at lower temperature
Leakage current	Less	More
Availability	Very easily available (obtained from sand) and cheap	Rare and costly
Use	Most widely used	Used less now

Important point: Silicon is the most widely used semiconductor material in modern electronic devices.

4. How a Pure Semiconductor Conducts

In a pure silicon crystal, each atom has 4 valence electrons. It shares one electron with each of its 4 neighbouring atoms. A shared pair of electrons is called a covalent bond. In this way every atom has a stable arrangement of 8 electrons around it.

Fig. 1: Pure (intrinsic) silicon crystal with covalent bonds

Diagram in words: The figure shows nine silicon atoms in three rows of three. Each atom is joined to its neighbours by lines with two dots on them. The two dots are the pair of shared electrons, that is, one covalent bond.

• At 0 K all the bonds are complete. There are no free electrons, so the crystal behaves like an insulator.
• At room temperature, heat energy breaks a few bonds. Each broken bond gives one free electron and leaves behind an empty place called a hole. This pair is called an electron-hole pair.
• A hole is the empty place left by a missing electron in a covalent bond. It behaves like a positive charge.

5. Intrinsic Semiconductor

Definition: A pure semiconductor, with no impurity added, is called an intrinsic semiconductor.

• Examples: pure silicon and pure germanium.
• The number of free electrons is equal to the number of holes.
• Its conductivity is low and depends mainly on temperature.
• It is rarely used directly to make devices.

6. Doping and Extrinsic Semiconductor

Doping: The process of adding a small amount of a suitable impurity to a pure semiconductor to increase its conductivity is called doping. The impurity that is added is called a dopant.

Extrinsic semiconductor: A semiconductor to which an impurity has been added is called an extrinsic semiconductor. Extrinsic semiconductors are of two types:

1. N-type semiconductor, made by adding a pentavalent impurity (5 valence electrons).
2. P-type semiconductor, made by adding a trivalent impurity (3 valence electrons).

7. N-Type Semiconductor

A pentavalent impurity such as phosphorus, arsenic or antimony is added to pure silicon. The impurity atom has 5 valence electrons. Four of them form covalent bonds with four neighbouring silicon atoms. The fifth electron is not needed in any bond, so it becomes a free electron.

Fig. 2: N-type semiconductor (silicon doped with phosphorus)

Diagram in words: The figure is like the pure crystal, but the middle atom is replaced by a phosphorus atom (P). It makes four bonds with its neighbours. A single extra dot near it is the fifth electron, which is free to move.

• The impurity atom gives away one electron, so it is called a donor.
• Free electrons are the majority carriers and holes are the minority carriers.
• The conductivity increases greatly.
• The N-type material is still electrically neutral as a whole. “N” stands for negative charge carriers (electrons).

8. P-Type Semiconductor

A trivalent impurity such as boron, aluminium, gallium or indium is added to pure silicon. The impurity atom has only 3 valence electrons, so it can form only 3 complete covalent bonds. In the fourth bond one electron is missing. This empty place is a hole.

Fig. 3: P-type semiconductor (silicon doped with boron)

Diagram in words: The middle atom is replaced by a boron atom (B). It makes three complete bonds. The fourth bond, shown with a dashed line and an open circle, is incomplete. The open circle is the hole.

• The impurity atom is ready to accept an electron from a neighbouring atom, so it is called an acceptor.
• An electron from a nearby bond can fill the hole, but it leaves a new hole at its own place. In this way the hole moves through the crystal.
• Holes are the majority carriers and free electrons are the minority carriers.
• The P-type material is also electrically neutral as a whole. “P” stands for positive charge carriers (holes).

9. Comparison Tables

Intrinsic and Extrinsic Semiconductors

Point	Intrinsic Semiconductor	Extrinsic Semiconductor
Meaning	Pure semiconductor with no impurity	Semiconductor in which impurity has been added (doped)
Examples	Pure silicon, pure germanium	Silicon or germanium doped with phosphorus or boron
Charge carriers	Number of electrons is equal to number of holes	Number of electrons is not equal to number of holes
Conductivity	Low	Much higher
Depends on	Mainly on temperature	Mainly on the amount of impurity added
Types	No types	N-type and P-type
Use	Rarely used directly	Used for making diodes, transistors and ICs

P-Type and N-Type Semiconductors

Point	P-Type Semiconductor	N-Type Semiconductor
Impurity added	Trivalent (3 valence electrons)	Pentavalent (5 valence electrons)
Examples of impurity	Boron, aluminium, gallium, indium	Phosphorus, arsenic, antimony
Name of impurity	Acceptor	Donor
Majority carriers	Holes	Free electrons
Minority carriers	Free electrons	Holes
Electrical nature	Electrically neutral	Electrically neutral

10. Applications of Semiconductors

• Diodes: Used as rectifiers to change AC into DC in power supplies and battery chargers.
• Transistors: Used as amplifiers and as electronic switches.
• Integrated circuits (ICs): Used in computers, mobile phones and control circuits.
• Solar cells: Convert sunlight directly into electricity.
• LEDs: Used in lamps, displays and indicators.
• Thyristors (SCR, TRIAC): Used for speed control of motors, light dimmers and fan regulators.
• Sensors: Thermistors and photodiodes are used to sense temperature and light.

11. Everyday Examples to Remember

• Mobile phone and computer: Contain integrated circuits made on silicon.
• LED bulb: Light is produced by a semiconductor device.
• Solar panel: Silicon solar cells convert sunlight into electricity.
• Fan regulator and light dimmer: Use semiconductor devices such as TRIACs to control power.

12. MCQ and True/False Facts

• Semiconductors have 4 valence electrons. Silicon and germanium are the commonly used semiconductors.
• At 0 K a pure semiconductor acts as an insulator.
• A semiconductor has an almost empty conduction band and an almost filled valence band (at low temperature).
• The resistance of a semiconductor decreases when the temperature increases.
• Doping means adding a small amount of impurity to a pure semiconductor.
• In an intrinsic semiconductor, the number of electrons is equal to the number of holes.
• A pentavalent impurity gives an N-type semiconductor (donor, majority carriers are electrons).
• A trivalent impurity gives a P-type semiconductor (acceptor, majority carriers are holes).
• N-type and P-type materials are both electrically neutral.

13. Important Questions for Examinations

Short-answer questions:

1. Define semiconductor. Mention any four applications of semiconductors.
2. Define semiconductor. Discuss its engineering applications.
3. Write the characteristics of semiconductors.
4. Name the commonly used semiconductor materials. Compare silicon and germanium.
5. Define doping.
6. Differentiate between intrinsic and extrinsic semiconductors.
7. Differentiate between P-type and N-type semiconductors.

Long-answer question (for practice):

1. Explain intrinsic and extrinsic semiconductors. Describe N-type and P-type semiconductors with diagrams.

14. Quick Revision

• Semiconductor: Conductivity is between that of a conductor and an insulator, and it can be controlled.
• Examples: Silicon and germanium. Silicon gap is about 1.1 eV and germanium gap is about 0.67 eV.
• Hole: Empty place left by a missing electron in a covalent bond; behaves like a positive charge.
• Intrinsic: Pure semiconductor; electrons = holes.
• Doping: Adding a small amount of impurity to a pure semiconductor.
• N-type: Pentavalent impurity (donor); majority carriers are electrons.
• P-type: Trivalent impurity (acceptor); majority carriers are holes.
• Temperature effect: Resistance decreases as temperature increases.

Related topics to study next: Classification of Electrical Engineering Materials, Conducting and Semiconducting Materials, Insulating Materials.

Conclusion: Semiconductors such as silicon and germanium conduct less than metals but more than insulators. Their conductivity can be controlled by doping, which produces N-type and P-type materials. These materials are the basis of diodes, transistors, integrated circuits and many other electronic devices.