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Physics · CDS

PC11 — Semiconductors & Electronic Devices

📖 PC11 · CDS General Science — Physics

This chapter covers the physics underlying all modern electronic devices — from transistors to logic gates. CDS questions focus on the distinction between conductors, insulators, and semiconductors; the p-n junction diode; and basic logic gates with truth tables. While lower-weightage, when questions appear they are often straightforward and scoring.

📌 CDS focuses on: Conductors vs semiconductors vs insulators (energy band); intrinsic vs extrinsic semiconductors (n-type, p-type); p-n junction diode — forward and reverse bias; rectification; basic logic gates (AND, OR, NOT) and their truth tables; NAND and NOR as universal gates.

1. Conductors, Semiconductors & Insulators

1.1

Energy Band Theory — Why Materials Conduct Differently

Fig. 1 — Energy band theory. Conductors have overlapping bands (electrons flow freely). Semiconductors have a small gap (partial conduction). Insulators have a large gap (no conduction under normal conditions).

📍 n-Type Semiconductor

Silicon/Germanium doped with pentavalent atoms (Phosphorus, Arsenic, Antimony)
Each dopant atom has 5 valence electrons — 4 form bonds, 1 is free
Majority carriers: free electrons (negative charge carriers)
Minority carriers: holes
"n" = negative majority carriers

📍 p-Type Semiconductor

Silicon/Germanium doped with trivalent atoms (Boron, Aluminium, Gallium)
Each dopant has 3 valence electrons — forms 3 bonds, creates a "hole" (vacancy)
Majority carriers: holes (behave like positive charge carriers)
Minority carriers: electrons
"p" = positive majority carriers

2. p-n Junction Diode

2.1

Forward & Reverse Bias — Rectification

Fig. 2 — p-n junction diode: forward bias (p to +, n to −) allows current to flow; reverse bias blocks current. This one-way current flow is used in rectifiers to convert AC to DC.

Rectification: A diode allows current in only one direction. Used in rectifier circuits to convert alternating current (AC) to direct current (DC). Half-wave rectifier: uses one diode — blocks half the AC cycle. Full-wave rectifier: uses four diodes in bridge configuration — converts both halves of AC cycle. All chargers (phone chargers, laptop adapters) contain rectifier circuits.

3. Logic Gates

3.1

Basic & Universal Logic Gates with Truth Tables

The building blocks of all digital circuits — computers, phones, and everything digital

Fig. 3 — The three basic logic gates with symbols and truth tables. AND: output is 1 only when all inputs are 1. OR: output is 0 only when all inputs are 0. NOT: output is always opposite of input.

📝 CDS PYQ

Semiconductors & Electronics

Q1. A p-n junction diode is forward biased when:

(a) p-side to negative and n-side to positive terminal
(b) p-side to positive and n-side to negative terminal
(c) Both p and n sides to positive terminal
(d) No bias is applied

Answer: (b) p-side to positive, n-side to negative terminal
Forward bias: connect the positive terminal of the battery to the p-side and the negative terminal to the n-side. This reduces the potential barrier at the junction, allowing current to flow. In reverse bias (opposite connection), the barrier increases and no significant current flows — only a tiny leakage current. This unidirectional property makes diodes useful as rectifiers.

Q2. The output of a NAND gate is 0 only when:

(a) All inputs are 0
(b) All inputs are 1
(c) At least one input is 1
(d) Inputs are different

Answer: (b) All inputs are 1
NAND = NOT + AND. The AND gate gives output 1 only when all inputs are 1. The NOT then inverts this to 0. For any other combination of inputs, AND gives 0, which NOT inverts to 1. So NAND output is 0 ONLY when both inputs are 1; otherwise output is always 1. NAND is a universal gate — all other gates can be built from NAND gates alone.

Q3. n-type semiconductor is formed by adding which type of impurity to silicon?

(a) Trivalent (3 valence electrons)
(b) Pentavalent (5 valence electrons)
(c) Divalent (2 valence electrons)
(d) Monovalent (1 valence electron)

Answer: (b) Pentavalent
Adding a pentavalent atom (5 valence electrons — e.g. phosphorus, arsenic, antimony) to silicon (tetravalent, 4 valence electrons) creates an n-type semiconductor. Four of the five valence electrons form bonds with silicon; the fifth is a free electron — the majority charge carrier. For p-type, trivalent atoms (e.g. boron) are added, creating "holes" (missing electrons) as majority carriers.

📚 Formula Sheet & Key Facts — PC11

📍 Semiconductor Types

n-type: pentavalent dopant; free electrons
p-type: trivalent dopant; holes
Silicon band gap ≈ 1.1 eV
Ge band gap ≈ 0.7 eV
R decreases on heating (unlike metals)

⚡ Diode

Forward bias: p(+), n(−) → current flows
Reverse bias: p(−), n(+) → no current
Si forward voltage ≈ 0.7 V
Rectifier: converts AC to DC
Half-wave: 1 diode; Full-wave: 4 diodes

📋 Logic Gates

AND: Y=A·B; output 1 only if both 1
OR: Y=A+B; output 0 only if both 0
NOT: Y=Ā; inverts input
NAND: output 0 only if both inputs 1
NOR: output 1 only if both inputs 0

🔋 Universal Gates

NAND and NOR are universal gates
Any circuit can be made from NAND alone
Any circuit can be made from NOR alone
Transistor: amplifier or switch
BJT: NPN or PNP type

⚡ Quick Revision — PC11

📍 Semiconductor Basics

Conductor > Semiconductor > Insulator (conductivity)
n-type: electrons; p-type: holes
Pentavalent → n-type doping
Trivalent → p-type doping
R decreases with temperature (unlike metals)

⚡ Diode Facts

Forward: p(+) → current flows
Reverse: no current flows
Uses: rectifier, LED, solar cell, detector
Rectifier: AC to DC conversion
Si diode: ~0.7V forward drop

📋 Logic Gates Quick

AND: output 1 only when all inputs = 1
OR: output 0 only when all inputs = 0
NOT: output = opposite of input
NAND = NOT(AND); NOR = NOT(OR)
NAND and NOR: both universal gates

🚨 CDS Traps

n-type: pentavalent (NOT trivalent)
p-type: trivalent (NOT pentavalent)
Forward bias: p to +ve terminal
NAND output 0: only when both inputs 1
NOR output 1: only when both inputs 0

📝 Practice Exercise

E-01

Two inputs A=1, B=0 are fed into a NOR gate. The output is:

(a) 1
(b) 0
(c) Depends on the circuit
(d) Undefined

E-02

Silicon is a semiconductor. Its electrical conductivity:

(a) Increases with temperature
(b) Decreases with temperature
(c) Remains constant
(d) Only changes above 500°C

E-03

Which gate gives an output of 1 only when all inputs are 1?

(a) OR
(b) NOT
(c) NAND
(d) AND

E-04

A full-wave rectifier uses how many diodes?

(a) 1
(b) 2
(c) 4
(d) 8

Answers: E-01: (b) 0 [NOR = NOT(OR); OR(1,0)=1; NOT(1)=0] | E-02: (a) Increases — more electron-hole pairs form at higher temp, increasing conductivity (opposite to metals) | E-03: (d) AND gate — output is 1 only when all inputs are 1 | E-04: (c) 4 diodes — bridge rectifier uses 4 diodes in a bridge configuration to utilise both halves of AC

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