Unit 2 Notes in English, FEEE 2004, Polytechnic 2nd Semester

 

2. OVERVIEW OF BASIC (ANALOG) & DIGITAL ELECTRONICS

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2.1 Introduction to Semiconductors

Semiconductors:

A semiconductor is a material that has electrical conductivity between that of a conductor and an insulator. Semiconductors are the foundation of modern electronics, including devices like transistors, diodes, and integrated circuits. The two most commonly used semiconductor materials are Silicon (Si) and Germanium (Ge).

2.1.1 Different Semiconductor Materials (Si, Ge)

1. Silicon (Si):

   • Bandgap: 1.1 eV
   • Common Usage: Most commonly used in modern electronics (transistors, solar cells, microprocessors).
   • Properties: Silicon is widely used due to its relatively stable properties and abundance in nature.
   • Example: Silicon Chips – The brain of microprocessors, used in computers, mobile phones, etc.

   Diagram:

   • Silicon Atomic Structure:
     • Silicon has 14 electrons, with an atomic number of 14. Its outer shell contains 4 valence electrons.

   
   Si: 1s² 2s² 2p⁶ 3s² 3p²
   (Each Si atom forms 4 covalent bonds with neighboring Si atoms)
   

2. Germanium (Ge):

   • Bandgap: 0.67 eV
   • Common Usage: Used historically in transistors and diodes.
   • Properties: Germanium has a smaller bandgap and is more sensitive to heat compared to Silicon.
   • Example: Germanium Transistors – Used in early transistor-based electronic equipment.

   Diagram:

   • Germanium Atomic Structure:
     • Similar to silicon, but with a smaller bandgap. Germanium’s outer shell also contains 4 valence electrons.

   
   Ge: 1s² 2s² 2p⁶ 3s² 3p²
   

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2.2 Doping (Impurities) in Semiconductors

Doping:

Doping is the process of adding specific impurities to a pure semiconductor material (like Si or Ge) to modify its electrical properties.

2.2.1 Intrinsic and Extrinsic Semiconductors

1. Intrinsic Semiconductor:

   • An intrinsic semiconductor is a pure semiconductor without any added impurities.
   • In intrinsic semiconductors, all the electrons are tightly bound in the valence band and are not free to move at normal temperature.

   Diagram:

   • Energy Band Diagram:
   • The valence band is full, and the conduction band is empty in an intrinsic semiconductor.
   • Bandgap between them is about 1.1 eV for silicon.

   
   Valence Band (Full)  |  Conduction Band (Empty) 
   (Energy gap of 1.1eV)
   

2. Extrinsic Semiconductor:

   • Extrinsic semiconductors are doped with impurities (called dopants) to increase their conductivity.
   • There are two types of extrinsic semiconductors:
     • n-type: Extra electrons are added by doping with donor atoms (e.g., phosphorus).
     • p-type: Holes are created by doping with acceptor atoms (e.g., boron).

   Diagram:

   • For n-type: Donor atoms (like phosphorus) donate free electrons.
   • For p-type: Acceptor atoms (like boron) create holes where electrons can move.

   
   n-type Semiconductor: Extra electrons are free to move, making the material negatively charged.
   p-type Semiconductor: The "holes" act as positive charge carriers.
   

2.2.2 Atomic Structure of Intrinsic and Extrinsic Semiconductors

• Intrinsic Semiconductors: The atoms are tightly bound and have no free charge carriers.
• Extrinsic Semiconductors: In n-type semiconductors, the added donor atoms provide free electrons, while in p-type, the acceptor atoms create holes that can carry charge.

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2.3 Conductivity

Conductivity in semiconductors depends on the number of charge carriers (electrons or holes) available in the material.

2.3.1 Carrier Transport: Diffusion & Drift Current, Mobility, Resistivity

1. Diffusion Current:

   • Diffusion occurs when charge carriers move from a region of high concentration to low concentration. This happens naturally due to the random motion of particles.

   Diagram:

   • The charge carriers (electrons/holes) move across the semiconductor to equalize concentration.

   
   Diffusion: High concentration → Low concentration
   

2. Drift Current:

   • Drift current occurs when carriers move under the influence of an electric field. In an applied electric field, electrons move toward the positive terminal, and holes move toward the negative terminal.

   Diagram:

   • Drift current occurs when the semiconductor material is placed in an electric field.

3. Mobility (μ):

   • Mobility refers to how easily charge carriers move through the semiconductor material under the influence of an electric field. Higher mobility leads to better conductivity.

4. Resistivity (ρ):

   • Resistivity is the material’s inherent property that resists the flow of electric current. A material with low resistivity conducts electricity well.

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2.3.2 Generation and Recombination of Charge Carriers, PN Junction

1. Generation:

   • Generation occurs when electrons in the valence band gain enough energy (from heat or light) to jump to the conduction band, leaving holes behind.

2. Recombination:

   • Recombination happens when an electron from the conduction band recombines with a hole in the valence band, neutralizing both the electron and hole.

3. PN Junction:

   • A PN junction is formed by placing p-type and n-type semiconductors together.
   • When a voltage is applied, forward bias allows current to flow, while reverse bias prevents current from flowing.

   Diagram:

   • Forward Bias: P-side is connected to the positive terminal, and N-side to the negative terminal.
   • Reverse Bias: P-side is connected to the negative terminal, and N-side to the positive terminal.

   
   Forward Bias: Current flows, Depletion region shrinks.
   Reverse Bias: Current doesn't flow, Depletion region widens.
   

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2.4 Active Components and Their Application

2.4.1 Diodes, VI Characteristics, Forward and Reverse Bias

1. Diode:

   • A diode is a two-terminal device that allows current to flow in one direction only.
   • Forward bias: The current can flow when the p-side is positive, and n-side is negative.
   • Reverse bias: The current is blocked except for a small leakage current.

2. V-I Characteristics:

   • Forward bias: When the diode is forward biased, current starts to flow after the voltage exceeds a threshold (usually 0.7V for Silicon).
   • Reverse bias: In reverse bias, no current flows except for a small leakage current.

   Diagram:

   • V-I Characteristics of a Diode:
   • The graph shows that in forward bias, current increases sharply after a certain threshold voltage.

   
   |       / 
   |      /
   |     /    
   |    /       
   |   /       
   |  /         
   ------------------->
   

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2.4.2 Bipolar Junction Transistors (BJT), PNP and NPN BJT, Characteristics

1. BJT:

   • A BJT is a three-layer semiconductor device with two pn junctions: emitter, base, and collector.
   • NPN: The base is p-type, and the emitter and collector are n-type.
   • PNP: The base is n-type, and the emitter and collector are p-type.

   Diagram:

   • NPN Transistor:
     • Emitter is n-type (negatively charged), base is p-type (positively charged), and collector is n-type.

   
   NPN Transistor:
   Emitter (n) → Base (p) → Collector (n)
   

2. Characteristics:

   • In the active region, the transistor amplifies signals.
   • In saturation region, it is "on," and in cutoff region, it is "off."

   Diagram:

   • Characteristic Curves of BJT: Shows how the current varies with voltage at different operating regions (active, cutoff, saturation).

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2.5 Boolean Algebra

2.5.1 Logic Gates

1. NOT Gate: Inverts the input signal (i.e., the output is the opposite of the input).
2. AND Gate: Output is 1 only if all inputs are 1.
3. OR Gate: Output is 1 if at least one input is 1.
4. NAND Gate: Output is the opposite of AND.
5. NOR Gate: Output is the opposite of OR.
6. EX-OR Gate: Output is 1 if inputs are different.
7. EX-NOR Gate: Output is 1 if inputs are the same.

Diagram:

• Truth Tables for Logic Gates:

A	B	NOT A	AND	OR	NAND	NOR	EX-OR	EX-NOR
0	0	1	0	0	1	1	0	1
0	1	1	0	1	1	0	1	0
1	0	0	0	1	1	0	1	0
1	1	0	1	1	0	0	0	1

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2.5.2 Binary Code of a Decimal Number

To convert a decimal number to binary, we repeatedly divide the decimal number by 2 and record the remainders.

Example: Convert decimal 13 to binary.

• 13 ÷ 2 = 6 remainder 1
• 6 ÷ 2 = 3 remainder 0
• 3 ÷ 2 = 1 remainder 1
• 1 ÷ 2 = 0 remainder 1

Binary Equivalent: 13 = 1101

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Important Practice Questions

1. Explain intrinsic and extrinsic semiconductors with diagrams.

• Solution: Intrinsic semiconductors are pure and have a full valence band. Extrinsic semiconductors are doped to increase conductivity, forming n-type and p-type semiconductors.

2. Describe the working of a diode in forward and reverse bias with its V-I characteristics.

• Solution: In forward bias, current flows when the voltage exceeds 0.7V (for silicon diodes). In reverse bias, the diode blocks current except for a small leakage current.

3. Explain the working principle of a BJT (NPN and PNP) with the help of characteristic curves.

• Solution: In NPN transistors, current flows from the collector to the emitter when the base is positively biased. The characteristic curves show how the current varies with voltage.

4. Simplify the Boolean expression for a given logic circuit.

• Solution: Use Boolean laws like De Morgan’s law, distributive law, and associative law to simplify the expressions.

5. Explain the difference between diffusion and drift current in semiconductors.

• Solution: Diffusion current occurs due to carrier concentration gradient, while drift current occurs due to the applied electric field.