Diodes, Transistors, and FETs: Fundamentals and Applications in Electronics, Thesis of Physics

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Mid Term

PHYSICS REVISION NOTES

Quantities

Electric Current:

Electric current can be defined as the flow of ions (charged particles) through a conductive medium,

where positive ions and negative ions move in opposite directions’

Unit of Current

Unit of current is ampere. A

Types of Current

Direct Current

It is a type of electric current where the flow of charges occurs in a single, constant direction.

In DC, the voltage remains steady over time.

Sources: Batteries, fuel cells, and solar cells

Uses: Laptops, smartphones, Solar Panels

Alternating Current:

Alternating Current (AC) is an electric current that changes direction periodically

The voltage in an AC circuit also oscillates between positive and negative values over time.

Sources: Power Plants, Generators, Household Outlet

Uses : household power supplies and large-scale power systems

Voltage:

Voltage is the measure of electric potential difference between two points in a circuit.

Unit: .Voltage is measured in volts (V).

Resistance

Resistance is the opposition that a substance offers to the flow of electric current.

Unit : Unit of resistance is ohms Ω.

Ohm's Law

states that the electric current (I) flowing through a conductor between two points is

directly proportional to the voltage (V) across the two points and inversely proportional to

the resistance (R) of the conductor. It can be expressed mathematically as:

V=IR

Electric Components: Electrical components are the fundamental building blocks of any

electrical circuit. They are devices that manipulate or control the flow of electric current.

Active Components: Electrical Components that generate electrical Power

Passive Components : Electrical Components that do not generate electrical Power.

Resistor

A resistor is a passive electrical

component that restricts the flow

of electric current in a circuit.

It does this by converting

electrical energy into heat

energy.

Capacitor

A capacitor is an electronic

component that stores electrical

energy in an electric field. It

consists of two conductive plates

separated by an insulating

material called a dielectric

Inductor

An inductor is a passive electronic

component that stores energy in

a magnetic field when electric

current flows through it

It consists of a coil of wire

Combinational Circuit:

A combination circuit is an electrical circuit that combines both series and parallel connections. This

type of circuit offers a more complex arrangement compared to simple series or parallel circuits.

To solve combination circuits we can use VDR and CDR. Each combination circuit is unique and its

not possible to solve all of them by a single method like is series or parallel

Kirchhoff's Current Law:

The sum of all currents entering a node equals to the sum of all currents leaving it

I

1

=I

2

+I

3

Node

:

Node in a circuit is a point where 2 or more components connect. It is junction &has its own

equation

Kirchhoff's Voltage Law:

The sum of all voltages in a closed loop is equal to zero

(-V)+V

1

+V

2

=0 or V+(-V

1

)+(-V

2

Voltage Divider Rule:

The voltage divider rule is a simple and widely used electrical principle to calculate the voltage

across a particular resistor in a series circuit.

This rule applies only to resistors in a series configuration.

Current Divider Rule:

The current divider rule helps calculate the current flowing through a specific branch of a parallel

circuit.

Series Circuit

A series circuit is an electrical circuit where all

components are connected end-to-end, forming

a single path for the current to flow. This means

the same current passes through each

component in the circuit.

I=I

1

=I

2

=I

3

V ≠V

1

≠V

2

≠V

3

Req=R

1

+R

2

+R

3

Parallel Circuit

A parallel circuit is an electrical circuit that has

multiple paths for electricity to flow through.

The current is divided between the paths

Electric components are connected across

multiple paths.

I ≠I

1

≠I

2

≠I

3

V=V

1

=V

2

=V

3

1/Req=1/R

1

+1/R

2

+1/R

3

CIRCUIT DIRAGRAM FOR PROBLEM 5>

Problem 4 :

A combination circuit consists of a 10-ohm resistor in

series with a parallel combination of three resistors: a 20-

ohm, a 30-ohm, and a 40-ohm resistor. The total voltage

across the circuit is 120V. Find:

a) The equivalent resistance of the parallel combination.

b) The total resistance of the circuit.

c) The total current flowing through the circuit.

d) The voltage drop across the 10-ohm resistor.

e) The current flowing through the 20-ohm resistor.

Problem 5

A circuit has 5 resistors r1=19 ,r2=85,r3=87 r4=

,r5=37,EMF=7.5V, Find

a) The equivalent resistance of the circuit.

b) The total current flowing through the circuit.

c) The voltage drops across each resistor.

Problem 1: Simple Loop

Consider a simple circuit with a 12V battery

connected to two resistors in series: a 5-ohm resistor

and a 7-ohm resistor.

Find:

a) The total resistance of the circuit.

b) The total current flowing through the circuit.

c) The voltage drop across each resistor.

Problem 2: Multiple Loops

Consider a circuit with three resistors connected in a

loop: R1 = 10 ohms,R2 = 20 ohms,R3 = 30 ohms .A

12V battery is connected across R1, and a 6V battery

is connected across R2 (opposite polarity to the 12V

battery Find

: a) The current flowing through each resistor

.b) The voltage drop across each resistor.

Problem 1: Simple Node Analysis

Consider a circuit with three nodes: A, B,

& C.

• Node A is connected to a 10V

voltage source and a 10-ohm

resistor.

• Node B is connected to the 10-ohm

resistor from node A, a 20-ohm

resistor, and a 30 - ohm resistor.

• Node C is connected to the 20-ohm

and 30-ohm resistors from node B.

FIND : The current flowing

through each resistor.

Problem 2: More Complex Node Analysis Consider a circuit

with four nodes: A, B, C, & D.

• Node A is connected to a 20V voltage source and a 10-

ohm resistor.

• Node B is connected to the 10-ohm resistor from node A,

a 20-ohm resistor, and a 30-ohm resistor.

• Node C is connected to the 20-ohm resistor from node B,

a 40-ohm resistor, and a 50-ohm resistor.

• Node D is connected to the 30-ohm resistor from node B,

the 40-ohm resistor from node C, and a 60-ohm resistor

connected to ground.

• Find: The voltage at each node & current flowing

through each resistor.

Materials based on their ability to conduct electric current.

CONDUCTOR

Conductor is a material that can

easily conduct electrical current.

Valance electrons are very loosely

Bond to atom good conductors

have 1 valence electrons

INSULATOR

Insulator is a material that

does not conduct electrical

current under normal

conditions valence electron

are tightly bound to the atom

SEMICONDUCTOR

Semiconductor is a material whose

electrical conductivity is in between

conductors &insulators their valance

electrons are not tightly or as

loosely bond to atom

Importance of Semiconductor:

Semiconductors are used instead of conductors in electronics because their r conductivity can be

precisely controlled through doping. They can easily switch between conducting and insulating

states, enabling transistor operation

Importance of silicon

Silicon is used in electronics because it has excellent semiconductor properties, is abundant (widely

available and cost-effective, can be easily doped to control its conductivity

N and P type semiconductors have limited number of free electrons in conduction band and limited

number of holes in balance band.

Electron Current:

When voltage is applied to an intrinsic silicon the free electron or attracted towards the positive and

this movement of electron is called electron current.

Doping:

The increase in number of free electrons or holes this call doping. it is the process of adding small

amounts of impurities to a pure semiconductor, like silicon, to change its electrical properties. It

increases the number of charge carriers, making the material more conductive.

Diodes

Diodes are semiconductor devices that allow current to flow in one direction only. They consist of a

P-type and an N-type semiconductor, each created by doping which are joined together to form a P-

N junction.

Depletion Region:

The depletion region is a barrier that forms at the junction between the P-type and N-type

parts of a diode. some electrons from the N-type side move to the P-type side, filling in holes. er. As

they do, they leave behind immobile charged ions—positive on the N side and negative on the P

side—which form the depletion region. This region doesn’t conduct current unless a sufficient

external voltage is applied to overcome the barrier.

Barrier Potential:

Electric field is formed in depletion region. This region doesn’t conduct current unless a

sufficient external voltage is applied to overcome the barrier. depletion region blocks current flow

until enough voltage is applied to push past it. The voltage required to break this barrier is called

barrier potential

Barrier Potential for silicon=0.7v | Barrier Potential for germanium=0.3v

N type semiconductors

Means to increase the number of free electrons in

conduction band

It requires pentavalent impurity for example

phosphorus

P type semiconductors

Means to increase the number of holes in valance

band

It requires trivalent impurity for example Boron or

gallium

Application of diodes:

DC Power Supply:

A DC power supply is an electronic device that Alternating Current into DC.

Rectification

Rectification is the process of converting alternating current (AC) into direct current (DC).

Half-wave rectification

Half-wave rectification is a process that converts alternating current (AC) to direct current

(DC) by allowing only one half of the AC waveform to pass through while blocking the other

half. This results in a pulsating DC output.

Disadvantage:

Only half of the AC input is utilized, leading to lower efficiency:

Half Wave Rectifier

A half-wave rectifier is a simple circuit used to convert an AC signal into DC It allows only one half of

the AC waveform (positive or negative) to pass through, while blocking the other half.

It uses a diode that allow current to flow in 1 direction and load resistor to consume output

Positive half cycle:

During Positive half Cycle the diode is forward biased acting as closed switch allowing current to

flow, and the output follows the input voltage.

Negative half cycle

During negative half Cycle is reverse biased blocking current, &the output voltage is zero.

• Ideal Model : The output voltage during the positive half-cycle equals the input voltage.
• Practical Model : The output voltage is slightly less than the input by about 0.7V due to the

diode’s forward voltage drop.

Full Wave Rectification

In a full-wave rectification, both halves of the AC input signal are used to produce a DC output.

A bridge rectifier

A bridge rectifier is a specific type of circuit configuration that converts alternating current (AC) into

direct current (DC) using four diodes arranged in a bridge formation. This setup allows the

rectification of both halves of the AC waveform, effectively doubling the output frequency of the

rectified signal compared to a half-wave rectifier.

Positive Half Cycle

During the positive half of the AC input, two diodes D1 and D2 in the bridge rectifier become

forward biased and conduct current. While D3 an D4 act as reverse biased and block it. The current

flows through the load resistor in one direction, producing a positive output voltage.

Negative Half Cycle

During the negative half of the AC input, the other two diodes (D3 and D4) become forward biased

and conduct current. While D1 and D2 act as reverse Bias and Block current. The current again flows

through the load resistor in the same direction as in the positive half-cycle, maintaining a positive

output voltage.

Ideal Model

In the ideal case, during both the positive and negative half-cycles of the input, the bridge rectifier

allows the full input voltage to pass through to the load.

Output Voltage : The output voltage across the load equals the peak input voltage during both half-

cycles, resulting in a continuous pulsating DC.

Practical Model

Voltage Drop : Since each conducting path has two diodes, the output voltage is reduced by

2 × 0.7V = 1.4V (for silicon diodes) from the peak input voltage.

Output Voltage : The output is slightly less than the input by 1.4V during each half-cycle.

Voltage Divider Biased Limiter

We can limit the value of output signal to what we want but if we want to limit it to a value

whenever we want in case of biased limiter, we will have to replace the biasing source from the

circuit however we don’t have to do that by using a voltage divider circuit we can change the value

of DC source

In this circuit we can change the value of the two

Resistors to control the value of V supply

So we can limit the value of out signal to what

we want and change it whenever we want

A Voltage Divider Bias Limiter is a circuit that combines a voltage divider network with a

diode limiter to control and restrict the voltage in a circuit.

The circuit of a negative voltage divider bias

limiter is given:

V3 is the voltage at the node closest to the diode in

the circuit.

V3 is considered output voltage because it directly

Influences the operation of the diode limiter

0.7 is added in case of positive and - 0.7 is subtracted

in case of negative voltage divider bias limiter

Value of V3 is given by

Numerical for limiters are given in the table above

Important not if the voltage to limited is greater then input voltage then the output

voltage would be same as input voltage and signal wont be clipped

Consider a positive

limiter circuit with a

10V peak-to-peak

input signal & a

silicon diode with a

forward voltage

drop of 0.7V.

Sketch the input

and output

waveforms.

Determine the

peak output

voltage.

Consider a negative

limiter circuit with a

10V peak-to-peak

sinusoidal input signal

and a silicon diode

with a forward voltage

drop of 0.7V.

Sketch the input and

output waveforms.

Determine the peak-

to-peak output

voltage.

Consider a biased

positive limiter circuit

with a 10V peak-to-peak

sinusoidal input signal, a

silicon diode with a

forward voltage drops of

0.7V, and a bias voltage

of 2V.

Sketch the input and

output waveforms.

Determine the peak

output voltage.

Consider a voltage

divider negative limiter

circuit with a 10V peak-

to-peak sinusoidal input

signal, a silicon diode

with voltage drop of

0.7V, a 1kΩ resistor

&2kΩ resistor.

Sketch the

input and output

waveforms

Determine the peak-to-

peak output voltage.

Transistor

A transistor is a semiconductor device used to amplify or switch current. It is one of the most

essential components in modern electronic devices.

Types of Transistors:

BJT is current-controlled ,

meaning the current at the base controls

the larger current between the collector

and emitter

FET is voltage-controlled,

meaning the voltage at the gate

controls the current between the

source and drain

Bipolar Junction Transistor (BJT) :

The Bipolar Junction Transistor (BJT) is a type of transistor that relies on the flow of both electrons

and holes hence the name "bipolar" to control and amplify electric current. BJTs are widely used in

analog circuits, switching applications, and amplifiers.

Structure of a BJT

A BJT is made up of three semiconductor regions:

Emitter (E): Heavily doped region that emits charge carriers (electrons or holes).

Base (B): Thin and lightly doped region that controls the flow of charge carriers.

Collector (C): Moderately doped region that collects the charge carriers from the emitter.

Junctions in BJTS:

These regions are separated by two important junctions that play a crucial role in how the transistor

works. These junctions are:

Emitter-Base Junction

Base-Collector Junction

Modes of Operation

Cutoff Region:

Both the emitter-base and base-collector junctions are reverse-biased.

The transistor is OFF, and no current flows.

Active Region:

The emitter-base junction is forward-biased, and the base-collector junction is reverse-biased.

The transistor amplifies signals in this mode.

Saturation Region:

Both the emitter-base and base-collector junctions are forward-biased.

The transistor is fully ON and behaves like a closed switch.

Breakdown Region:

The emitter-base junction is reverse-biased, and the base-collector junction is forward-biased.

Rarely used in practice.

Types of BJTS

1. NPN Transistor

2. PNP Transistor

.

Summary of transistor as amplifier

In an NPN & PNP transistors a small base current (IB) controls a larger collector current (IC). This

allows the transistor to amplify signals. When a small input signal is applied to the base-emitter

junction, it modulates the larger current flowing

from the collector to the emitter, in case of NPN

or from the emitter to the collector to, In case of PNP

resulting in signal amplification.

Both PNP and NPN transistors use the principle of controlling a larger current (IC) with a smaller base

current (IB) to achieve amplification. The key difference lies in the direction of current flow and the

polarity of the voltages applied to the terminals

β dc

Beta is the ratio of the collector current to the base current .It represents

how much the base current is amplified to produce the collector current.

βdc is used in circuit analysis to predict the relationship between the

base current and the collector current.

α

dc

Alpha is the ratio of the collector current the emitter current. It represents

the efficiency of the current transfer from the emitter to the collector.

It is always less than 1 (e.g., 0.95 to 0.99).

α&β Relation:

Working of NPN as Amplifyer

Emitter to Base:

The emitter is negatively charged (N-type), and the

base is positively charged (P-type). When a small

positive voltage is applied to the base, it allows

electrons to flow from the emitter into the base.

Base to Collector:

The base-collector junction is reverse-biased

(meaning the collector is more positive than the

base). This creates a large electric field that pulls

electrons from the base to the collector.

Collector Current

Most of the electrons that entered the base do not

stay there—they flow through the collector (which

is N-type and attracts the electrons), allowing a

larger current to pass from collector to emitter.

Small Base Current:

The small current at the base controls the large

current that flows from the collector to emitter. This

is called current amplification.

Summary for NPN:

Base current (IB) is small.

Collector current (IC) is much larger.

The current flows from collector to emitter when

the transistor is on.

Working of PNP as Amplifyer

Emitter to Base:

The emitter is positively charged (P-type), and

the base is negatively charged (N-type). When

a small negative voltage is applied to the base,

it allows holes (positive charge carriers) to flow

from the emitter to the base.

Base to Collector:

The base-collector junction is reverse-biased

(meaning the collector is negative compared

to the base). This causes the holes to flow

from the base to the collector.

Collector Current:

The collector (P-type) attracts the holes,

allowing a larger current to flow from emitter

to collector.

Small Base Current:

Just like in the NPN transistor, the small

current at the base controls a larger current

flowing from emitter to collector.

Summary for PNP:

Base current (IB) is small and negative.

Collector current (IC) is much larger.

The current flows from emitter to collector

when the transistor is on.

Same polarity (either both

positive or both negative) is

essential for proper biasing and

amplification in a PNP

transistor. If the polarities are

opposite, the base-emitter

junction would not be forward

biased, and the transistor would

not amplify.

Current And Voltage Analysis:

Here are the circuits we will analyze one uses PNP transistor to amplify the other uses NPN

What we are given

• V BB

& V CC

• R

B

& R

C

• β

dc.

What we Require:

• V

BC

, V

BE

, V

CE

• I

C

, I

E

, I

B

For Both PNP & NPN The base emitter junction is forward biased

Value of VBE is always +0.7V for NPN Transistor

Value of VBE is - 0.7V if VCC & VBB are Positive for PNP Transistor

Value of VBE is - 0.7V if VCC & VBB are Negative for PNP Transistor

Using KVL in Loop 1 to find value of I C

.

USING IB TO FIND VALUE OF IC

We were given value of β

dc

, we have calculated value of I

B ,

We can find Value of I

C

by

FOR NPN

- V BB

+ V B

+ V BE

= 0

- V

BB

+ I

B

R

B

+ V

BE

= 0

I

B

= V

BB

- V

BE

/R

B

FOR PNP

V BB

- V B - V BE

= 0

V

BB

- I

B

R

B

- V

BE

= 0

I

B

= V

BB

- V

BE

/R

B

Collector characteristic curves

Collector characteristic curves show the relationship between the collector current (IC) and the

collector-emitter voltage (VCE) for a transistor, while keeping the base current (IB) constant. These

curves are essential for understanding how a transistor behaves in different regions of operation.

These curves allow us to visualize how the transistor operates in its different regions (cutoff, active,

saturation, and breakdown).

Components of Curve:

Axis:

X-axis: Represents V

CE

(collector-emitter voltage), the voltage applied across the collector

and emitter terminals.

Y-axis: Represents I

C

(collector current), the current flowing from the collector to the

emitter.

Curve Families:

Ea ch curve corresponds to a specific value of I

B

, ranging from I

B

=0 (cutoff) to higher base

current values.

Region of Operations

Cutoff Region (Transistor OFF)

The transistor is in its OFF state because there is no base current (IB=0).

The base-emitter (BE) junction is reverse-biased , which means the voltage at the base is less than

the emitter VBE<0.

Similarly, the base-collector (BC) junction is also reverse-biased:

Since no current is flowing into the base (IB), there is no collector current (IC= 0 ), except for a very

tiny leakage current ICBO collector-to-base leakage current.

The transistor behaves like open switch, completely blocking current between collector & emitter.

On the Graph:

The flat line near the X-axis where IC≈0 approx. corresponds to the cutoff region.

This is seen for IB=0 in the family of curves.

How it transitions :

As a small base current (IB) is applied and increases, the transistor starts transitioning out of the

cutoff region into the saturation region.

Saturation Region (Transistor Fully ON)

The transistor enters saturation when the base current (IB) is sufficiently large, and the collector-

emitter voltage (VCE) is small less than 0.7v.

As VCC increases VCE also increases gradually with Ic this is displayed with curve in saturation region.

In this state, both the base-emitter junction & base-collector junction are forward-biased.

• The transistor behaves like a closed switch , allowing maximum current to flow from the

collector to the emitter.

• The collector current IC is no longer proportional to IB instead, it is limited by the external

circuit components (such as the collector resistor RC).

• There is very little voltage drop across the transistor because it offers minimal resistance.

On the Graph:

The steep, almost vertical rise near VCE≈ 0 represents the saturation region.

Here, ICis nearly at its maximum for a given IB, but the voltage across the collector and emitter is

very low

How it transitions :

As VCE increases beyond a certain threshold (0.7 for silicon BJTs), the transistor leaves the

saturation region and enters the active region. This happens when the base-collector (BC) junction

becomes reverse-biased.

Active Region (Amplification Region)

When VCE exceeds 0.7V the base collector junctions becomes reverse biased and transistor goes in

linear or active region IC levels off and remains constant for value of IB.

In this region, the transistor operates as an amplifier , where the collector current (IC) is proportional

to the base current (IB), based on the current gain (βdc):

IC=β x IB

On the Graph:

The horizontal (flat) portion of each curve corresponds to the active region. IC remains constant

even as VCE increases.

Each curve corresponds to a specific value of IB higher IB produces higher IC in the active region

How it transitions :

If VCE is increased excessively, the transistor may enter the breakdown region. This is usually

avoided in practical applications.

Breakdown Region (Transistor Overloaded)

When VCE reaches a sufficiently high voltage, the reverse-biased base-collector junction goes into

breakdown; and the collector current increases uncontrollably, independent of IB. A transistor

should never be operated in this breakdown region.

On the Graph:

The upward curve beyond the active region represents the breakdown region.