AHMED HEKAL

CHAPTER (8) MODERN ELECTRONICS

Mr. Ahmed Hekal

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Chapter 8

A. Definitions

1- Semi-Conductors:

- Materials that fall between conductors and insulators ,their conductivity increase by raising their

temperature

2- Crystal :

- regular arrangement of atoms in solid state

3- Hole:

- Free space left by electron in a broken bonds in the semi-conductor crystal

4- Dynamic (Thermal) Equilibrium

- It’s the state in which number of broken bonds in 1 sec equal number of mended bonds in the semi-

conductor crystal and this always make number of free electrons equal number of holes for each specific

temperature

5- Pure Semi-conductor:

- Semi-conductor in which concentration of n (free electrons) = concentration of P (holes) at any temperature

6- Impurity Atom:

- Atom of a trivalent element or pentavalent element added to a pure semi-conductor crystal to increase its

conductivity

7- Doping: (Adding impurities)

- Adding a trivalent element or pentavalent element to a pure semi-conductor crystal of tetravalent element

to increase concentration of free electrons or holes in it

8- P-type Semi-conductor

- Semi-conductor doped with a trivalent element to make the concentration of holes (p) greater than

concentration of free electrons (n)

9- N-type Semi-conductor

- Semi-conductor doped with a pentavalent element to make the concentration of free electrons (n) greater

than concentration of holes (p)

10- Law of Mass Action:

- The product of concentration of free electrons (n) and holes (p) = Constant , this constant doesn’t depend on

impurity type and it equals the square of concentration of free electrons Or concentration of holes in the

pure semi-conductor crystal at constant temperature

11- Diode:

- Two adjacent crystals one of them is n-type and the other is p-type

12- Diffusion Current:

- Current results in diffusion of free electrons from region n to region p and diffusion of holes from region p to

region n

13- Transition (depletion) Region:

- It’s the region that are free of charges carriers and it exists in the contact area between the two crystals in

diode

14- Drift Current:

- Current results in the internal electric field between positive ions towards (n) region and negative ions

towards (p) region in the contact area between the two crystals and it opposes the diffusion current

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15- Potential Barrier:

- The lowest potential in the contact area between the two crystals that prevent more diffusion of electrons

and holes to the lower concentration region of them

16- Distribution Factor (ratio) coefficient αe

- Ratio between the collector current to Emitter current when potential difference between base and

collector is constant

17- Amplification Factor βe

- Ratio between the collector current to base current when potential difference between emitter and

collector is constant

18- Analog Electronics

- Electronics that deal with natural quantities as they are, and convert them to electrical signals of continuous

decimal numbers (1, 2, 3…)

19- Digital Electronics

- Electronics that deal with natural quantities but convert them to non-continuous codes of zeros and ones

where zero represents low logic and one represents high logic

20- Logic Gates

- Parts of electronic circuits of modern devices that perform the logic operations on the digital signals (like

NOT, AND, OR )

B. What’s meant by? 1- The Potential Barrier of diode = 0.3 V

- It means that the lowest potential in the contact area between the two crystals that prevent more diffusion

of electrons and holes to the lower concentration region of them = 0.3 V

2- Distribution coefficient αe = 0.98

- Ratio between the collector current to Emitter current when potential difference between base and

collector is constant = 98 : 100

3- Amplification Factor βe = 46

- Ratio between the collector current to base current when potential difference between emitter and

collector is constant equals = 46 :1

C. Diagrams Relation between Diagram Law and slope

A. Current Intensity and Potential difference at forward and backward bias-connection in diodes

Diode allows current when forward connection and prevents current when backward connected

B. Vout and Vin when using Transistor as a key

When Vin is high Vout is low and vice versa

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C. IC and IE when V between Base and collector is constant

Slope = αe = Ic/IE Ic = αe IE

D. IC and IB when V between Emitter and collector is constant

Slope = Ic/IB

D. Deductions 1- Ratio of Amplification in Transistor

- αe = Ic/IE Ic = αe IE

- IB= IE - Ic

- Ic/IB = αe IE / (IE - αe IE )

- αe/(1- αe)

E. Usages: 1- Specialized electronics devices

Used as sensors for the for environmental factors as heat, light, pressure and pollution

2- Diode 1- Open key if it’s backward connected and closed key if it’s forward connected

2- Rectifying AC to DC to charge cars batteries and mobiles chargers

3- Transistor 1- As Amplifier 2- As a Switch

4- Analog to Digital Converter - Converts continuous electrical signals to digital non-continuous signals in the receiving device

5- Digital to Analog Converter - Converts digital non-continuous to continuous electrical signals in the receiving device

6- Analog Devices - Electronics that deal with natural quantities as they are, and convert them to electrical signals of continuous decimal numbers (1, 2, 3…)

1- Microphone 2- Legacy Video Camera 3- Legacy T.V.s

7- Digital Devices

1- Cell Phones (mobiles) 2- CDs 3- Computers 4- Digital channels

8- Logic Gates

Used in PC circuits

F. Comparisons

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1- N-Type and P-Type Crystals

N-Type P-Type

Concentration of charges carriers

Free electrons greater than holes

Holes greater than Free electrons

Type of impurity atom

Donor atom and it’s atom of pentavalent element as Antimony or Phosphorus shares with 4 bonds and lose one electron and became positive + ion

Acceptor atom and it’s atom of trivalent element as Boron or Aluminum shares only in 3 bonds and gains one electron and became negative - ion

Charges carriers Electrons Holes

Donors Impurities Acceptors Impurities

Shape of crystal after Doping

Impurity Valence Pentavalent atoms like Phosphorus(P) and Antimony (SB)

Trivalent atom as boron (B) and Aluminum (AL)

Impurity work action Shares with 4 electrons in forming bonds and one extra electron is left

Shares only with 3 electrons in bonds and there is one hole left needs an electron

Charges carrier type Free electrons Holes

Impurity atom after doping Became positive ions Became negative ions

Crystal Type after doping n-type p-type

Relation bet. n,p N > P P > N

At dynamic equilibrium N = p + ND+ P = n + NA

-

Diode Ohmic Resistance

Structure 2 adjacent n , p crystals Coil of wire of a specific resistivity

Charges Carriers Free electrons and holes Free electrons

Current Intensity Current flows in one direction and vanished يتالشى in opposite direction

Current can flow in the two directions

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Heat Effect Resistance decreases and conductivity increases

Resistance increases and conductivity decreases

Forward bias connection Backward bias connection

Work Action Crystal p is connected to positive pole of battery and crystal n is connected to negative pole of the battery

Crystal n is connected to positive pole of battery and crystal p is connected to negative pole of the battery

Effect of external voltage on diode Direction of external voltage of battery opposites direction of internal field in the depletion region so it weakens it

Direction of external voltage of battery same as direction of internal field in the depletion region so it weakens it

Voltage of Diode Less than potential barrier More than potential barrier

Thickness of depletion (transition) region

Decreases in cause of repulsion between similar poles

Increases in cause of attraction between different poles

Diode resistance Very small Very high

Current Intensity High (current passes) Low (current stops)

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PNP Transistor NPN Transistors

structure Base is negative - Collector and emitter are positive +

Base is positive + Collector and emitter are negative -

Symbol in electrical circuit

Similar properties NPN as an amplifier (common base) NPN as an amplifier (common emitter)

Connection method 1- Emitter E is connected with Base B forward connection 2- Collector C is connected with Base B Backward connection

Electrons Movement direction

1- Electrons are released from emitter of type (N) to Base of type (P) and spread for an interval of time until it’s picked up by Collector of type (N)

2- Collector N picks up electrons as it’s connected to positive pole of battery

Different properties NPN as an amplifier (common base) NPN as an amplifier (common emitter)

Circle Diagram

Operation Part of electrons are consumed in occupying the holes of Base, so collector current is less than emitter current (IC= αe

IE) and αe is always less than 1 so in this case transistor is not used in amplifying current but used in amplifying the electrical power

Electrons of collector are repulsed with negative pole of batteries so the two currents of electrons are met at the emitter and moves towards the collector so if a small electrical signal is placed in base current so its effect appears larger in

collector current by ratio of e where

e= IC/IB so transistors are used here in amplifying current and voltage and electrical power

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NOT Gate AND Gate OR Gate

Equivalent circuit A key connected in parallel When it’s opened, lamp is on and when it’s closed lamp is off

Two keys connected in series Lamp is not on except the two keys are closed

Two keys connected in parallel to each other Lamp is on if one of them is closed

Number of I/Ps and O/Ps One I/P One O/P

Two or more I/Ps One O/P

Two or more I/Ps One O/P

Truth Table

Input Output

0 1

1 0

Input Output

A B

0 0 0

0 1 0

1 0 0

1 1 1

Input Output

A B

0 0 0

0 1 1

1 0 1

1 1 1

Logic Operation Reverse Multiply (output is 1 when and only when the 2 inputs are 1)

Add (output is 1 if any one of the inputs was 1)

Symbol

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Transistor as a key in case of ON Transistor as a key in case of OFF

1- It’s connected in electrical circuit where emitter is common 2- VCC = VCE + ICRC

Where VCC is Battery voltage and VCE is potential difference between collector and emitter and IC is the collector current and RC is circuit resistance

When connecting Base B to a positive or high voltage, a large current IC is passing in collector circuit so value of ICRC is large and VCE is small (output is small value) It means that transistor passes current of base because (Vin > Vout)

When connecting Base B to a negative or low voltage, IC will decrease in collector circuit and ICRC is small so VCE increases (output is large value) It means that transistor doesn’t pass current of base because (Vout> Vin)

G. Briefly Notices 1. Conductivity for Semiconductors

- Semi-conductors are not a good conductors in low temperatures so it’s conductivity can be improved by 1- Raising temperature of semi-conductor 2- Doping (adding impurities of trivalent as Boron or pentavalent elements as Antimony

2. Properties of pure crystal (Pure Silicon)

1- The highest (last) level of each atom is complete by electrons at 0 kelvin

2- Electrons of internal levels are strongly coherent with nucleus but the outer electrons have higher

degree of freedom to move through the interatomic spaces inside the crystal

3- At low temperatures (specially at 0 kelvin), Bonds are intact سليمة so there are no free electrons inside

the crystal and no conductivity

4- By raising temperature some bonds are broken and some electrons are freed and when an electron

leaves its position it leaves a hole and this is not considered an ionization because atom will picks up

another electron rapidly and returns to neutral state

5- By continue raising temperature, number of free electrons and holes increases so conductivity increases

6- Electrons moves randomly inside the crystal to occupy holes that arises تنشأ عن breaking bonds

7- Energy needed to break a bond = energy resulting in building the bond whether this energy is thermal or

optical (light)

3. Types of Electronics components: 1- Simple components: as Resistance R , Induction Coil L , capacitor C

2- More complex components: Diodes, Transistors

3- Special components: photo-electric devices and control devices

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4. Diode: 1- In a crystal of type P , the concentration of holes p is larger than concentration of electrons n and in a

crystal of type n, the concentration of electrons n is larger than concentration of holes p

2- when the two crystals are contacted, holes p and electrons n are spreading from higher concentration

region to lower concentration so holes transfer from crystal p to crystal n and electrons transfer

from crystal n to crystal p and current passes called Diffusion Current

3- Holes of region p cannot cover all electrons in region n , and electrons of region n cannot cover all

holes of region p

4- Two regions empty of electrons and holes are generated around the contact area, these regions have

positive ions towards crystal n and negative ions towards crystal p these empty regions called

Transition or (depletion) Region

5- When crystal of type n lose some electrons it gains positive potential and crystal of type p gains a

negative potential because it receives electrons, so an electrical field generated and its direction from

crystal n (positive potential) to crystal p (negative potential) causing a current called Drift Current

6- By continue transferring electrons and holes from higher concentration region to lower concentration

region the potential difference between the two crystals increases until it reaches a value which

prevents this transfer of electrons from n to p and the Diffusion Current = Drift Current and

in this case the potential difference is called Potential Barrier

H. What’s happened in these cases? 1- Increasing number of broken bonds using heat , for the semi-conductor crystal

- Number of free electrons and number of holes increase so conductivity increases

2- Doping a pure Silicon crystal with atoms of Boron

- Boron atom gain an electron and became negative ion so concentration of holes increase inside crystal and

became crystal of type p and its conductivity increases

3- Doping a pure Silicon crystal with a pentavalent element atoms

- The pentavalent atom loses an electron and became a positive ion so the concentration of free electrons

increases and it the silicon became a crystal of type n and it conductivity increases

4- Contacting crystal of type p with a crystal of type n to form a diode

- Holes spread from p to n and free electrons spread from n to p , so positive ions increase towards n crystal

and negative ions increase towards p crystal this happens on the two sides of contact so a potential barrier

is generated and prevents transfer of electrons and holes

5- Connecting diode as forward-bias connection in the electrical circuit

- The external electrical field weakens the internal field and decreases the thickness of depletion region and

potential of diode decreases under the potential barrier and the resistance if diode decreases so a current

will pass

6- Connecting diode as backward-bias connection in the electrical circuit

- Increase internal electrical field and decrease thickness of depletion region, increase the potential barrier,

increase resistance of diode so no current will pass

7- Connecting diode to AC current

- Diode will rectify the direction of AC current, it will pass in one direction only and not pass in the other

direction

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I. Give reason 1- When raising temperature of semi-conductor its conductivity will increase

- Because raising temperature leads to increase the broken bonds and release more free electrons which

increases the electrical conductivity

2- The semi-conductor atom which has a broken bond isn’t considered an ion

- As soon as the bond is broken, the hole will picks up another electron from another bond or from a other

free electrons to be stable

3- The pure silicon crystal is completely insulating at 0 Kelvin

- Because at low temperatures all bonds are intact so there is no free electrons

4- At thermal equilibrium there is no increase in free electrons or holes

- Because number of broken bonds in 1 second equals number of mended bonds so number of free electrons

and number of holes is constant at a specific temperature

5- When adding atoms of Antimony to a crystal of silicon, its conductivity increases

- Because Antimony is pentavalent element and Silicon is tetravalent so antimony share silicon with 4

electrons in its outer shell and 1 electrons remain free which increases conductivity

6- Semiconductor crystal of type n or p is neutral

- Because in type p: sum of positive charges (p) = sum of negative charges for electron (n) + sum of negative

charges for Acceptor ions NA- , And

- in type n: sum of negative charges (n) = sum of positive charges for electron (p) + sum of positive charges for

Donor ions ND+

7- Semiconductors are used as sensors for environmental factors around them

- Because semiconductors are very sensitive for the surrounding factors (light, pressure, temperature,

radiation pollution )

8- In case of forward-bias connection the diode allows current flow

- Because the external electric field of the battery is in opposite direction of internal electric field of the diode

(at the contact region) so it weakens this field and the thickness of the depletion region decreases which

leads to decrease the potential barrier so electrical current will pass

9- In case of backward-bias connection the diode prevents the current flow

- Because the external electric field if the battery is in the same direction of internal electric field of the diode

(at the contact region) so it strengthens this field and the thickness of the depletion region will increase

which increases the potential barrier so electric current will not pass

10- Diode can be considered a circuit switch “key”

- Because in case of forward–bias connection current will flow “closed switch” and in backward-bias

connection current cannot flow “open switch”

11- Diode and electrical resistance are different

- If we used Ohmmeter in measuring electrical resistance ,reading doesn’t change if the current is reversed,

but in diode the current flows in one direction so reading of Ohmmeter is high in a specific direction of

current and it changes to zero if the current flows in the opposite direction

12- The effect of high temperature is different for diode and ohmic resistance

- By raising temperature more covalent bonds are broken which leads to increase number of free electrons

which increase the conductivity “free electrons” and decrease resistance, but in ohmic resistance by

increasing temperature its resistivity increase and conductivity decrease

13- Ohmmeter is used to check the diode operability

- Diode operation is perfect if it allows current in a specific direction (forward-bias) and if it prevents current

in the opposite direction (backward-bias)

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14- Diode is used in rectifying the direction of AC current

- Because diode allows current flow in one direction which is considered the first half of AC current cycle in

case of forward bias connection but it prevents the current in the other direction which is considered the

second half of AC cycle so the resultant voltage is uni-directional

15- The thickness of base should be very small in transistors

- To decrease the consumption of electrons that populates holes existing in base, this will allow the majority

of electrons to reach collector and the value emitter current to be near the value of collector current

16- Transistor is used as a switch “key”

- When connecting transistor “npn” in the common emitter circuit, so if the base voltage is positive or high

,current will flow in collector and transistor is considered a closed key “ON”, and if the base voltage is

negative or low the collector current is cut off so transistor is considered an open key “OFF”

21- Distribution constant “factor” αe is approximately 1 and the amplification constant “factor” βe is very high

- Because base is very thin and has a little impurities so it consumes a very small part of emitter current so

emitter current is approximately equal collector current and αe = IC/IE is approximately 1 , and base current is

very small βe= Ic/IB so βe is very high

22- Digital electronics is preferred more than analog electronics

- In digital electronics we can get rid of random currents and noise which resulting in random motion of

electrons of different voltage values(1,2,3,4,5,6,…) and digital electronics only deals with two values 0,1

So digital electronics produce a pure and regular signals

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Laws

Transistor

As a key

VCC= VCE + ICRC

As an amplifier:

Amplification Factor

βe = IC/IB = αe / (1 - αe)

Distribution Factor

αe = IC/IE = βe/ (1 + βe)

Emitter Current

IE = IC + IB

(AND) means: multiply (2 or more I/Ps and 1

O/P)

(OR) means: add (2 or more I/Ps and 1 O/P)

(NOT) means: reverse value (1 I/P and 1 O/P)

Laws

N-Type Crystal

Electrons concentration

n = ND+

Holes concentration

p = ni2/ND

+

Crystal returns to stable state when

NA-= ND

+

P-Type Crystal

Holes concentration

p = NA-

Electrons concentration

n = ni2/NA

-

Crystal returns to stable state when

NA-= ND

+

Mass Action Law

np = ni2

Where ni2 is the concentration of

electrons or holes in the pure silicon

crystal