Majority Charge Carrier & Minority Charge Carrier

Thermally Generated Charge Carriers

• There are no free charge carriers at 0 ^oK in the pure silicon / Germanium. However, when the room temperature increases, some of the covalent bonds are broken by heat energy. 
• This will result in electron – hole pairs are produced; it is called as thermally generated charge carriers. 

Mobile Charge Carriers & Immobile Ions

The concept of mobile charge carriers and immobile ions is explained here by P type semiconductor material and N type semiconductor materials

Mobile Charge Carriers and Immobile Ions in P type semiconductor

• The P type semiconductor material is formed by addition of acceptor type impurity atoms ( gallium, indium, aluminum, boron ) to pure silicon / germanium crystals. 

Mobility

 

The conductivity arises form mobile charge carriers of the semiconductor. 

Types of matter	Mobile charge carriers
Metal	Electrons
Ionized gas	Electrons & positive charged ions
Electrolyte	Position ions and negative ions

 

 In this theory, atomic model of Bohr is explained here.

Bohr’s Atomic Model

The atomic model is proposed by Bohr in 1913. The Bohr has made following assumptions.

• The atom has massive positive charged nucleus. 
• The electrons revolve round their nucleus in circular orbits. 

Atoms, Molecules & Ions

In this theory, definitions of atoms, molecules, monoatomic, diatomic, ions, anion and cations are given.

Atoms, Molecules and Ions

Atoms

• The smallest particle of any object is called as atoms. 

Atom

• According to Bohr , an atom is composed by number of electrons moves in circular or elliptical orbits around the center nucleus. 
• The nucleus consists of protons and neutrons.
• Mass of electron = 9.1 × 10 ^{– 31} kg
• Charge of electron = 1.6 × 10 ^{– 19} coulomb

Diffusion

Definition of Diffusion

• It is defined as the flow of charge from high density region to low density region without any external applied field. 
• The flow of charge is due to non - uniform distribution of charge carriers in the semiconductor crystal. 
• This will result in flow of current without any external applied field.

Drift & Drift Velocity

The directed motion of the charge carriers ( electrons + holes ) in the semiconductor done mainly by ( 1 ) Charge drift ( flow ) under the influence of electric field ( 2 ) Charge drift from high charge density to low charge density

Effect of Electric Field on Semiconductor Material

Semiconductor Material: No Electric Field

• When electric field is not applied to the semiconductor material at a temperature above 0 ^oK, the electrons as well as holes move randomly and collide with each other and other fixed ions within the crystal. 
• The net velocity of the charge carriers in any direction is equal to zero therefore no current flows through the crystal.

Semiconductor Material: Electric field Applied

• When electric field applied to the semiconductor, the charge carriers move in directed motion. 
• This will result in net velocity of charge carriers is called as drift velocity in the direction of applied field. 
• The electrons and holes move in the opposite direction but both produce current in the same direction due to their opposite charges.

Drift Velocity Formula

The drift velocity is directly proportional to the electric field E. The proportionally is called as mobility ( µ )

v α E

v = µ E    ……. ( 1 )

Where v = drift velocity ( meter / second )

           E = Electric field ( voltage / meter )

           µ = Mobility ( meter² / voltage – second )

Current Density Due to Charge Carriers

( 1 ) Current density due to electron drift

 J_e = e µ_e n E

 Where

       µ_e = Electron Mobility

       E = Electric field

       n = Electrons

( 2 ) Current density due to hole drift

 J_h = e µ_h p E

 Where

       µ_h = Hole Mobility

       E= Electric field

       p = Holes

Total current density due to electrons and holes carriers

J = J_e + J_h

  = e µ_e n E + e µ_h p E

  = eE ( µ_e n + µ_h p )

Drift Current

It is defined as the average velocity attained by the charge particles due to applied electric field.

I = envA  …… ( 2 )

Where

      e = Electron charge ( Coloumb )

      v = Electron drift velocity ( meter / second )

     A = Cross section area of conductor

      n = Number of free electrons per unit volume of conductor

            ( /meter³ )

from equation ( 1 ) and ( 2 )

I = enA ( µ E )

Where

      E = Electric field ( voltage / meter = V / L )

Therefore I = enAµ ( V / L )

          V / I = ( 1 / neµ ) L / a

               R = ( 1 / neµ ) L / a

Compare this equation with R = ρL / a

⸫ Resistivity ρ = ( 1 / neµ ) ohm – meter

   Conductivity σ = neµ  ( 1 / ohm – meter )

Summary

Drift velocity v α E                        v = µ E

Current density due to holes and electrons J = eE ( µe n + µh p ) Where µe = Electron Mobility µh = Hole Mobility n = Number of electrons P = Number of holes E = Electric field per meter e = Electric charge

Drift Velocity v = I / enA Where e = Electron charge ( Coulomb ) v = Electron drift velocity ( meter / second ) A = Cross section area of conductor n = Number of free electrons per unit volume of conductor ( /meter3 )

Resistivity ρ = ( 1 / neµ ) ohm – meter Conductivity σ = neµ  ( 1 / ohm – meter )

 

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Compare Silicon & Germanium

What is hole current?

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Single Phase Full Wave Bridge Rectifier

In this theory, the working of single-phase full wave bridge rectifier is given.

Bridge Rectifier

Bridge Rectifier Circuit Description

• There are four diodes D1, D2, D3 and D4 connected in the bridge configuration. 
• There is no need of center tapping transformer as that is required in the single-phase half wave rectifier. 
• The alternating supply through step down transformer is given to diagonally opposite end AC and load is connected to other two ends BD.

Working of Bridge Rectifier

Positive half cycle

• The end P of the secondary winding of transformer becomes P whereas S becomes negative during positive half cycle of the alternating supply.
• This will make diodes D2 and D4 forward biased and current flows through them. 
• The current flowing through path P – A – D2 – B – Q – LOAD – R – D – D4 – C – S. 
• As there are two diodes D2 and D4 conducts and both are in series, the voltage available at the load is equal to secondary winding voltage minus voltage drop in the diodes D2 and D4. 
• The diode D1 and D3 remains in the reverse biased condition during positive half cycle of the alternating supply.

Load voltage = Transformer secondary voltage

                      – Voltage drop across diode D2

                      – Voltage drop across diode D4

Voltage drop across diode = 0.7 V for Silicon

                                           = 0.3 V for Germanium                      

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Negative half cycle

• The end S becomes positive with respect to end P of the transformer secondary winding. 
• This will make diode D1 and D3 forward biased and current flows through them. 
• The current flowing through path S – C – D3 – B – Q – LOAD – R – D – D1 – A – P. 
• As there are two diodes D1 and D3 conducts and both are in series, the voltage available at the load is equal to secondary winding voltage minus voltage drop in the diodes D1 and D3. 
• The diode D2 and D4 remains in the reverse biased condition during positive half cycle of the alternating supply.

Load voltage = Transformer secondary voltage

                      – Voltage drop across diode D1

                      – Voltage drop across diode D3

Advantages of Bridge Rectifier

• No need of centre tapping transformer
• The peak inverse voltage is one half to that of single phase full wave
• Rectifier
• Low ripple factor

Disadvantages of Bridge Rectifier

• Four diode requires
• There are two diodes conducts which are in series during positive and negative half cycle of the alternating supply. 
• The output voltage is low as compared to single phase full wave rectifier because of voltage drop in the two diodes.

Why is it called as single-phase full wave bridge rectifier?

Single phase – The single-phase supply is given at the input side

Full wave – Diodes conduct during both positive and negative cycle   

                    of the input supply

Bridge – Four diodes are connected in the bridge configuration

Rectifier – It converts the input AC power into output DC power.

Rectifier Efficiency & Power Efficiency

In this theory, the concept of rectifier efficiency and power efficiency in the rectifier is given.

Example of rectifier efficiency and power efficiency

The input of the single phase half wave rectifier is 60 W whereas the output is 24 W. Find out rectifier efficiency and power efficiency.

Solution

Rectifier efficiency

    = ( Output DC Power / Input AC Power ) × 100%

    = ( 24 / 60 ) × 100%

Single Phase Full Wave Rectifier

In this theory, circuit explanation and working of full wave rectifier is explained here. As the rectifiers conducts during both positive and negative half cycle of the input alternating supply, it is called as full wave rectifier.

Full Wave Rectifier Circuit Description

• As the rectifiers conducts during both positive and negative half cycle of the input alternating supply, it is called as full wave rectifier.

Single Phase Half Wave Rectifier

 

• As the rectifier conducts only during positive half cycle of the alternating supply, it is called as half wave rectifier. 
• There is no current flows during negative half cycle of the alternating supply and the voltage across load is equal to supply voltage during positive half cycle and zero during negative half cycle.

Compare-Class A, Class B & Class C Power Amplifier

In this theory, comparison of Class A, Class B and Class C Power Amplifier on the different parameter basis is given. The comparison of the Class A, Class B and Class C Power amplifier is done on the basis of transistor bias and amplitude of the input signal.

RC Coupled Amplifier

Why is it called as RC Coupled Amplifier?

• The output of the first stage is connected to input of the second stage by coupling capacitor and shunt resistor therefore it is called as RC coupled amplifier. 
• The output of the amplifier is inverted if there is odd number of stages like 1, 3, or 5. 

Practical Amplifier Circuit

 

The practical amplifier circuit for faithful amplification is shown in the figure. The function of the each circuit element is described as below.

Biasing circuit

• The resistance R1, R2 and emitter resistance R_E provides biasing and stabilization circuit. 

Direct Coupled Amplifier

Why is it called as direct coupling?

• The output of the first stage is connected to input of the second stage without any coupling device therefore it is called as direct coupling amplifier.

Circuit Diagram

• The circuit diagram of three stage direct coupled amplifier is shown in the Figure. 

Common Emitter Transistor Amplifier

 

• The common emitter transistor amplifier is shown in the figure A. 
• The input signal is applied between base and emitter whereas the output is taken from collector and emitter. 

• The output voltage equation V_CE is given by

        V_CE = V_CC – i_cR_C

Voltage Gain & Bandwidth

 Gain of the Amplifier

• The ratio of the output quantity to the input quantity of the amplifier is called as Gain of the Amplifier.

Gain of Multi Stage Amplifier

• Let us consider three amplifiers having respective gain of G1, G2 and G3. The total gain of the amplifier is

Transformer Coupled Amplifier

 Disadvantages of RC Coupled Amplifier

• The main disadvantage of RC Coupled amplifier is that it consists of low voltage gain and low power gain.
• The input impedance of the amplifier is low whereas the output impedance is high. 
• When the output impedance of first stage is comes with in parallel with low impedance of input stage of second stage in the multistage amplifier circuit, the effective load is decreased. 

Multivibrator Interview Question Answer

What is meaning of the switching transistor?

Switching transistor : When a transistor is used as switch, it is called as switching transistor.

Give reason: The transistor has high efficiency as a switch in the OFF and Saturation condition.

Transistor OFF state

The collector leakage current flow through transistor during its OFF state therefore the power loss is small as compared to full load current. Due to this reason its efficiency is high during OFF state.