Unit IX: Electronic Devices

Electronic Devices introduction

Electronic Devices : Energy bands in conductors, semiconductors and insulators (qualitative ideas only) Semiconductor diode – I-V characteristics in forward and reverse bias, diode as a rectifier;
Special purpose p-n junction diodes: LED, photodiode, solar cell and Zener diode and their characteristics, zener diode as a voltage regulator.
Junction transistor, transistor action, characteristics of a transistor and transistor as an amplifier (common emitter configuration), basic idea of analog and digital signals, Logic gates (OR, AND, NOT, NAND and NOR).

Chapter–14: Semiconductor Electronics: Materials, Devices and Simple Circuits

1. Energy bands in solids

The energy bands in solids are very helpful in the study of behavior of the solids. As according to energy band theory when two atoms are brought together, then they experience interaction between the valance electrons, due to this interaction the splitting of outer energy level takes place and forms valance band and conduction band. The formation of energy bands in case of Si is as shown in fig.

Some terms related to band theory semiconductor.

(i)  Valance band: – The lower energy band occupied by most of the electrons and highest filled is called valance band

(ii)  Conduction band: – The upper energy band occupied by least electrons and partially filled is called conduction band.

(iii)   Forbidden energy gap:-The valance band and conduction band are separated by a certain energy gap; electron cannot exist in this gap. So it is called forbidden energy gap.

(iv) Fermi energy: – the maximum energy possessed by the electrons at zero Kelvin is called Fermi energy   

2 Energy gap in conductor, insulator and semiconductors 

Conductor, insulator and semiconductors on the basis of band theory are explained as below.

Conductors:-

In case of conductors, valance band and conduction band are overlapped with each other. There is no any forbidden gap between them so electron can easily jump from valance band to conduction band. Hence metals are good conductor of electricity. The resistivity of metals lies between 10^-2 to 10^-8 Ωm and conductivity lies between 10² to 10⁸ Sm^-1

Insulators:-

In case of insulator, there is large forbidden gap between valance band and conduction band (E_g>3eV), so electron cannot jump from valance band to conduction band. Hence insulators are bad conductor of electricity. The resistivity of insulator materials is about 10⁸ Ωm and conductivity of semiconductor materials is about 10^-8 Sm^-1

Semiconductors:-

In case of semiconductor, there is small forbidden gap between valance band and conduction band (E_g˂3eV), so electron can easily jump from valance band to conduction band, when a sufficient amount of energy is provided to the electron. The resistivity of semiconductors lies between 10⁵ to 10⁰ Ωm and conductivity lies between 10^-5 to 10⁰ Sm^-1. Here we can see that the resistivity and conductivity of semiconductors lies between metals and insulators.

• At 0K all the semiconductor materials behave as insulators, because all the electrons lie in the valance band. But at room temperature electron acquire some thermal energy and sift to conduction band so at room temperature semiconductor materials starts conducting.
• Si and Ge are the examples of semiconductor materials having band gaps Si=1.1 eV & Ge=0.72eV

• The conductivity of metals decreases with rise in temperature because of resistance created by moment of +vely charged ions while conductivity of semiconductor material increases with increase in temperature because electron may easily jump from valance band to conduction band at high temperature.

Classification of semiconductor materials on the basis of their chemical composition

• Elemental semiconductors e,g Si and Ge
• Compound semiconductors e,g inorganic semiconductors :- CdS , GaAs , CdSe , InP etc
• Organic polymers: polypyrrole, polyaniline, polythiophene etc.

Now a day the compound semiconductors are widely used.

3. Types of the semiconductors:-

There are of two type of the semiconductors (I) intrinsic semiconductor (ii) extrinsic semiconductor

(1) Intrinsic semiconductors

The pure form of the semiconductor is called intrinsic semiconductor. Silicon and germanium are the examples of pure semiconductors. We will study the conductivity in Si semiconductor. The electronic arrangement of Si is 1s²2s²2p⁶3s²3p².

It has four valence electrons, each form a covalent bond with its four neighboring atoms as shown in the figure. At absolute zero temperature, Si and Ge behave as insulators.  At room temperature, some electrons   jump from the valance band to the conduction band and produce a vacant space which is known as a hole. Hole has the ability of attracting electrons and the randomly moving free electron can get fixed in the hole.

Thus hole behaves as a positive charge though it is neither a real particle nor has any positive charge. The number    of free electrons

( ne )  and  holes  ( nh )  in  a  pure  semiconductor are same. So we can say that both hole and electrons are the charge carriers in intrinsic semiconductor.

4.Doping

The intrinsic semiconductor has a very low conductivity, so to increase the conductivity some impurities are added in the semiconductor. The process of adding the impurity is called doping.

• 1ppm of impurity (dopants atom) may increase the conductivity of semiconductor up to 10³

Essential requirements for a doping process:-

• The semiconductor material should be highly pure
• the size of the dopant atom should be comparable to semiconductor atom.
• The dopant atom should not destroy the crystal lattice.
• The concentration of the dopant atom should be small (in part per millions)

Types of the dopant:-

1 Pentavalent impurity: the 15 nth group element of the periodic table are called pentavalent impurities. For examples As, Sb, P

2 Trivalent impurity The 13 nth group element of the periodic table are called trivalent impurities. E,g:- B, Al, and In

Methods of the doping

1. by heating the pure semiconductor material in the environment of the impurity atom
2. by melting together

• by bombarding the impurity atom on pure semiconductor

5. Extrinsic Semiconductors (N and P type Semiconductors)

When the impurity is added in the pure semiconductor then the formed semiconductor is called extrinsic or impure semiconductor. Extrinsic semiconductors are of two types 1 P-type 2 N-type

• P type semiconductor

When trivalent (13-group) impurity is added in the pure semiconductor (14-group) then the formed semiconductor is called p type semiconductor. e,g :- SiAl

When Al is added into the Si then p type semiconductor is formed.

In Al three electrons are present in the outer most orbits, when it forms covalent bonds with four Si atoms then , Si show a deficiency of electron, this deficiency can be filled by breaking a covalent bond  hence holes & electrons  are created, electron shift toward deficiency and hole can be  filled by breaking another covalent bond in the semiconductor.  In this way holes seems to be moving in the semiconductor. Hence holes are the majority charge carriers in P type semiconductor.

• The accepter energy level is present just above the valance band in this semiconductor. In P type semiconductor a accepter impurity create a hole in valance band, which can be filled by a electron of covalent bond by gaining a very small energy (about 0.01-0.05 eV in Si). Thus accepter energy level is present just above the valance band in this semiconductor
• N type semiconductor

When Pentavalent (15-group) impurity is added in the pure semiconductor (14-group) then the formed semiconductor is called N type semiconductor.

For example when Sb is added into the Si then N type semiconductor is formed. In Sb five electrons are present in the outer most orbits, when it forms covalent bonds with four Si atoms then one electron of Sb remains free.  This electron may hit a covalent bond and removes a electron, the ejected electron may hit to another electron and the process is carried out again and again and conductivity of the semiconductor increases due to motion of electrons hence  we can say that electrons are the majority charge carriers.

The  free electron have energy level just below the conduction

band. So we can say that the donor energy level lies just below the conduction band.

4. Electrical conductivity in semiconductors

Let us consider a semiconductor material of length l area of cross section A, also suppose that n_e and n_h are the number of electrons and holes then current due to them can be given as

                              I_e =n_eeAv_e     

                      &     I_h = n_heAv_h

So the total current I= n_eeAv_e+ n_heAv_h  = Ae(n_ev_e+ n_hv_h)………………………(1)

      As                             R=ρ     &      I=                            

                               ⇒     I=      ⇒  I=     ⇒  I=    ……………………….(2)   

Comparing eqⁿ (1)& (2) we get

                                                       = Ae(n_ev_e+ n_hv_h)

                                                       =   = e(n_{e +} n_h )

                                                       =   = e(n_{e +} n_h )

                                                      Or     σ=   = e(n_{e +} n_h )

Hence conductivity in semiconductor is due to flow of both the electrons and holes.

Difference between Intrinsic & Extrinsic semiconductor

Difference between P Type & N Type semiconductors

SOME IMPORTANT POINTS

*Electron is more mobile than hole.

*Ohms law is not obeyed in semiconductors.

*C is not semiconductor because of E_g=5.54eV.

*Both p type & n type semiconductor are electrically neutral.

*N type is more mobile than P type semiconductor.

             9(b) semiconductor devices: electronic devices

5 P-N Junction

When a p type semiconductor is joined with a n type semiconductor by a suitable means then the formed device is called pn junction.

As soon as the pn junction is formed then the electron starts to diffuse toward p region where they combine with holes and get neutralized. Similarly holes moves toward n regions where they combine with the electrons and get neutralized. This process is called electron hole combination.

Due to this electron hole combination near the junction,   the  concentration of  –ve charge increase near the p region  and concentration of +ve charge increase near the n region, this  concentration of opposite  charge act as a battery called fictitious battery or depletion layer 0r junction barrier.  This junction act as barrier for the flow of minority charge carrier from p to n and from n to p. in case of Si & Ge the value of junction barrier is 0.7 eV & 0.3 eV  respectively. Here it should be noted that larger the doping concentration of the impurities atom, the depletion layer width will be small and barrier field will be strong and vice versa. Thus by simply changing the doping concentration we can obtain different type of semiconductor.

Circuit symbol of a pn junction diode

A pn junction diode may be represented as an arrow. The direction of the arrow is from p region (anode) to n region (cathode). Here the arrow shows the direction of flow of the conventional current.

6 Forward and reverse biased pn junction diode.

1. Forward Bias:-

A pn junction diode is said to be forward biased if positive terminal of the battery is connected to p region and negative terminal of the battery is connected to n region. As shown in the fig.

When pn junction diode is forward biased then the electron  from n region and hole from p region starts moving toward  junction barrier and the width of depletion layer starts  decreasing at a certain value of applied voltage  (knee voltage)the depletion layer disappears, at this time maximum current flow in the diode, this current is called forward current.

1. Reversed bias:-

A pn junction diode is said to be reversed biased if positive terminal of the battery is connected to n region and negative terminal of the battery is connected to p region. As shown in the fig.

When pn junction diode is reversed biased then the electron from p region and hole from n region starts moving away from junction barrier and width of depletion layer starts increasing, at  a certain value of applied voltage (zener voltage) the depletion  layer breaks up and no current flow in the diode, due to majority  charge carriers however small reverse current flow due to  minority charge carriers. This current due to minority charge carrier is called reverse current or minority current or leakage current.

7 Characteristic of the pn junction diode:-

The variation of the current and applied voltage across the junction diode is known as characteristic of the pn junction diode.

1. Forward biased characteristics:-

The graphical relation between forward biased voltage and forward current is called forward biased characteristics.

In case of forward biasing with rise in forward voltage, initially small current flow through the circuit up to threshold voltage or knee voltage, the value of knee voltage for Si is 0.7eV and for Ge the value is 0.2v, but after knee voltage the current starts increasing rapidly and diode have a small resistance.

2. Reversed biased characteristics:-

The graphical relation between reversed biased voltage and reverse current is called reversed biased characteristics.

In case of reverse biased pn junction diode with increase in reverse voltage the majority charge carrier moves away from the junction due to which width of depletion layer increases, at this stage the resistance of the pn junction becomes very large hence no current flow through the junction due to majority charge carriers.

However small current flows through the junction due to the minority charge carriers through the junction in a direction to the applied voltage. This small current through the junction is called reverse current or leakage current. At a certain value of applied voltage (zener voltage) the depletion layer breaks up at this point the maximum reverse current flows through the diode called reverse saturation current.

• PN junction diode is highly current sensitive because when a large amount of current passes through the diode then large amount of current produces in the diode which may damage the diode thus pn junction diode is highly current sensitive

Static and dynamic resistance of a junction diode

D.C or static resistance:-It is defined as the ratio of the d.c. voltage across the diode to the dc current flowing through it.

A.C. or Dynamic resistance:- It is defined as the ratio of the small change in voltage to the corresponding small change in current in the diode.

8 Junction diode as a rectifier:-

A device which converts alternating current into direct current is called rectifier

Principle: it is based on the principle that diode conduct when forward biased and does not conduct when reversed biased

Rectifier is of two types: – 1.  Half wave rectifier 2.  Full wave rectifier

1. Junction diode as half wave rectifier:-

The rectifier which converts half of ac into dc is called half wave rectifier as shown in fig, it consist of two coils primary coil (P)and secondary coil(s).  The   primary coil is connected to a source of alternating voltage and the secondary coil is connected to the diode through a load resistance across which we can obtain the output.

The working of half wave rectifier can be explained in two steps

• For +ve half of the input signal

Suppose for the positive half wave of input signal the terminal p₁   of primary coil  is at negative potential and p₂ is at positive  potential  due to this the terminal s₁ of the secondary coil becomes positive and s₂ becomes negative .In  this case the diode becomes forward biased ,hence diode will conduct so we will obtain output . As shown in fig

• For –ve half of input signal

Suppose for the positive half wave of input signal the terminal p₁   of primary coil is at positive potential and p₂ is at negative potential due to this the terminal s₁ of the secondary coil becomes negative and s₂ becomes positive .In this case the diode becomes reversed biased, hence diode will not conduct so we will obtain no any output for the –ve half of input signal. As shown in fig

Thus the output obtained by the half wave rectifier is dc and frequency of the output wave is same as that of ac.

1. Junction diode as Full wave rectifier:-

The rectifier which converts both half of ac into dc is called full wave rectifier, as shown in fig.

In full wave rectifier we uses two diodes instead of a single one, it consist of two coils primary coil (P) and secondary coil(s).  The   primary coil is connected to a source of alternating voltage and the terminals of the secondary coil are connected to two diodes through a load resistance across which we can obtain the output.

The working of full wave rectifier can be explained in the following two steps.

(i) For +ve half of the input signal

Suppose for the positive half wave of input signal the terminal p₁   of primary coil is at negative potential and p₂ is at positive potential, due to this the terminal s₁ of the secondary coil becomes positive and s₂ becomes negative .In this case the diode D₁ becomes forward biased ,hence  this diode will conduct so we will obtain output . As shown in fig

(ii) For –ve half of input signal

Now again suppose for the positive half wave of input signal the terminal p₁   of primary coil is at positive potential and p₂ is at negative potential ,due to this the terminal s₁ of the secondary coil becomes negative and s₂ becomes positive .In this case the diode D₂ will  conduct so we will again obtain  output for the –ve half of input signal . As shown in fig

Thus the output is obtained both for the positive and negative signals of the input so the diode is called full wave rectifier.

9 Special types of pn junction diode

1. Zener diode:-

A properly (heavily) doped pn junction diode which may works in breakdown reason during reverse biasing is called zener diode. The symbolic representation of zener diode is as given below. Here the starting point of the arrow represents the p type semiconductor and the end point represents the n type semiconductor.

Zener diode as a voltage regulator:-

Principle: when a diode is operated in the reverse breakdown region, the voltage across it remains practically constant for a large change in reverse current.

The circuit diagram of the zener diode as a voltage regulator is as shown in the fig.

The zener diode works only on or above the breakdown voltage. Now if voltage across the zener diode is larger than the reverse break down voltage then it acts as the voltage regulator. As explained below.

1. When the input voltage is equal to the reverse break down voltage then the diode will not conduct and voltage obtained across the diode will be equal to breakdown voltage.
2. But on increasing the input voltage, the resistance of the diode decreases so that the current increase in the same ratio such that the voltage across the zener diode remains constant. Hence Zener diode as a voltage regulator

1. Tunnel diode:-A diode having very small barrier width and very high impurity concentration in both p and n region is called tunnel diode
2. Photo diode:-

A photosensitive diode which converts light energy into electrical energy called photodiode

When light on the semiconductor then the electron absorb the light energy and jump to conduction band leaving behind a hole in the valance band, thus electron hole pair is created when light fall on the semiconductor. This electron hole pair constituent photocurrent

Uses:-

The photo diode are used to detect the radiation, in optical signals

1. Light emitting diode (LED):-

The spacial purpose PN junction diode which converts electric energy into light energy are called light emitting diodes.

In light emitting diode the p type semiconductor as upper layer is deposited by diffusions on N type semiconductor layer. As shown in fig.

When LED is forward biased then electron falls from conduction band to valance band and emits light of differnent radiations depanding upon the type of materials used to construct diode.

GaAs produces infrared radiation, GaP produses red or green light, GaAsP produses red or yellow light.

Circuit symbol of LED

Uses:-

• These are used in digital display, indicator lamps.
• These are used in calculators and watches.
• These are used in optical communications ,remote control , optical mouse for computers, in traffic lights , in burglar alarm etc

1. Solar cell:

A pn junction diode which converts light energy into electrical energy is called solar cell. It is made up of PN junction diode whose P region is very lightly doped and N region is heavily doped. And photosensitive metals are placed in contact of both P and N region. The upper p region is covered with a transparent glass plates. As shown in fig.

When a light fall on the PN junction diode then hole from the N region diffuses toward the junction and electron from the P region diffuse toward junction. At junction electron hole combination takes place and gives electric current in the external circuit which can be flow through the load resistance.

Solar cell symbol

Uses of solar cell

These are used in street light, solar geezers, satellites, for charging the batteries, etc

10 Junction transistor:-

A two junction three terminal device formed by doping a thin layer of one type semiconductor into two thick layer of other type of semiconductors is called junction transistor. The transistor is used to transfer the resistance.

There are three terminals of the transistor as given bellow

1. Emitter
2. Base

• Collector

Emitter:–The terminal of the transistor which is heavily doped and emits the charge carriers is called emitter.

Base:-The terminal of the transistor which is lightly doped and controls the flow of the charge carrier from emitter to collector is called base.

Collector:-The terminal of the transistor which is moderately doped and collects the charge carrier is called collector.

Junction transistor is of two types:-

• NPN Transistor:-

When P type semiconductor is doped between two n type semiconductors than NPN transistor is formed. The circuit diagram and symbol of the NPN transistor is as shown in fig.

Working of NPN transistor:-

When base emitter junction of transistor is connected in forward biased and the collector base junction is connected in reverse biased then transistor will start working.

Due to forward biasing of the emitter base junction the electron from the N region of the junction move toward base .At base 5% electron whole combination takes place which causes 5% base current, remaining electron move toward collector terminal which is reversed biased, at where electrons are neutralized by the positive potential of Vcc and results into 95% collector current.

During this process the deficiency of holes in P region is fulfilled by breaking a covenant bond. By which a pair of whole and electron is created. Whole remains in P region and electron move in the external circuit toward V_EE.

The total current in the circuit can be given by the Kirchhoff’s law as

I_e =I_c +I_b

(ii)PNP transistor:-

When N type semiconductor is doped between two P type semiconductors than PNP transistor is formed. The circuit diagram and symbol of the PNP transistor is as shown in fig.

Working of PNP transistor:-

When base emitter junction of transistor is connected in forward biased and the collector base junction is connected in reverse biased then transistor will start working.

Due to forward biasing of the emitter base junction the hole from the P region of the junction move toward base . At base 5% electron whole combination takes place which causes 5% base current, remaining holes move toward collector terminal which is reversed biased at where holes are neutralized by the electrons coming from Vcc and results into 95% collector current.

During this process the deficiency of holes in emitter region is fulfilled by breaking a covenant bond. By which a pair of whole and electron is created. Hole moves toward base and electron move in the external circuit toward V_EE.

The total current in the circuit can be given by the Kirchhoff’s law as

I_e =I_c +I_b

1. Why in transistor base is doped thin and light?

Ans. Because the number of carrier in the base should be small so that only small combination of electron and holes can takes place and collector current should not decrease

1. in transistor reverse biasing is high as compared to forward biasing. Why?

Ans. Because it increase the attraction to enter the charge carriers in the collector regions

11 Transistor configuration

There are three type of transistor configuration

In common base configuration base is common in both input and output, in common emitter configuration emitter is common in both input and output, in common collector configuration collector is common in both input and output as shown in fig.

12 Characteristic of transistor:-

To study the characteristic of a transistor, transistor should be connected in a circuit in which one terminal is common to other two. There are basically three types of transistor characteristics

 Common emitter transistor characteristics:-

The graph between voltage and current, when emitter of transistor is common to both input and output circuits is known as common emitter characteristics of a transistor.

Common emitter characteristics are of three types:-

1. Input characteristics:-

The variation of input current I_b with input voltage V_BE at constant output voltage V_CE, called input characteristics.

In common emitter configuration input current increases with increase in the input voltage keeping output voltage constant, larger the output voltage more is increase in input current. As shown in fig.

Input resistance:-

The ratio of small change in V_BE to the small change in I_b is called input resistance

As      r_o=d V_BE/ dI_b      at constant Vc

Output characteristics:-

The variation of output current I_c with output voltage V_CE at constant input current Ib is called output characteristics.

In common emitter configuration with rise in output voltage, output current increases and becomes constant and remains always less than the constant input current. As shown in fig.

Output resistance:–The ratio of small change in output voltage V_CE to the small change in collector current Ic is called output resistance.

As                at constant I_b

Transfer characteristics

The graph between the input current I_b and output current I_c is called transfer characteristics. As shown in diagram with increase in input current output current increases. The current gain in the common emitter transistor may be calculated as

at constant V_c

14 Common base transistor characteristics:-

The graph between voltage and current, when base of transistor is common to both input and output circuits are known as common base characteristics of a transistor.

Common base characteristics are of three types:-

Input characteristics:-

The variation of input current I_e with input voltage V_BE at constant output voltage V_CE, called input characteristics.

In common base configuration input current increases with increase in the input voltage keeping output voltage constant; larger the output voltage more is increase in input current. As shown in fig.

Input resistance:-

The ratio of small change in V_BE to the small change in I_e is called input resistance

As      r_i=d V_BE/ dI_e      at constant Vc

Output characteristics:-

The variation of output current I_c with output voltage V_CB at constant input current Ie is called output characteristics.

In common base configuration with rise in output voltage, output current increases and becomes constant and remains always less than the constant input current. As shown in fig.

Output resistance:-

The ratio of small change in output voltage V_CE to the small change in collector current Ic is called output resistance.

As                at constant I_e

Transfer characteristics

The graph between the input current I_e and output current I_c is called transfer characteristics. As shown in diagram with increase in input current output current increases. The current gain in the common base transistor may be calculated as

at constant V_c

15 Transistor as an amplifier:-

A device which increases the amplitude of the input signal is called amplifier. Amplifier is made by the use of transistor. Amplifiers are of three types

1. Common base transistor amplifier:-
2. Common emitter transistor amplifier
3. Common collector transistor amplifier

16 Common base transistor amplifier:-

Common base transistor is of two types

1Common base NPN transistor as an amplifier:-

In this amplifier the signal is applied between emitter and base terminal and the output signal is taken collector and base terminal

Principle of amplifier

When base emitter junction of the transistor is connected in forward biased and base collector junction is connected in reverse biased then common base transistor act as an amplifier.

Working of amplifier

According to Kirchhoff’s rule the output voltage of the amplifier may be given as

Vo= Vcc-IcR

The working of the transistor may be explained in the following two steps.

1. For positive half wave of the input signal

When positive half of input is applied across the base emitter junction of the amplifier then this junction becomes reversed biased. Due to which input current decreases so that collector current also decreases. Now according to the above equation high output is obtained.

2. for negative half wave of the input signal

When negative half of input is applied across the base emitter junction of the amplifier then this junction becomes forward biased. Due to which input current increases so that collector current also increases. Now according to the above equation high output is high.

Here we can see that both input and output are in same phase in case of common base transistor.

Current gain: The ratio of the collector current to the emitter current is called current gain of the common base amplifier.

I,e

Voltage gain: The ratio of the collector base voltage to the emitter base voltage is called voltage gain

I,e

Power gain: The ratio of the output power to the input power is called the power gain.

I,e

2 Common base pnp transistor as an amplifier:-

In this amplifier the signal is applied between emitter and base terminal and the output signal is taken collector and base terminal

Principle of amplifier

When base emitter junction of the transistor is connected in forward biased and base collector junction is connected in reverse biased then common base transistor act as an amplifier.

Working of amplifier

According to Kirchhoff’s rule the output voltage of the amplifier may be given as

V_o= -V_cc+I_cR_L

The working of the transistor may be explained in the following two steps.

1 For positive half wave of the input signal

When positive half of input is applied across the base emitter junction of the amplifier then this junction becomes forward biased. Due to which input current increases so that collector current also increases. Now according to the above equation high output is obtained.

2 for negative half wave of the input signal

When negative half of input is applied across the base emitter junction of the amplifier then this junction becomes reversed biased. Due to which input current decreases so that collector current also decreases. Now according to the above equation high output is low.

Here we can see that both input and output are in same phase in case of common base transistor.

Current gain: The ratio of the collector current to the emitter current is called current gain of the common base amplifier.

I,e

Voltage gain: The ratio of the collector base voltage to the emitter base voltage is called voltage gain

I,e

Power gain: The ratio of the output power to the input power is called the power gain.

I,e

17 Common emitter amplifier:-

In this amplifier the input signal is applied between base emitter circuit and output is taken from collector emitter circuit

1Common emitter NPN transistor as an amplifier:-

In this amplifier the signal is applied between emitter and base terminal and the output signal is taken collector and emitter terminal

Principle of amplifier

When base emitter junction of the transistor is connected in forward biased and collector emitter junction is connected in reverse biased then common emitter transistor act as an amplifier.

Working of amplifier

According to Kirchhoff’s rule the output voltage of the amplifier may be given as

Vo= Vcc – IcR_L

Where Vo is output voltage, Vcc is common collector supply, Ic is collector current and R_L is load resistance.

The working of the transistor may be explained in the following two steps.

1 For positive half wave of the input signal

When positive half of input is applied across the base emitter junction of the amplifier then this junction becomes forward biased. Due to which input current increases so that collector current also increases. Now according to the above equation low output is obtained.

2 for negative half wave of the input signal

When negative half of input is applied across the base emitter junction of the amplifier then this junction becomes reversed biased. Due to which input current decreases so that collector current also decreases. Now according to the above equation the output is high.

Here we can see that both input and output are in opposite phase in case of common emitter transistor.

Current gain:The ratio of the collector current to the base current is called current gain of the common emitter amplifier.

I,e

Voltage gain: The ratio of the collector emitter voltage to the emitter base voltage is called voltage gain

I,e

Power gain The ratio of the output power to the input power is called the power gain.

I,e

1Common emitter pnp transistor as an amplifier:-

In this amplifier the signal is applied between emitter and base terminal and the output signal is taken collector and emitter terminal

Principle of amplifier

When base emitter junction of the transistor is connected in forward biased and collector emitter junction is connected in reverse biased then common emitter transistor act as an amplifier.

Working of amplifier

According to Kirchhoff’s rule the output voltage of the amplifier may be given as

V_o= -V_cc+I_cR_L

The working of the transistor may be explained in the following two steps.

1 For positive half wave of the input signal

When positive half of input is applied across the base emitter junction of the amplifier then this junction becomes reversed biased. Due to which input current decreases so that collector current also decreases. Now according to the above equation low output is obtained.

2 for negative half wave of the input signal

When negative half of input is applied across the base emitter junction of the amplifier then this junction becomes forward biased. Due to which input current increases so that collector current also increases. Now according to the above equation high output is high.

Here we can see that both input and output are in opposite phase in case of common emitter transistor.

Current gain: The ratio of the collector current to the base current is called current gain of the common emitter amplifier.

I,e

Voltage gain: The ratio of the collector emitter voltage to the emitter base voltage is called voltage gain

I,e

Power gain: The ratio of the output power to the input power is called the power gain.

I,e

18 Transistor as an oscillator

Oscillator is a device which converts DC energy into the AC energy. In oscillator energy oscillates between the electric and magnetic fields.

Principle of an oscillator

When some fraction of output is fed back to the input of amplifier via positive feedback network then constant amplitude oscillation is obtained at output of amplifier which is shown in block diagram below along with the circuit diagram

Working

When base emitter junction of the transistor is connected in forward biased and collector emitter junction is connected in reversed biased then transistor becomes in action.

In oscillator the parallel combination of inductor and the capacitor is connected across the base emitter junction of the transistor. A high inductive reactance inductor L’ is connected in series with Vcc and then connected with collector.

When switch s is closed then low collector current begins to flow through L’ and the changing magnetic flux linked with L and induced emf is produced across it, which charge the capacitor and increase  forward biased condition of base emitter junction simultaneously.

Thus collector current increases to maximum and emf across the inductor decreases to zero. Now again capacitor starts discharging and emf starts increase in inductor in opposite direction which discharge the capacitor till collector current becomes zero. Thus the charging and discharging of  capacitor repeated again and again and constant amplitude oscillation is obtained.

The frequency of oscillation depends on the value of L and C , which can be given by

19 Transistor as a switch

Switch is a device which open or close the circuit. The circuit diagram of common emitter transistor as a switch is as shown in fig.

According to Kirchhoff’s law the input and the output of the transistor may be given as

V_i= I_bR_b + V_be

                   V_o= V_cc – I_cR_L

As when we increase input voltage from zero then output voltage also changes.

In case of Si semiconductor the barrier potential is about 0.6 to 0.7 eV

Case1. If Vi< 0.6 eV then base emitter junction is reversed biased and collector current cannot flow through the base.  I,e   Ic=o

Then from equation 2         V_o=V_cc

Thus for low input, high output is obtained and transistor is said to be in cut of state or we can say the switch is off.

Case2. If 0.6< Vi<1.0 e V then forwarded biased condition of the input junction increases and collector current start flowing through base and gradually increases and becomes maximum at 1 volt.  Now from equation 2, Vo decreases gradually and becomes minimum. In this stage the transistor is said to be in active state.

Case 3 if Vi >1eV then forward biased condition of the base emitter junction is maximum so that collector current is maximum called saturation current . Now from eq. 2  output voltage is minimum. Here the transistor is called in on state.

Thus when Vi is low then output is high and transistor is in off state. And when Vi is high then output is low then transistor is said to be in on state. Thus transistor acts as a switch.

For a better switch the active region must be small.

20 Advantage of semiconductors over vacuum tube:-

Advantage:-

Semiconductors are very small in size

These requires low voltage to operate

These are shock proof

The cost of production is small

Disadvantage:-

Semiconductor device are heat sensitive, they damage due to overheating.

The noise level in semiconductor device is very high.

Semiconductor device have poor response in high frequency range

To read/download class 12 unit 1o communication system  click on the link given below

unit 10 communication system