What is a Transistor? PNP Transistor and NPN Transistor, BJT

(Last Updated On: July 6, 2020)

Transistor

Transistor / Bipolar Junction Transistor BJT:

Transistor is called a Bipolar junction transistor BJT because the function of a transistor is based on both majority charge carriers (F.b) and minority charge carriers (R.b). It is the first condition carrying the transistor as Bipolar junction transistor BJT.

When two PN-junction diodes are placed back to back or opposite to each other, we get a transistor with two junctions. This is the second condition carrying the transistor as a Bipolar junction transistor.

This arrangement gives us two types of Transistors

  • PNP type Transistors and
  • NPN Type transistors

PNP type transistor:

Transistor

 

NPN Type Transistor:

Transistor

For PNP type Transistor a thin layer of N-type Semi conductor is sandwiched between two P-type Semi conductors. Similarly, for NPN-type transistor a thin layer of P-type Semi conductor is sandwiched between two N-type Semi conductors.

Now, each transistor whether if it is a PNP type or NPN type Semi conductor has three regions namely Emitter, Base, and Collector. The structure and Symbol of both PNP and NPN transistors are given below.


Structure of PNP and NPN Transistors:

Transistor

PNP and NPN Transistor Symbol:

Transistor

The Arrow head is always at emitter and shows the direction of conventional current.

For PNP-type transistor the arrow head points from emitter to base hence we can say that emitter is +ive with respect to base.

For NPN type transistor the arrow head points form base to the emitter hence we can say that base is base is +ive with respect to emitter.

Hence every transistor consists of three regions or electrodes (emitter, base and collector), let’s discuss each one of it.



Emitter:

This is the left hand section/region of a transistor. It is heavily doped. The function of emitter is to supply majority charge carries to base. For PNP type transistor emitter supplies holes charges to base while for NPN type transistor emitter supplies electrons to base.

Base:

It is the middle section/region of a transistor it is very thin (10m-16) as compared to emitter and collector. The basic function of base is to control the flow of charges. It is lightly doped.

The emitter base junction is forward biased. The collector base junction is reversed based (having high resistance). As shown in the fig below

Transistor

Collector:

It is the right hand section/region of the transistor or in simple words the region located opposite to the emitter is called collector the function of a collector is to collect majority charge carries through base.

The size of the collector is more as compared to base and emitter because the collector has to dissipate much power.

Collector is not heavily doped like emitter nor it is lightly doper like base but it is doped between emitter and base.

Transistors are made by the process of

  • Growing
  • Alloying
  • Diffusion

In transistor “T” is for transfer and “R” is for Resistance i.e. (T/r is transfer of resistance) It transfers emitter base resistance (low resistance) to collector base resistance (high resistance). Hence we can say t/r transfer low resistance to higher resistance.

Transistor is a current controlled three terminal semi-conductor device. The current is controlled by forward biasing and reversed biasing.


There are two types of circuits

  1. Analog circuits
  2. Digital circuits

In analog circuits the basic function of a transistor is to amplify signal so it is used as an amplifier. While in digital circuits the T/r is used as switch or gate.

Biasing of a Transistor :

For the proper working of a transistor it is essential to supply voltage of correct polarity Across its two junctions, the junctions are

  1. Emitter base junction

  2. Collector base junction

For correct biasing emitter base junction is forward biased and collector base junction is reversed biased. As we have two types of transistor so we will discuss the biasing and working of both the transistors.

Biasing and working of PNP Transistor t/r:

Structure:

Transistor

Symbol:

Transistor

Considering the emitter-base junction as shown in the fig. the +ive terminal of the battery (VEE)is connected with the P-type emitter. So the emitter-base junction is forward biased (having less resistance). The emitter is made up of the P-type semiconductor. And we know that P-type semi-conductor consists of the holes as majority carriers these holes are pushed or pulled into the base due to the +ive terminal (VEE) of the battery as we know similar charges repel each other so the +ive terminal of the battery will repel the holes toward the base and due to this repulsion holes emitter current “IE”. The direction of the conventional current and emitter current is show in the fig above. The middle region/section of the T/r is called a base. The base is lightly doped. It is very thin as compared to the emitter and collector. The base is made up of N-type semiconductors. Due to its smaller size the base will have few electrons. These electrons combine with few holes that came from the emitter. So there will be extremely less recombination of holes and electrons. That is almost about 2 to 5% of the total holes (100%) that come from the emitter. So this will constitute the base current “IB”. Thus the remaining 95 to 98% holes that come from emitter will not recombine due to the less amount of electrons in the base. Now what happens to these holes which are still in base to understand this now we come to the collector-base junction.

The –ive terminal of the battery “Vcc” is connected with the base P-type collector so the collector base junction is reversed biased. As we can see in the fig. as the collector base junction is reversed biased so there will be no flow of the holes from collector to base but the flow of holes will be in opposite direction that is from base to collector and this movement is due to the –ive terminal of the battery as we know different charges attract each other so the –ive terminal “Vcc” will attract all the 95 to 98% holes from the base. The holes provided by the emitter goes to collector through base this will constitute the collector current ”Ic” collector   current is equal to emitter current because base current is very very small but in actual practice emitter is equal to the sum of base current and collector current that is

IE = IB + Ic


Biasing and working of NPN t/t

Structure:

Transistor

Symbol:

Transistor

Considering the emitter base junction as shown in fig above the –ive terminal “Vee” of the battery is connected with the N-type emitter so the emitter base junction is forward biased (having less resistance). The emitter is made up of the N-type semi-conductor and we know that N-type semi-conductor consist of electrons as majority carries these electrons are pushed or pulled into the base due to the –ive terminal of the battery “Vee” as we know similar charges repel each other so the –ive terminal of the battery will repel electrons and due to this repulsion the electron will cross the junction this will constitute the emitter current “Ie”.

The middle region/ section of the t/r is called base as shown in fig. base is lightly doped it is very thin as compared to emitter and so due to its smaller size base have few holes as it is made up of the P-type semi-conductor so there will be extremely less recombination of holes and electrons. It is almost about 2 to 5% of the electrons that came from the emitter this will constitute the base current  “IB” thus the remaining 95 to 98%  electrons that come from emitter will not recombine with the holes due to their small amount now we will see what happened to these 95 to 98% electrons which are in base. To understand this now we come to collector base junction.

Considering the collector base junction as shown in the fig the +ive terminal of the battery ((Vcc))  Is connected with the N-type semi-conductor so the collector base junction is reversed biased (have high resistance) as the collector base junction is reversed biased so there will be no flow of electrons from collector to base the flow of electron will be in opposite direction that is from base to collector and this flow of electron from base to collector is just because of the +ive terminal (Vcc) of the battery so the +ive terminal (Vcc) of the battery will attract all the 95 to 98% electron from the base. The electron provided by the emitter goes to collector through base this will constitute the collector current “Ic” the collector current is equal to the emitter current because the base current is very very small but in actual practice emitter current is equal to the sum of base current and collector so that

IE = IB + IC


Important Biasing Rules:

A transistor will work if and only if its emitter base junction is forward biasing and collector base junction is reversed biasing.

So the important biasing rule is to keep the emitter base junction as forward biase and collector base junction as reversed biased. The different potentials are designated by double subscripts as shown in fig. the first subscript represent the point or terminal that is more +ive or less –ive then the pt or terminal represented by the second subscript.

For PNP type T/r:

First we apply the important biasing rule on this T/r so that we keep its emitter base junction as forward biased and collector base junction as reversed biased as shown in the fig.

Transistor

in PNP the letter N is being negative for collector and base the arrow head points from emitter to base mean that emitter is +ive with respect to base, moreover it is clear from the above figure. collector is more –ive than base or with respect to base.

Now let us come to the potential difference. Potential difference b/w emitter and base is written as VEB but not VBE  and the potential difference b/w base and collector is written as VBC but not VCB as show in the figure below.

Transistor


Now considering NPN T/r:

Apply the same biasing rule

Transistor

In NPN the letter P is being +ive for collector and base and arrow head pts from base to emitter which shows that base is +ive with respect to emitter. Moreover it is clear from the fig collector is more (+ive) than base.

So the potential difference b/w base and emitter will be VBE not VEB and the potential difference b/w collector and base is VCB not VBC because collector is more +ive with respect to base.

Gain of Transistor:

Gain of transistor is the ratio of output signal to input signal. Mathematical

Gain = output signal /input signal

The signal may be current voltage or power

Current Gain of transistor:

When transistor is biased than maximum part of majority carries if emitter reaches to collector these majority carries cause collector currents which passes through the lead connected hence this collector current is the output current and the emitter current is the input current this the ratio of collector current to the emitter current is called current gain of a transistor mathematical

Current gain is denoted by

= Ic / Ie

= IC / IE

Current again is also denoted by A or alpha dc ( ) its value is always less than one like it can be (.99) but it cannot be 1

Mathematically current gain

The negative sign shown that Ic is gain out of T/r while Ie is coming into the T/r.

Considering only magnitude of currents we can write

Alpha dc ( ) = Ic / Ie

This dc alpha can be written only  and is equal to Ic/Ie and is called as forward current.

Transfer ration or amplification factor and is denoted by hFB  here F is for forward and B is for common base that is base is grounded note that  is obtained by common base configuration or common base circuit or common base connection. dc show that ratio of Ic and Ie is only and only for dc values actually value of  show the quality of a transistor, higher the value of alpha better will be the transistor; a transistor is considered to be better if collector current Ic is equal or closely equal to the Ie(emitter current) but in practice it is not possible hence we can say that ideal value of alpha is 1 and its better value ranges from 0.95 to .0999 incidentally there is an Ac alpha for a transistor that is denoted by    is defined as the ratio of change in collector current

To the emitter current  mathematically we can write

It is also ( ) called as short circuit gain of a T/r and is denoted by hfb

Note that for all practical purpose

Current gain is also defined as the ratio of collector current to the base current it is denoted by Bdc  mathematically

Bdc=IC/IB

It is only obtained by common emitter circuit it is also called as common emitter dc forward ratio it is denoted by hFE maximum value of β is 500. Similarly for AC current βac =


Voltage gain of transistor:

A transistor works as transfer of resistance for proper biasing emitter base junction is kept forward biased and collector base junction is kept reversed biased. Due to forward biased emitter base resistance (RFB) become very less that is 40 to 800Ω and it will pass more current and the voltage drop will be low.

And collector base resistance (RCB) due to reversed biased become very very high and it is upto 106Ω and it will pass very very less current and will have high voltage drop.

Voltage drop across emitter base junction = Vin = IERFB

Voltage drop across collector base junction Vout=ICRCB

Since the ratio of output voltage to the input voltage is called voltage again

Mathematically

Voltage gain = Vout / Vin

Putting values

Voltage gain = ICRCB / IEREB

= IC/IE. RCB / REB

=where IC / IE equal to

= RCB / REB

RCB/REB  is the ratio of output resistance

To input resistance

Vgain= .Ar

Current gain=  IC / IE = Iout / Iin


Power gain of T/r:

Power gain of T/r is denoted by AP

Ap = Pout / Pin

AP = I2out Rout / I2in Rin

When Iout = IC and Iin = IE

Similarly

AP = I2C. Rout / (IE)2. Rin = (IC / IE)2. Rout / Rin

2 Ar

So power gain =AP = 2 Ar

Transistor Configuration:

Basically there are three types of circuit connection for the operation of transistor and is called as transistor configuration namely

  1. Common base configuration (CB)
  2. Common emitter configuration(CE)
  3. Common collector configuration(CC)

The term common is used to denote the terminal or electrode or region that is common to both input and output circuit since a transistor is three terminal device so one of its terminal has to be common electrode is generally grounded now consider the three configuration

Common Base configuration:

here the base terminal is common to both input and as will as to output circuit the structure and the symbol representation for the common base configuration is given as

In this configuration emitter current (IE) is the input current and collector current (I) is the output current the input signal is applied b/w emitter and base while is taken from collector and base.

Current gain of transistor is denoted by

∝=A1= dc=-IC / IE

The nagatice sign with IC current  show that this current is going  out of tr so considering only magnitude of current

dc = IC / IE


Common emitter configuration:

here the emitter terminal common to both input and output circuits.

Base terminal is taken as input and the collector terminal is taken as output the arrangement is shown as

Structure:

Transistor

Symbol:

Transistor

The ration of dc collector current to dc base current is called d.c β(βdc) or just β mathematically

Βdc = β = IC / Iβ

At is also called as common emitter d.c forward transfer ratio and is denoted by hFE it si possible for B to have as high value as 500

While analyzing a.c operation of a T/r we use Ba.c , Bac is the change in collector (∆IB)

Mathematically

Ba.c = ∆IC / ∆IB

It is also called as common emitter a.c forward current ration and is written as hFE  as we know that

IE= IC + I—————-(1)

B= IC / IB

= IC= B.IB

Put in equation (1)

= IE = BIB + IB

IE= IB(B+1)

Or

IE= (1+B)IB


Common collector Configuration:

Here collector terminal is common to both input as well as to output terminal its structure and symbol are given as

Structure:

Transistor

Symbol:

Transistor

The input signal is applied b/w base and collector while the output is taken from emitter and collector. IB is the input current and IE is the output current

Current gain of T/r = output current / input current———–(1)

=IE / IB ­———————(1)

x and / both side by IC

equation (1) will become

=IE/IB x IC/IC = IE/IC . IC/IB —————-(a)

Where IC / IB = β and IE / IC = 1/∝

Putting these value in equation (a)

= 1/∝.β = β/∝ = current gain of T/r

For common collector

Relation b/w ∝ and β :

As we know that for a T/r IE = IC + IB ——————-(i)

By definition ∝ = IC/IE ———————(ii)

By definition

β = IC /IB  —————–(iii)

put value of IB in Equation (ii)

β = IC / IE – IC

/ numeration and denominator by IE

β = IC/IE /IE – IC /IE

= IC /IE/1-IC/IE

=∝/1-∝

β= ∝/1-∝

β= β/1+β

as IB= IC + IB

∝= IC / IE  =  IC / IC+ IB

/ numerator and denominator by IB

∝ = IC / IB  / IC + IB /IB

= β / IC /IB+IB/IB

= β/β+1

∝=β/1+β



Practical Use of a Transistor:

Driving a Relay

A PNP or NPN type transistor can be used to control a relay, which can be then used to control high or low power electrical loads. In this example I am going to use an NPN type transistor.

Relays can be controlled through a low voltage signal and can be used to control a high power circuit with a lower power circuit. To make a relay Operate, first of all, we will have to energize the relay coil by simply passing the current through the relay coil. Most of the relays which are available in the market are 12v and 24v. I will be using a 12v relay. 12v relay cannot be directly controlled by the controller, that’s why electromechanical relay needs a driver circuit. A driver circuit simply consists of an NPN or PNP type transistor and a resistor. Depends on the designer whether he/she wants to use the PNP or NPN. I will be using an NPN transistor in my Proteus simulation.

For the Driver circuit to design, first of all, we find the relay coil resistance using a digital multimeter. The relay voltage is already known which is 12v. then by using the Ohm’s law

V = IR

we can find the current, which will be needed to energize the relay coil to make enough magnet to attract the contact. After finding the relay coil current then select any general NPN type transistor whose collector current is greater than the relay coil current. In my case, I will use 2n2222 type transistor because it is cheap, easily available in the market and moreover it can handle much more current than the calculated value which was 32ma.

A 10k resistor is connected with the base of the 2n2222 NPN transistor as it’s a bjt bipolar junction transistor and it’s a current controlled device so that’s why a 10k resistor must be added to limit the current. then we get the below circuit. through this circuit, we can control an electromechanical relay through a microcontroller.

Transistor

 

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About the Author: Engr Fahad

My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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