Unit – III

Bipolar junction transistor

3.1 Introduction of transistor, construction

*BJT:-The BJT is constructed with the three draped semiconductors region’s separate by two Pn junctions as shown below

-Basic epitaxial planner structures.

-Three terminal with region’s are called emitter, base and collector.

-The physical representation of the two types of BJT’s,

One type consists between two regions separated by a P region (npn) and other type consists of two p regions separated by an n region (pnp).

-The Pn junction joining the base region and the emitter region is called the base emitter junction.

-The Pn junction joining the base region and the collector region is called the base collector junction.

-The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector regions.

3.2 transistor operations

*Base Transistor Operation:-

In order for the transistor to operate properly as an amplifier the two pn junction must be correctly biased with the external D.C vtg.

-The next figure shows the proper bias arrangement for both the npn and pnp transistors for active operation as an amplifier.

-In both the cases the base emitter

(BE) junction is forward biased & the base collector junction (BC) junction is reverse biased

• As from above figure consider n-p-n transistor. The forward bias from base to emitter narrow’s the BE depletion region and the reverse bias from base to collector widens the BC depletion region shown in figure.
• The heavily doped N-TYPE emitter region is full with conduction band(frep)  electron’s that easily diffuse through the forward biased BE junction into the p-type base region where they become minority carrier’s same as forward biased diode region
• The base region is lightly doped & very thin so that it has a very limited number of holes.
• Those only a small percentage of all the e-flowing the BE junction can combine with the available holes in the base.
• The relatively few recombined flow out of the base lead as valance electrons, forming as small base current.
• Most of the e flowing from the emitter into the thin lightly dooped base region do not recombine but diffuse into the BC depletion region.
• The BC depletion region diffuse e is being pulled across the reverse biased BC junction by the attraction of the collector supply vtg.
• The electrons now move through the collector region, out through the collector lead into the +ve terminal of the collector vtg source. This forms the collector electrons current.
• The collector current is much larger than the base current.
• This is the reason transistor exhibit current gain.

3.3 BJT characteristics, load line, operating point, leakage currents, saturation and cut off mode of operations

Transistor Current:-

Transistor’s Characteristic’s and parameters:-

-The transistors is connected to d.c bias vtg for both the npn & pnp types VBB forward biases the base emitter junction &Vcc reverse biases the base collector junction.

*Biasing conditions for different regions of operation:-

Transistor’s Configuration:- 

1)              Common Base configuration(C.B)

2)              Common emitter Configuration(C.E)

3)              Common Collector Configuration(C.C)

Common Base configuration (C.B):-

-The I/p is applied between the emitter and the base. The base acts as a common terminal between the I/p and o/p.

-The input vtg is therefore VEB and the input current is IE.

-The output is taken between the collector and the base therefore the output vtg is VCB and the output current is IC.

* Current relation’s in CB configuration:-

-The collector current is IC of the common base configuration is given by

Ic=Ic(INI)+ICBO

-Where the Ic(INI) called the injected collectors current and it is due to the number of electrons crossing the collectors base junction.

-ICBO :- This is the reverse saturation current flowing due to the minority carrier’s between the collector and base when the emitter is open

-ICBO flow’s due to the reverse biased collector’s base junction. As ICBO negligible as compared to Ic(INI) we can neglect it in practice.

.’. Ic=Ic(INI)………………………practically

Ic=ICBO………………with emitter open

Emitter is open

ICBO

Collector is to base control

-Since the ICBO flow’s due to terminally generated minority carrier’s it increases with increase the temperature.

-It doubles it’s value for every 100c rise in temperature.

-Current amplification factor or current gain (ddc):-

Current amplification factor or current gain is the ratio of collector current due to the injection to the total emitter current

αd.c = Ic(INI)

-The value of the ddc for CB configuration will always be less than 1.This is because

Ic(INI)<IE.

-Typically the value of d.d.c ranges between 0.95 to 0.995 depending upon the thickness of the base region.

-Larger the thickness of the base region smaller the value of the d.d.c

Ic(INI)=d.d.c.IE

Hence the expression for IC is given by

IC=αd.c IE + ICBo--------------------I

But the ICBo is negligibly small

ICd.d.c IE

* Expression for IB:-

IE=IB+IC

IE=αd.cIE+ICBo+IB…………………from I

IB=(1-αd.c)IE-ICBo

Neglecting ICBo

IB=(1-αd.c)IE

• Characteristics of a transistors in a common base configuration:-

1. Input Characteristic:-

A.  Input Characteristic: is always a graph of input current verses input vtg. For common base (CB) configuration input current is the emitter current (IE) & I/p vtg. Is the emitter  to the base vtg (VEB)

The I/p Characteristic is plotted at a constant O/p vtg. VCB

B. Output Characteristic:

Output Characteristic is always a graph of O/p current versus O/p Vtg.

For the CB configuration the O/p current is collector current (IC) of the output voltage is collector to base vtg. (VCB)

Output Characteristic is plotted for a constant value of I/p current (IE)

O/p Characteristic of a n-p-n transistor in CB Configuration

Dynamic O/p resistance (ro)

Ro = / I constant

In the active region Ic does not depend on VCB. It depends only on the I/p current IE. That is why the transistor is called as a current controlled or current operated device.

Feature of CB configuration :

1. Common terminal : base
2. Input current : IE
3. O/p current IC
4. I/P Vtg. : VEB
5. O/P Vtg. VCB
6. Current gain : ( less than 1)
7. Vtg. Gain : medium
8. Input resistance : very low (20-)
9. O/P resistance : very high (1 m-2)
10. Application : as preamplifier

Load line Analysis

• The load line solution was found by superimposing the actual characteristics on a plot of network equation involving the same network variables.
• It is known as load line analysis as load of network defined the slope of the straight line connecting the points defined by network parameters.
• The smaller the load resistance, the steeper the slope of the network load line.

Fig.: DC Load Line (Ref. 2)

• The output equation is given by,

• The characteristic curve is drawn as

Fig.: DC Load Line characteristic curve (Ref. 2)

• When Ic = 0mA then

• If we choose VCE = 0V then,

Numerical

Determine the values of VCC RC and RB of a fixed bias configuration for a given load line and defined Q point.

Solution:

Region of Operation

3.4 Eber-moll‘s model.

This model of transistor can operate in forward active mode. The circuit has two diodes with two current sources connected as shown below. The diodes represent base-emitter and base collector. The directions of current are as shown in the figure below. The minority carriers flow through the base region.

Fig: Eber’s Moll model for NPN bipolar junction transistor

IE = IF – αR IR

IB = (1- αF) IF + (1- αR) IR

IC = -IR + αF IF

The Eber’s Moll model parameters are related as

IE,sαF = IC,sαR

The above relation is called as reciprocity relation. But for case when there is no minority carrier diffusion due to uniform base doping and equal voltages of base-emitter and base-collector junctions the relation becomes

IF(VBE) αF = IR (VBC+VBE) αR

Where

IF = Forward current of diode

IR = reverse current of diode

αF = Forward transport factor

αR = Reverse transport factor

IE,s = Saturation current of base-emitter diode

IC,s = Saturation current of base-collector diode

References

1. S. Salivahanan, N. Suresh Kr. & A. Vallavaraj, ―Electronic Devices & Circuit‖, Tata McGraw Hill,

2008

2.     Millman, Halkias and Jit, ―Electronic devices and circuits‖ McGraw Hill

3.     Boylestad&Nashelsky, ―Electronic Devices & Circuits‖, Pearson Education, 10TH Edition.

4.     Sedra& Smith, ―Micro Electronic Circuits‖ Oxford University Press, VI Edition

5.     Robert T. Paynter, ―Introducing Electronic Devices & Circuits‖, Pearson Education, VII Edition, 2006