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Unit-II BIPOLAR JUNCTION TRANSISTOR
INTRODUCTION
The transistor was developed by Dr.Shockley along with Bell Laboratories team in 1951
The transistor is a main building block of all modern electronic systems
It is a three terminal device whose output current, voltage and power are controlled by its input current
In communication systems it is the primary component in the amplifier
An amplifier is a circuit that is used to increase the strength of an ac signal
Basically there are two types of transistors
Bipolar junction transistor
Field effect transistor
The important property of the transistor is that it can raise the strength of a weak signal
This property is called amplification
Transistors are used in digital computers, satellites, mobile phones and other communication systems, control systems etc.,
A transistor consists of two P-N junction
The junction are formed by sand witching either p-type or n-type semiconductor layers between a pair of opposite types which is shown below
Fig: transistor
TRANSISTOR CONSTRUCTION
A transistor has three regions known as emitter, base and collector
Emitter: it is aregion situated in one side of a transistor, which supplies charge carriers (ie., electrons and holes) to the other two regions
Emitter is heavily doped region
Base: It is the middle region that forms two P-N junction in the transistor
The base of the transistor is thin as compared to the emitter and is alightly doped region
Collector: It is aregion situated in the other side of a transistor (ie., side opposite to the emitter) which collects the charge carrirs
The collector of the transistor is always larger than the emitter and base of a transistor
The doping level of the collector is intermediate between the heavy doping of emitter and the light doping of the base
TRANSISTOR SYMBOLS
The transistor symbol carries an arrow head in the emitter pointing from the P- region towards the N- region
The arrow head indicates the direction of a conventional current flow in a transistor
The direction of arrow heads at the emitter in NPN and PNP transistor is opposite to each other
The PNP transistor is a complement of the NPN transistor
In NPN transistor the majority carriers are free electrons, while in PNP
transistor is forward biased and collector base junction is reverse biased
The emitter – base junction is forward biased only if V is greater than barrier potential which is 0.7v for silicon and 0.3v for germanium transistor
The forward bias on the emitter- base junction causes the free electrons in the N – type emitter to flow towards the base region. This constitutes the emitter current. Direction of conventional current is opposite to the flow of electrons
Electrons after reaching the base region tend to combine with the holes
If these free electron combine with holes in the base, they constitute base current ().
Most of the free electrons do not combine with the holes in the base
This is because of the fact that the base and the width is made extremely small and electrons do not get sufficient holes for recombination
Thus most of the electrons will diffuse to the collector region and constitutes collector current. This collector current is also called injected current, because of this current is produced due to electrons injected from the emitter region
There is another component of collector current due to the thermal generated carriers.
This is called as reverse saturation current and is quite small
OPERATION OF PNP TRANSISTOR
Operation of a PNP transistor is similar to npn transistor
The current within the PNP transistor is due to the movement of holes where as, in an NPN transistor it is due to the movement of free electrons
In PNP transistor, its emitter – base junction is forward biased and collector base junction is reverse biased.
The forward bias on the emitter – base junction causes the holes in the emitter region to flow towards the base region
This constitutes the emitter current ( ).
The holes after reaching the base region, combine with the electrons in the base and constitutes base current.
Most of the holes do not combine with the electrons in the base region
This is due to the fact that base width is made extremely small, and holes does not get sufficient electrons for recombination.
Thus most of the holes diffuse to the collector region and constitutes collector region
This current is called injected current, because it is produced due to the holes injected from the emitter region
There is small component of collector current due to the thermally generated carriers
This is called reverse saturation current.
Common base configuration (CB)
The input is connected between emitter and base and output is connected across collector and base
The emitter – base junction is forward biased and collector – base junction is reverse biased.
The emitter current, flows in the input circuit and the collector current flows in the output circuit.
The ratio of the collector current to the emitter current is called current amplification factor.
If there is no input ac signal, then the ratio of collector current to emitter current is called dc alpha
The ratio of change in the collector current to change in the emitter current is known as ac alpha
= Common-emitter current gain = Common-base current gain
= IC = IC
IB IE
The input characteristics look like the characteristics of a forward-biased diode. Note that VBE varies only slightly, so we often ignore these characteristics and assume:
Common approximation: VBE = Vo = 0.65 to 0.7V
The higher the value of better the transistor. It can be increased by making the base thin and lightly doped
The collector current consists of two parts transistor action. Ie., component dependind upon the emitter current , which is produced by majority carriers
The leakage current due to the movement of the minority carriers across base collector junction
CHARACTERISTICS OF CB CONFIGURATION
The performance of transistors determined from their characteristic curves that relate different d.c currents and voltages of a transistor Such curves are known as static characteristics curves There are two important characteristics of a transistor Input characteristics Output characteristics
INPUT CHARACTERISTICS
The curve drawn between emitter current and emitter – base voltage for a given value of collector – base voltage is known as input characteristics
Base width modulation (or) Early effect
In a transistor, since the emitter – base junction is forward biased there is no effect on the width of the depletion region However, since collector – base junction is reverse biased as the reverse bias voltage across the collector – base junction
small collector current flows even when emitter current this is the collector leakage current
SATURATION REGION
collector current flows even when the external applied voltage is reduced to zero. There is a low barrier potential existing at the collector – base junction and this assists in the flow of collector current
(II) COMMON – EMITTER CONFIGURATION
The input is connected between base and emitter, while output is connected between collector and emitter Emitter us common to both input and output circuits.
The bias voltage applied are Vce and Vbe. The emitter-base junction is forward biased and collector-emitter junction is reverse biased. The base current Ib flows in the input circuit and collector current Ic flows sin the output circuit. CE is commonly used because its current, Voltage, Power gain are quite high nd output to input impedance ratio is moderate The rate of change in collector current to change in base current is called amplification factor B. The current gain in the common-emitter circuit is called BETA (b). Beta is the relationship of collector current (output current) to base current (input current).
Two voltages are applied respectively to the base and collector with respect to the common emitter.
Same as the CB configuration, here in the CE configuration, the BE junction is forward biased while the CB junction is reverse biased. The voltages of CB and CE configurations are related by:
The base current is treated as the input current, and the collector current is treated as the output current:
Solving this equation for collector current, we get the relationship between the output collector current and the input base current:
Here we have also defined the CE current gain or current transfer ratio
which is approximately the ratio of the output current and the input current. The two parameters α and β are related by:
Characteristics of CE configuration
i) Input Characteristics
Same as in the case of common-base configuration, the junction of the common-emitter configuration can also be considered as a forward biased diode, the current-voltage characteristics is similar to that of a diode:
The Curve drawn between base current and base-emitter voltage for a given value of collector-emitter voltage is known as input characteristics.
The input characteristics of CE transistors are similar to those of a forward biased diode because the base-emitter region of the transistor is forward-biased. Input Resistance is larger in CE configuration than in CB configuration.
Thus, the current gain is greater than unity.
Saturation Region
With low values of Vce, the transistor is said to be operated in saturation region and in this region, base current Ib does not correspond to Ic,
Cut off Region
A small amount of collector current Ic flows even when Ib=0, This is called emitter leakage current.
iii) Common Collector Configuration :
Input is applied between base and collector while output is applied between emitter and collector. The collector forms the terminal common to both the input and output. GAIN is a term used to describe the amplification capabilities of an amplifier. It is basically a ratio of output to input. The current gain for the three transistor configurations (CB, CE, and CC) are ALPHA(a), BETA (b), and GAMMA (g), respectively.
i) Input Characteristics
To determine the i/p characteristics Vce is kept at a suitable fixed value. The base collector voltage Vbc is increased in equal steps and the corresponding increase in Ib is noted. This is repeated for different fixed values of Vce.
ii) Output Characteristics
Current components in a Transistor
As a result of biasing the active region current flows to drift and diffusion in various parts of transition. Due to forward bias across input junction, there across three phenomena. a) The generation and Recombination of electrons and holes Let, n -> Electron concentration P -> Hole concentration Tn -> Life time of electron Tp -> Life time of Holes no -> Equilibrium density of electrons po -> Equilibrium density of Holes
reverse-biased diode, and the collector current Ic would equal the reverse saturation current ICO. If IE ≠ 0, then
From figure, we note that
Ic = Ico - IpC
For a p-n-p transistor, Ico consists of holes moving across Jc from left to right (base to collector) and electrons crossing Jc in the opposite direction.
Since the assumed reference direction for Ico in figure is from right to left, then for a p-n-p transistor, Ico is negative. For an n-p-n transistor, Ico is positive.
Emitter Efficiency:- (γ)
The emitter, or injection, efficiency γ is defined as
γ ≡ Current of injected carriers at JE
Total emitter current
Transport Factor:- (β)*
The transport factor β*^ is defined as
β*^ ≡ injected carrier current reaching Jc
injected carrier current at JE
In the case of a p-n-p transistor we have
β*^ = IpC / IpE
Large – signal current Gain:- (α)
We define the ratio of the negative of the collector-current increment to the emitter-current change from zero (cutoff) to IE as the large-signal currant gain of a common-base transistor, or
α = - Ic – Ico / IE
since Ic and IE have opposite signs, then α, as defined, is always positive Typical numerical values of α lie in the range of 0.90 to 0.995.
α = IpC / IE
= IpC / IpE. IpE / IE
α = β*^ γ
IC = - α IE + Ico
Ic = - α IE + Ico (1- eVc^ / Vr)
Description of Ebers-moll model
The current equations derived above is interpreted in terms of a model shown in the figure. This model of transistor is known as Eber Moll model of transistor. From the diagram applying Kirchhoff’s current law at the collector node, we get IC= - αN*IE + ICO (1 – eVCB/Vt) Where α N is the current gain of common base transistor mentioned above in normal mode of operation, V BC is the base to collector voltage, I co is the reverse saturation current of base collector junction. Similarly at emitter and base node by applying Kirchhoff’s current law IE= - αIIC+IEO(1 – eVBE/Vt), IE+IB+IC = 0 Where αI is the inverted current gain of common base transistor with roles of collector and emitter interchanged, V BE is the base to Emitter voltage, I co is
allow only currents order of reverse saturation currents. Since D1 and D2 are in series same current should flow through both of them then only currents order of reverse saturation currents flow through their junctions. It is obvious that this is not the case with the transistor in active region (because of the internal design of transistor). The forward current entering the base is sweeped across into collector by the electric field generated by the reverse bias voltage applied across the base collector junction.
Base width modulation:-
As the applied base-collector voltage (VBC) varies, the base-collector depletion region varies in size. This variation causes the gain of the device to change, since the gain is related to the width of the effective base region. This effect is often called the "Early Effect"
An NPN bipolar transistor can be considered as two diodes connected anode to anode. In normal operation, the emitter-base junction is forward biased and the base-collector junction is reverse biased. In an npn-type transistor for example, electrons from the emitter wander (or "diffuse") into the base
- Base width has been assumed to be constant
- When bias voltages change, depletion widths change and the effective base width will be a function of the bias voltages
- Most of the effect comes from the C-B junction since the bias on the collector is usually larger than that on the E-B junction
- Base width gets smaller as applied voltages get larger
Early Effect: Common Base Input Characteristic
I E
I (^) F 0 ( eqVEB^ /^ kT^ 1) R I (^) R 0 ( eqVCB^ /^ kT 1) Ebers-Moll
Assuming – VCB > few kT/q and W/LB << 1
W cosh I^ ^ DE^ DB^ ^ LB^ ^ ^ DB F 0 ^ qA^ LE n E 0 ^^ LB p B (^0) W ^ qA^ W p B 0 sinh (^)
L B (^)
I (^) E I (^) F 0 eqVEB^ /^ kT^ qA
D W
B (^) p B 0 e qVEB / kT
- Exponential prefactor will increase as VCB increases (W decreases)
