BIPOLAR JUNCTION TRANSISTOR

A bipolar junction transistor is a three terminal semiconductor device in which the operation depends on the interaction of majority and minority carriers.

Transistor refers to Transfer Resistor

i.e., signals are transferred from low resistance circuit into high resistance circuit.

BJT BIPOLAR JUNCTION TRANSISTOR consists of silicon crystal in which a layer of ‘N’ type silicon is sandwiched between two layers of ‘P’ type silicon.

The semiconductor sandwiched is extremely smaller in size.

In other words, it consists of two back to back PN junction joined together to form single piece of semiconductor crystal.

These two junctions gives three region called Emitter, Base and Collector.

There are two types of transistors such as PNP and NPN.

The arrow on the emitter specifies whether the transistor is PNP or NPN type and also determines the direction of flow of current, when the emitter base junction is forward biased.

Emitter:

It is more heavily doped than any of the other region because its main function is to supply majority charge carriers to the base.

Base:

It forms the middle section of the transistor.

It is very thin as compared to either the emitter or collector and is very lightly doped.

Collector:

Its main function is to collect the majority charge carriers coming from the emitter and passing through the base.

In most transistors, collector region is made physically larger than the emitter because it has to dissipate much greater power.

Operation of Transistor

The basic operation will be described using the pnp transistor.

The operation of the pnp transistor is exactly the same if the roles played by the electron and hole are interchanged.

BIPOLAR JUNCTION TRANSISTOR BJT

One p-n junction of a transistor is reverse-biased, whereas the other is forward-biased.

Both biasing potentials have been applied to a pnp transistor and resulting majority and minority carrier flows indicated.

Majority carriers (+) will diffuse across the forward-biased p-n junction into the n-type material.

A very small number of carriers (+) will through n-type material to the base terminal. Resulting IB is typically in order of microamperes.

The large number of majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal.

Majority carriers can cross the reverse-biased junction because the injected majority carriers will appear as minority carriers in the n-type material.

Applying KCL to the transistor :

I_E = I_C + I_B

The comprises of two components – the majority and minority carriers

IC = ICmajority + ICOminority

I_CO – I_C current with emitter terminal open and is called leakage current.

Common Base configuration

Common-base terminology is derived from the fact that the :

base is common to both input and output of the configuration.

base is usually the terminal closest to or at ground potential.

All current directions will refer to conventional (hole) flow and the arrows in all electronic symbols have a direction defined by this convention.

Note that the applied biasing (voltage sources) are such as to establish current in the direction indicated for each branch.

To describe the behavior of common-base amplifiers requires two set of characteristics: o Input or driving point characteristics.

Output or collector characteristics

The output characteristics has 3 basic regions:

Active region –defined by the biasing arrangements

Cutoff region – region where the collector current is 0A

Saturation region- region of the characteristics to the left of

V_CB = 0V

The curves (output characteristics) clearly indicate that a first approximation to the relationship between IE and IC in the active region is given by

I_C ≈IE

Once a transistor is in the ‘on’ state, the base-emitter voltage will be assumed to be

V_BE = 0.7V

In the dc mode the level of I_C and I_E due to the majority carriers are related by a quantity called alpha

α = I_C / I_E

I_C = α I_E + I_CBO

It can then be summarize to IC = αI_E (ignore I_CBO due to small value)

For ac situations where the point of operation moves on the characteristics curve, an ac alpha defined by

Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector. The value of is typical from 0.9 ~ 0.998.

Common Emitter configuration

It is called common-emitter configuration since :

emitter is common or reference to both input and output terminals.

emitter is usually the terminal closest to or at ground potential.

Almost amplifier design is using connection of CE due to the high gain for current and voltage.

Two set of characteristics are necessary to describe the behavior for CE; input (base terminal) and output (collector terminal) parameters.

Input characteristics for CE configuration

I_B in microamperes compared to milliamperes of I_C.

I_B will flow when V_BE > 0.7V for silicon and 0.3V for germanium

Before this value I_B is very small and no I_B.

Base-emitter junction is forward bias

Increasing V_CE will reduce I_B for different values.

Output characteristics for CE configuration

For small V_CE (V_CE < V_CESAT, I_C increase linearly with increasing of V_CE

V_CE > V_CESAT I_C not totally depends on V_CE — > constant I_C

I_B(uA) is very small compare to I_C (mA). Small increase in I_B cause big increase in I_C

I_B=0 A — > I_CEO occur.

Noticing the value when I_C=0A. There is still some value of current flows.

Common Collector configuration

Also called emitter-follower (EF).
It is called common-emitter configuration since both the
Signal source and the load share the collector terminal as a common connection point.
The output voltage is obtained at emitter terminal.
The input characteristic of common-collector configuration is similar with common-emitter. configuration.
Common-collector circuit configuration is provided with the load resistor connected from emitter to ground.
It is used primarily for impedance-matching purpose since it has high input impedance and low output impedance.
For the common-collector configuration, the output characteristics are a plot of I_E vs V_CEfor a range of values of I_B.

Small Signal Amplifier

When the input signal is so weak as to produce small fluctuations in the collector current compared to its quiescent value, the amplifier is known as Small Signal Amplifier.

In other words, as the name indicates, the input applied to the circuit is V_in << V_th. It has only one amplifying device.

Α = I_C / I_E

I_C = α I_E + I_CBO

Voltage and current equation for hybrid parameters:

V₁ = h₁₁i₁ + h₁₂V₂

I₂ = h₂₁i₁ + h₂₂V₂

The values of h-parameters:

h₁₁ = V₁/ i₁

h₁₂ = V₁ / V₂

h₂₁ = i₂ / i₁

h₂₂ = i₂ / V₂

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