04_How a BJT work

7/29/2019 04_How a BJT work

1/14

How a BJT (Bipolar Junction Transistor)

Works

Its all in the doping

Fig 4.1 How a Transistor is Doped.

The way a transistor works can be described with reference to fig 4.1 which shows the

basic doping of a junction transistor and Fig 4.2 the method of operation of the device.

The operation of the transistor is very dependent on the degree of doping of the variousparts of the semiconductor crystal. The N type emitter is very heavily doped to provide

many free electrons as majority charge carriers. The lightly doped P type base region isextremely thin, and the N type collector is very heavily doped to give it a low resistivity

apart from a layer of less heavily doped material near to the base region. This change inthe resistivity of the collector close to the base, ensures that a large potential is present

within the collector material close to the base. The importance of this will becomeapparent from the following description.

Fig. 4.2 How a Transistor Amplifies Current.


2/14

During normal operation, a potential is applied across the base/emitter junction so that the

base is approximately 0.6v more positive than the emitter, this makes the base/emitterjunction forward biased.

A much higher potential is applied across the base/collector junction with a relativelyhigh positive voltage applied to the collector, so that the base/collector junction is heavilyreverse biased. This makes the depletion layer between base and collector quite thick

once power is applied.

As mentioned above, the collector is made up of mainly low resistivity material with a

layer of high resistivity material next to the base/collector junction. This means that mostof the voltage between collector and base is developed across this high resistivity layer,

giving a high voltage gradient near the collector base junction.

When the base emitter junction is forward biased, a small current will flow into the base.

Therefore holes are injected into the P type material. These holes attract electrons acrossthe forward biased base/emitter junction to combine with the holes. However, because theemitter region is very heavily doped, many more electrons cross into the base region than

are able to combine with holes. This means there is a large concentration of electrons inthe base region and most of these electrons are swept straight through the very thin base,

and into the base/collector depletion layer. Once here, they come under the influence ofthe strong electric field across the base/collector junction. This field is so strong due to

the potential gradient in the collector material mentioned earlier, that the electrons are


3/14

swept across the depletion layer and into the collector material, and so towards thecollector terminal.

Varying the current flowing into the base, affects the number of electrons attracted from

the emitter. In this way very small changes in base current cause very large changes in

the current flowing from emitter to collector, so current amplification is taking place.

Go to Animated Version of How a BJT Works.

Top of Page

Transistors

The name "Transistor" came from the word transfer resistor. These were thesemiconductor devices which replaced the Vaccum tubes which lead to lot of

developments in Semiconductor technology. The transistors as we know are the three

terminal devices, but they are not the first devices with the three leads or terminals.The vacuum tube were the first three terminal devices.The vacuum tube were initially used for amplification of the electrical signals, whether it

is current or voltage. With the invention of vacuum tube the long distance transmissionand reception of radio signals was possible.

But it had its own disadvantages. Due to the disadvantages, scientist and physicists werelooking for the alternative of vacuum tubes. While working at Bell laboratories, John

Bardeen and Walter Brattain, in 1947 Invented the very significant electronic component
http://www.learnabout-electronics.org/bipolar_junction_transistors_05.phphttp://www.learnabout-electronics.org/bipolar_junction_transistors_04.phphttp://www.learnabout-electronics.org/bipolar_junction_transistors_04.phphttp://www.learnabout-electronics.org/bipolar_junction_transistors_05.php


4/14

called transistor.Let us study more about this transistor in this section.

What is a Transistors?

Back to Top

It is a semiconductor device having two junctions and three terminals.

The transistors have three leads or terminals:

1. EMITTER2. BASE3. COLLECTOR

Emitter (E) is a heavily doped region of the device and is a supplier of majority charge

carriers to the base.

Base (B) is made thin and is lightly doped. This is done to reduce the recombinationprocess.

Collector (C) is moderately doped and collects majority carriers through base.
http://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.html


5/14

Transistors are heart of today's circuits.Transistors are the device which are primarilyused for two purposes in today electronic circuits:

1. Amplifier: it can amplify the current or voltage of the input. Amplification is theprocess of increasing the strength of the signal by using external source.

2.

Switch: The transistors are used in various modern electronic circuits. It can beused as switch to "ON" or"OFF" the flow of the current in the circuit.

History of Transistors

Back to Top

The Vacuum tube was invented by Lee de Forest, an American physicist and inventor in

1906. He had 180 patents to his credit. This invention helped in the development ofcomputers. The vacuum tubes were used in those days computers. The vacuum tubes

were used in computers up till late 1940's and early 1950's.

But soon the it was discovered that the there are several disadvantages of using vacuumtube in electronic circuit.

1. Tend to leak too much.2. The metal used by vacuum to emit electrons got burned out frequently.3. They tend use too much of power to run.4. They are bulky in size so the circuits using them are also bulky and are not

portable.5. Due to the bulk size the devices occupy too much of space.

Due to these disadvantages, scientist and physicists were looking for the alternative ofvacuum tubes. While working at Bell laboratories, John Bardeen and Walter Brattain, in

1947 were experimenting to understands the nature of charged particles at the metal andsemiconductor interface. By this experiment they came to know that by placing two point

contacts near to one another, they could make a three terminal device - this device is thethe first "point contact" transistor. For the invention of the transistor both these physicist

received the Nobel prize for physics in 1956. The name "Transistor" came from transferresistor.

How Transistors Work?Back to Top

We know that transistors are three terminal devices. The transistors are used to amplifythe input signal or to act as switch for electronic circuits. This task is accomplished by

using one of the terminal to control the flow of current between the other two terminals.
http://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.html


6/14

Let us look at the transistor, it has three terminals:

1. Base (B),2. Collector (C),3.

Emitter (E) as its three terminal.

Base (B) is used as the terminal to control the flow of current between the Collector (C)and Emitter (E).

The VEE supply is used to forward bias emitter base junction. As a result significantcurrent flows, once the potential barrier is exceeded. The majority charge carriers diffuse

from emitter into base and this results in emitter current IE as indicated in the abovediagram. Once these reach the base, very few electrons or hole undergo recombination

process and rest diffuse through collector base junction due to potential on the collectorside. In a PNP the diffused holes reach the collector and at the same time an electron

from the emitter enters into the positive pole of VEE thereby creating a hole in the emitter.Thus the current in PNP is caused by holes and the current in external circuit by

electrons.Due to very less recombination process, the base current IB = IC - IE flows.

Apply Kirchhoff current law,IE = IB + ICwhere IE = Emitter Current

IB = Base CurrentIC = Collector Current.

The action of NPN is similar to that of PNP.

Types of Transistors

Back to Top

The letters N and P here refers to the layers of semiconductor material used to make thetransistor.

The transistor are of two types:


7/14

1. NPN: when a p-type is sandwiched between two n-type, the resulting transistor isa n-p-n transistor. Here N denotes N-doped semiconductor.

2. PNP : A p-n-p transistor is obtained by sandwiching a n-type semiconductorbetween two p-type semiconductors. Here P denotes P-doped semiconductor.

Symbolically it is represented as:

Graphene Transistors

Back to Top

The modern transistors are based on semiconductor technology. The numbers of

transistors on the single chip are quickly reaching its physical limits. The scientists allover world are working on alternative of semiconductor.In 2006, Walter de Heer and his team had successfully built an all-graphene transistors.

The graphene is one atom thick pure carbon layer.The advantage of using graphene as a substitute is that it offer very little resistance to the

flow of electrons so it can perform large number of operations without heating much andmoreover it is good thermal conductor so the heat can be dissipated from it very quickly.

Germanium Transistors

Back to Top

Germanium is used as semiconductor in transistors and other semiconductor devices. Thegermanium was the king in old days when it is the major constituent of all the

semiconductor devices but today, it is majorly used in fiber-optic systems and infraredoptics. The germanium is replaced by pure silicon. Today it is used in less than 10% of

semiconductor devices. These transistors are replaced by silicon transistor because thegermanium transistors are prone to failure.


8/14

Power Transistors

Back to Top

These transistors can be used to amplify the power of the input signal. The power

transistors are general purpose transistors, only difference is that they are biased such thatthe input power is amplified.

Field Effect Transistors

Back to Top

A Field effect transistor has two layers of semiconductor material, as shown in figure.The two layers are such that they form a channel through which electricity flows.

FET has three terminals, namely

1. Gate2. Source3. Drain.

Gate is the channel that modulates the conductivity. Source is a terminal where themajority carriers enter the channel. Drain is the terminal where the majority carriers

leave the channel.

The voltage connected gate, interferes with the current flowing from source to drain.

Hence the gate controls the flow of current in the channel, i.e. between source and drain.

By increasing or decreasing voltage at gate, the current in the channel can be controlled.

The Field effect transistors are of two types-

1. Junction Field Effect Transistor(JFET)2. Metal Oxide Semiconductor Field Effect Transistor(MOSFET).

MOSFET are used mostly in all the modern electronics circuits.

NPN Transistors

Back to Top

The bipolar transistors are of two types:

1. NPN2. PNP.
http://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.html


9/14

NPN transistor has a layer of P-doped semiconductor (the "base") sandwiched betweentwo N-doped layers. When a small current is applied to the base, it is amplified to

produce a large collector and emitter current. The NPN transistors are most commonlyused in electronic circuitry.

PNP Transistors

Back to Top

PNP transistors is a bipolar transistor. It has a layer of N-doped semiconductor (the"base") sandwiched between two P-doped layers. When a small current is applied to the

base, it is amplified to produce a large collector and emitter current.

Transistors as Switches

Back to Top

Given the transistor figure:

Let us study the working of Transistor as a switch in the following steps:

1. A small Positive voltage at Base, Positive voltage to collector and Negativevoltage to emitter is being applied.

2. The voltage at Base is slightly positive than that of Emitter and the voltage atCollector is more positive than that of Base region.

3. Due to this arrangement the base region attracts electrons from the emitter regionand the collector attracts the electrons from the base region. So, the electrons flow

from emitter to collector and as we know that the flow of electrons is opposite tothat flow of current and hence the current flows from Collector to Emitter.

4. From this we can conclude that the transistor acts as switch facilitating the flow ofcurrent from the Collector to Emitter.

5. Now take the case that the Base region is not connected to voltage supply, so thethere would not be any charge flow from Collector to Emitter and hence the

circuit acts as switch blocking the flow of current from the Collector to Emitter.


10/14

RF Transistors

Back to Top

These types of transistors are used in amplification of RF signals. These are special type

of power amplifiers, which amplifies weak Rf signals at the specific frequency. Thesetransistors should be very high precision and reliability.

Thin Film Transistors

Back to Top

A thin-film transistor, or more generally TFT, is a FET (field-effect transistor). It isconstructed by depositing thin films of a active layer of semiconductor material,

dielectric layer and metallic contacts over a substrate.

A most commonly used substrate in TFT is glass. The TFTs are primary used in liquidcrystal displays. In the conventional transistors the semiconductor material is used as thesubstrate while in TFT generally glass is used as substrate.

Bipolar Transistors

Back to Top

A Bipolar (junction) transistor (BJT) bipolar junction three-terminal electronic devicewhich has two types of charge carriers namely electrons and holes whereas in Field effect

transistors (FET) one type of charge carriers. Hence Bipolar transistors as a device

controls the current whereas FET act as a Voltage controller.The name of Bipolar transistor came from the fact that its operation involves bothelectrons and holes. In BJT, Charge flows through it are due to diffusion of both the

electrons and holes across the junction between two regions of different doping.The BJT are of two types

1. NPN transistor2. PNP transistor.

Transistor Configurations

Back to Top

The three types of circuit connections for operating a transistor:

1. Common Emitter (CE) Configuration2. Common Base (CB) Configuration3. Common Collector (CC) Configuration.
http://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.htmlhttp://physics.tutorvista.com/electricity-and-magnetism/transistors.html


11/14

The common electrode is generally grounded and is common to the input and outputcircuit.

These are the Configuration circuits in NPN transistors:

These are the Configuration Circuits in PNP transistor:


12/14

Transistor Characteristics

Back to Top


13/14

To study the transistor characteristics a transistor is either operated in common emitter

configuration or in common base configuration. Let us take an npn transistor operated oncommon emitter mode. As discussed earlier a transistor works only when the input side is

forward biased and output side is reverse biased. Ammeter are connected in series withbase and collector to measure base current and collector current respectively. Voltmeters

are connected in parallel to measure the input voltage (VBE) and output voltage (VCE). Tostudy the characteristics, the output parameter, namely VCEis made constant and the

variation of input current with input voltage is measured and the same plotted in thegraph. (VBE v/s IB). A family of curves may be drawn by varying VCE. The graph

resembles the characteristics of a forward biased P-N Junction.

Ri = VBEIB

Now to study the output characteristics, input current is kept constant and the variation of


14/14

out voltage and output current are measure and a graph is plotted (adjacent page). Itresembles characteristics of a reverse biased P-N junction diode.

The output impedance can be found from the graph. Output impedance is the ratio of

output voltage to output current at a constant input current. That isRo = VCEICThe current ratio between Collector and base is given by

Current gain = IcIB which is nearly a constant.

Documents

04_How a BJT work