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Transistors

The name "Transistor" came from the word transfer resistor. These were the semiconductor 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 transmission and reception of radio signals was possible.
But it had its own disadvantages. Due to the disadvantages, scientist and physicists were looking for the alternative of vacuum tubes. While working at Bell laboratories, John Bardeen and Walter Brattain, in 1947 Invented the very significant electronic component called transistor.
Let us study more about this transistor in this section.

 

What is a Transistors?

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It is a semiconductor device having two junctions and three terminals.
The transistors have three leads or terminals:
  1. EMITTER
  2. BASE
  3. 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 recombination process.

Collector (C) is moderately doped and collects majority carriers through base.

Transistors are heart of today's circuits.Transistors are the device which are primarily used for two purposes in today electronic circuits:
  1. Amplifier: it can amplify the current or voltage of the input. Amplification is the process of increasing the strength of the signal by using external source.
  2. Switch: The transistors are used in various modern electronic circuits. It can be used as switch to "ON" or "OFF" the flow of the current in the circuit.

History of Transistors

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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 of computers. 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 vacuum tube 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 of vacuum 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 and semiconductor 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 the the 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 transfer resistor.

How Transistors Work?

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We know that transistors are three terminal devices. The transistors are used to amplify the 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.
                                    N P N Common Emitter Configuration

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 significant current 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 above diagram. 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 collector side. 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 Current
IC = Collector Current.

The action of NPN is similar to that of PNP.

Types of Transistors

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The letters N and P here refers to the layers of semiconductor material used to make the transistor.
The transistor are of two types:
  1. NPN: when a p-type is sandwiched between two n-type, the resulting transistor is a n-p-n transistor. Here N denotes N-doped semiconductor.
  2. PNP : A p-n-p transistor is obtained by sandwiching a n-type semiconductor between two p-type semiconductors. Here P denotes P-doped semiconductor.
                      p n p Junction and n p n Junction

Symbolically it is represented as:

                                p n p and n p n Junction Symbolically

Graphene Transistors

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The modern transistors are based on semiconductor technology. The numbers of transistors on the single chip are quickly reaching its physical limits. The scientists all over 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 and moreover it is good thermal conductor so the heat can be dissipated from it very quickly.

Germanium Transistors

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Germanium is used as semiconductor in transistors and other semiconductor devices. The germanium 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 infrared optics. 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 the germanium transistors are prone to failure.

Power Transistor

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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 that the input power is amplified.

Field Effect Transistors

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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. Gate
  2. Source
  3. Drain.

Gate is the channel that modulates the conductivity. Source is a terminal where the majority 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

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The bipolar transistors are of two types:
  1. NPN
  2. PNP.
NPN transistor has a layer of P-doped semiconductor (the "base") sandwiched between two 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 commonly used in electronic circuitry.

PNP Transistors

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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

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Given the transistor figure:
                                          Biasing in NPN Transistor

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 Negative voltage to emitter is being applied.
  2. The voltage at Base is slightly positive than that of Emitter and the voltage at Collector is more positive than that of Base region.
  3. Due to this arrangement the base region attracts electrons from the emitter region and 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 to that 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 of current from the Collector to Emitter.
  5. Now take the case that the Base region is not connected to voltage supply, so the there 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.

RF Transistors

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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. These transistors should be very high precision and reliability.

Thin Film Transistors

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A thin-film transistor, or more generally TFT, is a FET (field-effect transistor). It is constructed 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 liquid crystal displays. In the conventional transistors the semiconductor material is used as the substrate while in TFT generally glass is used as substrate.

Bipolar Transistor

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A Bipolar (junction) transistor (BJT) bipolar junction three-terminal electronic device which 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 both electrons 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 transistor
  2. PNP transistor.

Transistor Configurations

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The three types of circuit connections for operating a transistor:
  1. Common Emitter (CE) Configuration
  2. Common Base (CB) Configuration
  3. Common Collector (CC) Configuration.
The common electrode is generally grounded and is common to the input and output circuit.
These are the Configuration circuits in NPN transistors:
                                  P N P Common Emitter Configuration
                                     P N P Common Base Configuration
                                   P N P Common Collector Configuration
These are the Configuration Circuits in PNP transistor:
                                   N P N Common Emitter Configuration
                                      N P N Common Base Configuration
                                  N P N Common Collector Configuration

Transistor Characteristics

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Circuit of n p n Transistor

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 on common 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 with base 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). To study 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 the graph. (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.

Input Characteristics of npn Transistor


Ri = $\frac{\delta V_{BE}}{\delta I_{B}}$
Now to study the output characteristics, input current is kept constant and the variation of out voltage and output current are measure and a graph is plotted (adjacent page). It resembles characteristics of a reverse biased P-N junction diode.

Output Characteristics of n p n Transistor

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 is
Ro = $\frac{\delta V_{CE}}{\delta I_{C}}$The current ratio between Collector and base is given by,
Current gain = $\frac{I_{c}}{I_{B}}$ which is nearly a constant.

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