Transistor was called as “Transit Resistor” or “Transfer Resistor” initially. These are the elemental unit of electronic circuits. They are so heavily used that you can not imagine a PCB without this component. This article will discuss about what is Transistor, its configuration, classification, working principle, applications, advantages and disadvantages.
What is Transistor
Transistors are electronic devices which forms the basic and major component of any electronic circuits. Earlier Transistors were made of Germanium which were temperature sensitive and slowly were replaced with silicon. Silicon Transistors are cheaper to manufacture. They are elementary units of microchips and computers.
Fig. 1 – Physical View of Transistor
It can also be defined as a three terminal device which consists of three distinct layers where two similar layers sandwich the layer of the opposite type is called a ‘Transistor’.
The layers could be two ‘’P” type sandwiching one “N” type or two “N” type sandwiching one “P” type as shown in the Fig.2. The left terminal is called as Emitter, the right terminal is called as Collector and the area in the middle is called as Base.
Classification of Transistors
Primarily Transistors are of three types namely:
- Point Contact transistor
- Field Effect Transistor (FET)
- Bipolar Junction Transistor (BJT)
Fig. 2 – Classification of Transistors
They were the first Transistors invented which used Germanium as its semiconductor, and two Phosphor Bronze wires were passed through it. High current pulses were used to fuse the wires & this caused Phosphorus to diffuse from the wires in to the Germanium which created P-type regions around the points. A PNP structure was formed, exhibiting negative resistance.
Field Effect Transistor (FET)
FET’s are three terminal devices having Gate, Source, Drain and the Substrate is considered as the fourth terminal. They are voltage controlled devices that have control on the size and the shape of the channel. Since they operate on single channel, they are labelled as unipolar Transistors. They are further categorized as:
Junction Gate Field-Effect Transistor is a three terminal device which does not require biasing current and entirely voltage controlled. They are termed as Depletion mode devices having N-type and P-type channel.
Metal Oxide Silicon Field Effect Transistor is also unipolar transistor having Gate, Source, Drain and Substrate. The two modes of operation of MOSFET’s are Enhancement Mode and Depletion Mode having N-type and P-type channel.
Bipolar Junction Transistor (BJT)
Junction transistors are generally called BJT or Bipolar Junction Transistors having three terminals emitter, base and collector. The current controls the transistors i.e. a small current flowing through the base causes large current to flow from emitter to collector. The BJT’s turns “ON” by the input current which is applied to the base.
They are further categorized as:
- NPN Transistors
- PNP Transistors
Fig. 3 – Basic Structure & Circuit Symbol of NPN – PNP Transistors
A NPN Transistor is a type of BJT which is composed of three layers where ‘P’ doped layer is sandwiched between two ‘N’ doped layers.
A PNP Transistor is a type of BJT which is composed of three layers where ‘N’ doped layer is sandwiched between two ‘P’ doped layers.
Transistor Circuit Configurations
Transistor has three terminals namely Emitter, Base and Collector terminals but when it is connected in a circuit, four terminals are required. Two terminals are required for the input and the other two for the output. Hence one terminal of the Transistor is made common to both input and output terminals. The Emitter and the Collector is invariably forward and reverse biased respectively.
There are three ways of connecting a Transistor in a circuit, namely:
- Common Base connection (Common Base Circuit)
- Common Emitter connection (Common Emitter Circuit)
- Common Collector connection (Common Collector Circuit)
Common Base Circuit
As the name suggests, this type of circuit has Common Base for both input and output. The input signal is applied between Emitter and Base and output is obtained from Collector and Base. In this type of circuit, the Emitter Current (IE) which is also the Input current is high. Hence input Resistance is low.
Fig. 4 – Common Base Circuit
Because of the Reverse Voltage at the Collector, the output Resistance is high. Common Base connection is rarely used as it has no current Gain.
Common Emitter Circuit
It is the most widely used circuit in all Transistor applications. This circuit as shown in the Fig.5 has Common Emitter for both input and output. The input signal is applied between Base and Emitter and output is obtained from Collector and Emitter. Because of small Base Current (IB), the input Resistance is very high.
Fig. 5 – Common Emitter Circuit
The output Resistance of CE circuit is low and current Gain is large because Collector current (IC) is much larger than the Base Current (IB). The voltage gain is high in this circuit.
Common Collector Circuit
In this type of circuit as shown in the Fig.6 Collector is made common to both input and output. The input signal is applied between Base and Collector while output is obtained between the Emitter and Collector.
Fig. 6 – Common Collector Circuit
Input Resistance is high and output Resistance is low when compared to other circuits. There is no voltage gain. This circuit finds its use in impedance matching.
How does Transistor Work
To understand the working principle of a Transistor, let us consider NPN Transistor as shown in the Fig.6. The junction between Emitter and Base is known as Emitter-Base junction which is forward biased. Similarly, junction between the Base and Collector is known as Collector-Base junction which is reverse biased.
Fig. 7 – Basic Circuit of NPN Transistor
The forward bias causes the electrons or current in the N-type Emitter to flow towards the Base.The Emitter is heavily doped so that large number of charge carriers (electrons) is infused into the Base region. This current from the Emitter region is called as Emitter current IE. As the current flows through the P-type Base, few electrons combine with the holes. This constitutes Base current IB . The Base is very thin and lightly doped which helps the remaining charge carriers to move towards Collector region.
Reverse bias on the Collector attracts electrons. The Collector is moderately doped and allows the remaining electrons to flow through it. Hence we can conclude that the Emitter current is the sum of the Base Current and Collector Current i.e. IE = IB + IC
Applications of Transistor
The applications include:
- Transistors are used in oscillators and modulators as amplifiers.
- They are used in digital circuits as switches.
- Transistors are used in Radio-frequency circuits for wireless systems.
- Transistor switches are used in Burglar alarms, industrial control circuits, memories and microprocessors.
- They are used in Sub Wordline Driver (SWD) to produce low frequency currents.
- MOSFET’s are used in Chopper circuits.
- JFET, MOSFET can act as a passive element like Resistor.
Advantages of Transistor
The advantages are:
- Transistors are compact in size.
- They provide high voltage gain and requires less supply voltage.
- They do not require heating power as they do not have filament.
- Transistors have higher life expectancy than vacuum tubes.
- Controlling high power circuits is easier.
- Circuit design is simpler.
Disadvantages of Transistor
The disadvantages are:
- Leakage current is amplified in the Common Emitter circuit.
- They have lower Power dissipation below 300W.
- Transistors except FET (Field Effect Transistors) have low input impedance.
- They are temperature dependent.