The transistor is a device that amplifies a signal consisting of voltage or current. It is a three terminal semiconductor device and its terminals are collector, base, and emitter. Let’s see further what is a transistor, how it works, and its applications in electronics.
To understand what is transistor and its working, and its applications in electronics, consider a tap, connected to a water tank. When the knob of that tap is tight then water doesn’t fall, but as we turn the knob on, water starts to fall. In the case of a transistor, the one end of the tap which is is collector, and another end from where water falls, is the emitter and the knob is the base. So, through the base terminal, we can control the current flow from collector to emitter.
Using this property we can design many circuits from it. Such as amplifiers, electronic switches, oscillators and many more.
There are two types of transistors.
- Bipolar junction transistor (BJT)
- Field effect transistor (FET)
They further divided both types of transistors into sub-categories. Let’s see the first type of transistor which is the bipolar junction transistor.
What is bipolar junction transistor?
A bipolar junction transistor is a type of transistor that has three layers of semiconductor. It has three terminal base, collector, and emitter. It is a current controlled device. That means the current through the base terminal to the emitter terminal decides the current through the collector terminal to the emitter terminal. Read further to know what is a bipolar junction transistor, its structure, why we call it bipolar and junction transistor, and what is its application in electronics.
Internal structure of a bipolar junction transistor (BJT)
Bipolar junction transistor has three doped regions. All three are structured such as they form two p-n junctions between them. Therefore, it is clear that there can be two types of junction transistors as shown in the figure below.
- n-p-n transistor: – It has two n- type semiconductor separated by a p- type semiconductor (fig a.(i)). Symbol of the n-p-n transistor is shown in figure (b)(i).
- p-n-p transistor: – In this, two blocks of p- type semiconductor are separated by a block of a n-type of semiconductor(figure (a)(ii)). Symbol of p-n-p transistor is shown in figure (b)(ii).
The width of all three blocks of a transistor is different. Their doping levels are also. The arrow used in the symbol of an n-p-n or p-n-p transistor is the direction of the current flowing in it. Now, let’s briefly discuss the three blocks of semiconductors in it.
- Emitter: – A semiconductor block with medium size but highly dopped. It gives a large amount of majority charge carriers for current flowing.
- Collector: – A larger size and simple dopped compared with emitter. It collects a major amount of charge vessels provided by the emitter.
- Base: – It is present in the center of the both blocks (emitter and collector) and separates both. It is extremely thin and less dopped.
How a BJT works (what is going on inside it)?
We know that in the case of forming a p-n junction, a depletion region forms across the junction. In a transistor, depletion regions are formed across the emitter-base and base-collector junctions. To know the working principle of a transistor, we have to know the nature of depletion regions formed on these junctions. When proper biasing is applied on its terminals, the charge carriers start moving across different regions. It is biased differently for different purposes.
In the figure above, base-emitter is in forwarding bias produced by VEE and base-collector is in reverse bias produced by VCC. In this biasing transistor is inactive state and acts as an amplifier. Voltages between the emitter-base and collector-base are denoted by VEB and VCB respectively.
There is a high density of majority carriers in the highly doped emitter region that is holes in the p-n-n transistor and electrons in the n-p-n transistor. These carriers enter into the base region.
In a p-n-p transistor, the base carrier is the electron. So, majority of holes which is coming from the emitter region, neutralize the electrons of the base region. Since the base-collector junction is in reverse bias, these holes, which appear as minority charge carriers on this junction, easily cross the junction and reach into the collector region. Holes present in the base region either move towards the base terminal to neutralize with electrons coming from outside, or to reach the collector, it crosses the junction and reaches the collector terminal. The base is made thin so that the holes do not go to the base terminal and cross the junction.
For a transistor
The principal is same in the n-p-n transistor but currents got reverse.
Configurations of Transistor (Ways to connect a transistor in circuit)
Since transistor has three terminals- emitter, base, and collector. In any transistor circuit, one terminal should be common. So, it can be used in three configurations.
- First configuration is the Common emitter (CE) configuration where emitter terminal is common.
- Second configuration is Common base (CB) configuration where base is common.
- Third and last configuration is Common collector (CC) configuration where collector is common.
The common emitter is the most used circuit and the most used transistor is n-p-n silicon transistor. In the figure below, the circuit diagram in CE configuration and its input/output characteristics are shown for a silicon transistor.
Applications of transistor in electronics
There are plenty of applications of a transistor in electronics. Here are the most common applications of a transistor. Transistors are mostly used as electronic switches, amplifiers, oscillators, etc.
Bipolar junction transistor (BJT) as a switch
The figure given below is the circuit diagram for the transistor as a switch. The circuit is in a common emitter configuration.
Applying KVL in the input and output terms.
You can clearly see that if collector IC current increases then output voltage VO will decrease. When the input voltage is less(almost equal to zero) then it is in cut-off state and no collector current (IC) is flowing, in that condition output voltage VO is equal to VCC. As Vi increases, the collector current starts flowing and output voltage starts decreasing, it is the active state of a transistor. When VI is enough to work it in saturated state, in that condition base and collector current is maximum. So, the output voltage is minimum. Let’s understand it by a graph given below.
By the graph, you can observe that output voltage VO is high till input voltage Vi is low. If the input voltage is that much so it can take the transistor into the saturated state then output is low. So, this is the logic for working it as a switch.
bipolar junction transistor (BJT) as an amplifier
It is important to operate a transistor as an amplifier, it should be inactive state. Input voltage should be fixed in between active states.
If a small cyclic with VS amplitude signal source is connected in series with the power supply VBB then IB current will flow with a small cyclic difference. As result, the collector current will also flow with the same cyclic difference which is equivalent to base current IB. DC voltages on input and output are filtered using capacitors so we can measure the ac changes in output.
The circuit is given below for a transistor amplifier.
We have learned –
- Types of transistor
- What is bipolar junction transistor
- Types of bipolar junction transistor
- How a bipolar junction transistor works
- Configurations of BJT
- BJT as switch
- BJT as amplifier