Transistor: Types, Principle, Applications, Examples

Table of Contents

What is a Transistor?

A transistor is a three-terminal semiconductor device used to amplify or switch electrical signals and power. It is one of the most essential tools in electronics. The power provided to one terminal is regulated by the other terminal, which amplifies or switches the signal as required. They are utilized in digital or analog circuits and can be incorporated in ICs.

Semiconductor materials such as pure silicon or germanium are utilized to make transistors. Transistors were discovered in 1947 by three scientists: John Bardeen, Walter Brattain and William Shockley and were the greatest invention of the time. In 1956, they were awarded the Nobel Prize for their great achievement. Similarly, in the 1960s, the Metal-Oxide-Semiconductor-Field-Effect-Transistors (MOSFET) were developed that made it possible to place billions of transistors inside one chip. Today, every microchip requires transistors to operate any ordinary device like a calculator or complex automobiles.

Types of Transistors

Based on their usage in circuits, transistors are mainly categorized into two types: Bipolar Junction Transistors (BJT) and Field Effect Transistors (FET).

Bipolar Junction Transistors (BJTs) – It has three terminals: Emitter, Base and Collector. A small current is passed through the base and the emitter terminal which controls the larger flow of current through the base and the collector terminal. BJTs are also of two types: NPN and PNP, depending upon the type of material used.

In the BJTs, there are three types of configurations known as common-base (CB), common-emitter (CE) and common-collector (CC). The base terminal is common between input and output terminals in the common base construction. Similarly, the collector terminal is common in the common collector configuration whereas the emitter terminal is common between the input and output terminals in the common emitter configuration.

Field Effect Transistors (FETs) -There are three terminals, Gate, Source and Drain in the Field Effect Transistor. The voltage provided to the gate can control the flow of current between the source and the drain. FET is a type of unipolar transistor that uses either an N-channel or a P-channel FET for conduction. FETs are mainly used in low-noise amplifiers, buffer amplifiers, and analog switches.

Beyond these two, there are several other transistors like Junction Field Effect Transistor (JFET), Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET), Darlington Transistors and Phototransistors etc.

How Transistors Work?

The basic principle of a transistor is to control the flowing of current through one channel by changing the intensity of a smaller current flowing through a second channel. The operation of a transistor is highly influenced by its type. In a Bipolar Junction Transistor (BJT), a small current is applied to the base that controls the flow between the collector and emitter terminals. In a Field Effect Transistor (FET), a voltage is applied to the gate terminal that controls the flow of current between the source and drain.

For illustration here is the brief description of the working of NPN configuration of the BJT. The emitter in an NPN transistor is made of n-type material, therefore the majority of the charge carriers are electrons. When the junction of the base-emitter is forward biased, electrons flow from the n-type region to the p-type region, while minority charge carrier holes travel towards the n-type region.

When they meet, they mix and allow current to flow across the junction. When the junction is reverse-biased, the holes and electrons go away from it, creating a depletion region between the two positions where no current flows. When a current passes through the base and emitter, electrons escape the emitter and enter the base, as seen above. Normally, electrons unite when they reach the depletion area.

However, the doping level in this region is really low, and the base is extremely weak. This means that the majority of the electrons can go over the zone without reuniting with holes. As a consequence, the electrons will go towards the collector. This allows them to move across the reverse-biased junction, while the current flows in the collector circuit.

The same concept applies to the PNP transistor, with the only difference that the current carriers are holes and the voltages are opposite.

The Invention of the Transistor

A vacuum tube known as thermionic triode was constructed in 1907 which enabled the amplification of radio technology and long-distance telecommunications. However, the triode was a challenging device that consumed a lot of power. The collaboration of John Bardeen, William Shockley, and Walter Brattain made the invention of the transistor in 1947. It replaced the use of large vacuum tubes in amplifying and switching signals which transformed electronics, making portable, more efficient and highly reliable electronic equipment. Their combined discovery received the Nobel Prize in Physics in 1956.

Vacuum tubes were bulky and were very difficult to use in the devices. Therefore, scientists started searching for a better alternative. Scientists in the Bell Laboratories were researching germanium and silicon. These materials were found to be conducting electricity under special conditions. Thus, theories were developed on these materials stating that more efficient amplifiers could be built from these materials.

On December 16, 1947, John Bardeen and Walter Brattain built the first point-contact transistor using germanium which controlled the flow of electrons at the surface of the semiconductor. Later, Shockley in 1948 discovered the Bipolar Junction Transistor which was found to be very practicable and more efficient to use. It became the basis for all contemporary transistors.

Transistor Structure and Materials

Typically, a transistor contains three layers of semiconductor materials or terminals that are used in the connection of a circuit and thus carry the current. A potential difference or current is applied to one pair of the terminals which controls the flow of the applied voltage or current through the other pair of terminals. The three terminals are called the emitter, base and the collector. Collector is the positive terminal of the transistor while the emitter is the negative terminal. The base region serves as an activating terminal for the transistor.

Transistors are composed of semiconductor materials which have special properties for the conduction of electricity. Silicon and germanium are two best examples which are frequently used as semiconducting materials. Germanium was widely used in the early devices. Later, it got replaced by silicon because of its thermal instability. Still it is being used in high-speed applications. Silicon is used mostly as it is abundant, stable and cost-effective. Today, other advanced materials such as gallium arsenide (GaAs) and silicon carbide (SiC) are being used to get better performance.

A transistor is a three-layer semiconductor component layered between two identical types of semiconductor which is also known as transistor fabrication. The structure varies with the type of transistor. The mostly used transistor materials are PNP and NPN transistors.

P-N-P Transistor: In this configuration one n-type material is placed between two p-type materials. Two crystal diodes are connected in series in a PNP transistor. The right side of the diode is the collector-base diode while the left side is called the emitter-base diode.
N-P-N Transistor: In this configuration one p-type material is placed between two n-type materials. N-P-N transistors are mostly used to amplify weak signals into powerful signals. In an NPN transistor, electrons travel from the emitter to the collector region which causes the current to flow through the transistor.

Applications/Uses of Transistors

Transistors are widely applied in almost all modern devices. Some of its applications are listed below:

As a Switch: For controlling DC power to a load, BJT transistors can be applied as a form of switching. The regulated current flows between the emitter and base, whereas the switched (controlled) current flows between the emitter and collector. They are also used widely in microprocessors and signal processing.
As an Amplifier: A transistor can amplify current. Vbe will act as a bias voltage created at the base-emitter junction. Electrons begin to flow from the emitter because of the forward biasing of the base-emitter junction to reunite with holes in the base. This causes the base to become negatively charged. If the base current Ib is increased slightly, hole-electron recombination becomes neutral, and the collector current Ic increases. As a result, the base’s current Ib changes slightly.
Microphone: The electrical output of the microphone changes depending on the sound waves because the base current Ib fluctuates due to the microphone’s low alternating voltage, implying that a small change in Ib can induce a huge change in Ic. When the microphone’s output is passed into the transistor as input, the fluctuating collector current Ic goes into the speaker, and we realize that changing the transistor’s input causes a significant variation in the transistor’s output. The transistor amplifies the microphone’s electronic signal. The frequency remains constant, but the loudspeaker’s sound wave has larger amplitude than the sound waves delivered into the microphone.
Medical Devices – Transistors are essential electronic components in pacemakers and imaging techniques.
Automobiles -Transistors are also used in engine control units and infotainment systems.

Analog vs. Digital Transistors

The use of a transistor varies according to its functions. The two basic transistors based on their functions are digital and analog transistors.

Digital Transistors – Digital transistors are basically used for switching operations and also in obtaining continuous signal amplification like audio and radio applications. They operate in cut-off and saturation regions thus are ideal for logic gates, microprocessors and flip-flops. We can get fast switching speeds and clear ON/OFF states also with low power consumption.
Analog Transistors – Analog transistors are basically used for obtaining continuous signal amplification like audio and radio applications. They operate in the active region to create an appropriate correlation between input and output signals. Amplifiers, oscillators, analog signal processing, etc utilise analog transistors.

How to Test a Transistor?

To test a transistor we can study the following guides.

Using a Multimeter

Plug the probes into the multimeter. The black probe gets into the common terminal, while the red probe gets into the terminal labeled for testing diodes.
Set the button on the diode test function.
Use alligator clamps instead of probe tips.

If base, emitter and collector are well determined then we can use following tips

Attach the black probe to the base of the transistor.
Place the red probe to the emitter. Check the multimeter’s screen and note if the resistance is low or high.
Insert the red probe to the collector. The screen should give the same reading as touched to the emitter.
Pull the black probe and fix the red probe at the base.
Apply the black probe to the emitter and collector. Check the readings on the multimeter’s screen and compare with the earlier readings.
If the previous measurements were both low and the present readings were both high or vice-versa, the transistor is working otherwise it is dead.

If the three terminals cannot be determined, following tips can be used.

Attach the black probe to any one of the terminals.
Tap the red probe on top of the other two terminals.
If the indicator shows high resistance when all of the leads get touched, we can say it is the base and a nice NPN transistor.
If the indicator gives two different readings for the other two leads, attach the black probe to a separate lead and redo the procedure.
Once we clamp the black probe to each of the three terminals and don’t receive the same high resistance reading when tapping the other two terminals with the red probe, we have either a faulty transistor or a PNP transistor.

Common Transistor Problems and Troubleshooting Them

Problems

Short Circuit – Despite the input, the transistor acts like it is always active. High voltage, high current, or overheating could be the cause of this.
Leakage Current – A damaged transistor may allow excessive current to flow. This might be due to a faulty component or excessive heat.
Failure to Switch Properly – A damaged gate or base junction may fail to switch properly. An incorrect resistor or wrong base voltage may be the cause for it.
Burnt Components – Any internal damaged components, broken leads or poor soldering in the transistor cannot conduct properly even if it is properly biased.

Troubleshooting

Test the transistor properly before using it in testing devices like multimeters.
Check properly whether the transistor is burnt, discolored or the leads are broken.
Also check other components like resistors, capacitors, diodes etc. that can make a transistor faulty in the circuit.

DIY Transistor Projects for Beginners and Hobbyists

If anyone is interested in learning more about transistors or beginners and hobbyists, going through project works related to transistors, here are some DIY transistor projects to gain some skills on the usage of transistors.

Simple LED – A BJT NPN transistor (BC547) is used in LEDs to turn the flash on and off. Other components like 1kΩ resistor, 10kΩ resistor, 100µF capacitor and a battery are used for the circuit. The capacitor charges and discharges as a timer circuit.
Audio Amplifier– A BJT NPN transistor (BC547) is also used in audio amplifiers. A speaker, 10kΩ resistor, 100Ω resistor, 100µF capacitor, audio source (phone, MP3 player) are other components required for the project. To power the speaker, the transistor amplifies a weak audio signal coming from the source.
Touch Sensor Circuit – NPN transistor (BC547), LED, 470Ω resistor, touch plates (foil or metal plates) and a battery are required for the project. A tiny current flows when the plates are touched that turns the transistor on and flashes the LED.

The Future of Transistors

Transistors can revolutionize the electronics with the advancements in the devices and circuits. Some of the predicted and ongoing researches on transistors are given below:

Nanotransistors – They are built at an atomic scale to attain ultra-fast computing.
Graphene Transistors – They are ultra-thin transistors designed to gain superior conductivity and flexibility.
Quantum Transistors – On advancements in quantum computing, quantum transistors are being developed that utilize electron-spin for the data storage and computation. These might be the ultra-fast and low-energy devices used in computing systems.
Neuromorphic Transistors- In the era of AI, neuromorphic transistors can be used to mimic or replicate the efficiency of the human brain.

Conclusion

Transistors turned electronics and remained to prove vital to advances in technology. Their uses in computers, communication, medical devices, and automation have made them extremely important in modern times. With extra research and development, transistors are going to improve, extending the edges of technology even more.

References

Amos, S. W., & James, M. R. (1963). Principles of Transistor Circuits. American Journal of Physics, 31(4), 309-310.

Amos, S. W., & James, M. (2013). Principles of transistor circuits: introduction to the design of amplifiers, receivers and digital circuits. Elsevier.

Lundstrom, M. S. (2022). Transistors! (Vol. 1). World Scientific.

Pulfrey, D. L. (2010). Understanding modern transistors and diodes. Cambridge University Press.

Schwierz, F. (2010). Graphene transistors. Nature nanotechnology, 5(7), 487-496.

Cao, W., Bu, H., Vinet, M., Cao, M., Takagi, S., Hwang, S., … & Banerjee, K. (2023). The future transistors. Nature, 620(7974), 501-515.

https://builtin.com/hardware/transistor

https://www.geeksforgeeks.org/electronics-engineering/what-is-transistor/