A bipolar junction transistor (BJT) is a current-driven semiconductor element that uses a small amount of current to control the flow of a larger electric current. In contrast to a unipolar transistors, such as a field-effect transistors which uses only one kind of charge carrier, a BJT uses both electrons and electron holes as charge carriers. BJT Transistors can be used to amplify weak electrical signals and are also used as oscillators or as switches.
With the rapid development of electronic technology over the past ten years, there has been a huge increase in the use of electronic components in many high-tech industry products. Among these many newly developed electronic components, transistors play the most critical role. Many electronic components are passive components such as resistors, capacitors, and inductors. The reason why they are called passive components is they do not exert an effect on the electronic signal, that is, they do not amplify or change the electronic signal. Components that can exert a change to electronic signals are called active components. Chips are designed to use integrated electronic circuits, (mainly semiconductors transistors) to modulate or switch electronic signals to achieve the desired function. They are key components in many high-tech products.
What is a Transistor?
A transistor is semiconductor device that controls and regulates the flow of electrical signals. By changing the amount of conduction or insulation of an electric current, it can change the voltage or flow of the current. It can perform as either a switch or an amplifier. A key components in most electronic devices today, the transistor was invented in 1947. Many developments have been made with transistors since then, and one major development has been the bipolar junction transistor (BJT).
Parts of the transistor:
Transistors are made up of three layers of semiconductor material (terminals) that connect to external circuits and carry current. Voltage or current applied to two of the terminals of the transistor controls the flow of current passing through one of those terminals and a third terminal.
BJT transistors have the following three terminals:
- Base: Used to enable current transmission
- Collector: The positive pole of the transistor
- Emitter: The negative pole of the transistor
Type of Transistors:
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Bipolar Junction Transistor (BJT): In a BJT, the electric current flowing into the transistor controls the devise. The three terminals of a BJT are base, emitter, and collector. A very small current flowing between the base and emitter can control a larger current flowing between the collector and emitter. BJTs include P-N-P transistors and N-P-N transistors.
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Field Effect Transistor (FET): In a FET, the voltage going into the transistor controls the devise. For FET’s, the three terminals are Gate, Source, and Drain. The voltage at the gate terminal can control the current between the source and drain, FET is a unipolar transistor, and can be either an N-channel FET or P-channel FET. FET’s main applications are in low noise amplifiers, buffer amplifiers, and analog switches.
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Other types: There are many types of FET transistors, including MOSFETs, JFETs, ITFETs, FREDFETs, OFEs, and more.
The main function of the BJT transistor:
In a BJT transistor there are two P-N junctions. In the normal operation of a BJT, the base-emitter junction is forward biased and the base-collector junction is reverse-biased, and when current flows through the base-emitter junction, current will flow into the collector circuit. Because the BJT transistor can act as either P-N or N-P, it is also called a NPN transistor.
How does NPN transistors work?
The emitter of an NPN device is made of n-type material, so the majority carriers are electrons. When the base-emitter junction is forward biased, electrons will move from the n-type region to the p-type region, leaving the minority carriers empty. The holes move towards the n-type region. When they meet, they will combine, allowing current to flow through the junction. When the junction is reverse biased, the holes and electrons move away from the junction, and now a depletion region is formed between the two regions, and no current flows through it.
When current flows between the base and emitter, electrons will leave the emitter and flow into the base. Usually, electrons combine when they reach the depletion region. But the low doping level in this region and the thin base means that most electrons can pass through this region without recombining with holes, and the electrons drift toward the collector. By flowing through the effectively reverse-biased junction, the current flows through s the collector circuit.
What are the characteristics of a transistor?
- Input Characteristics: While keeping the output voltage constant, the input current varies as a function of the input voltage.
- Output characteristics: While keeping the input current constant, the relationship between the output current and output voltage changes.
- Current transfer characteristics: While keeping the voltage constant, the output current varies with the input current.
What are the advantages of transistors?
- Lower cost and smaller size.
- Less mechanical sensitivity.
- Low operating voltage.
- Long life.
- No power consumption.
- Switch quickly.
- More efficient circuits can be developed.
- Used to develop a single integrated circuit.
What are the limitations of transistors?
- Transistors lack higher electron mobility.
- Transistors are easily damaged when electrical and thermal events occur.
- Transistors are affected by electronic fields and radiation.
Technological development of transistors:
With the introduction of new equipment for the miniaturization of transistors, the demand for the reduction in size of transistors has increased, as well as the requirements for improvement of component characteristics. With the rapid development of high-tech products, from computers to smartphones, the public has shown a higher and higher demand for the computing speed of chips. This has accelerated the demand for transistors with enhanced performance. In this nano-generation of transistor processing, the introduction of stress technology (strain) has led to changes in material characteristics, increasing the functional density of these devices.
With the evolution of Very-large-scale integration (VLSI) technology, future development of transistors will focus on artificial intelligence (AI). The development of artificial intelligence relies on the machine's self-deep learning technology, big-data analysis, and processing capabilities. Huge databases enable the machine to achieve intelligent responses.
The development of artificial intelligence requires the processing of huge amounts of data and high-speed computing capabilities. Due to these requirements, the hardware of supercomputers plays an indispensable role in artificial intelligence. Developing of computers with ultra-high-speed computing capabilities will be a focus of future development.
The Internet of Things (IoT) is also another highly valued application for IC technology. The IoT can connect countless electronic products through the internet for remote control and collection of data. The vast possibilities of these technological developments will only be realized by the continued development of semiconductor electronic components. Therefore, there is still unlimited development potential for transistors in the future.
