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What Is a Transistor?
A transistor is a small electronic component, or semiconductor, that can be used as an amplifier or as a switch. It does this by converting the input current and controlling the voltage running through the circuit. Transistors are found in an array of electronic circuits and applications.
Our detailed guide will explain all that there is to know about transistors, including the history and definition of a transistor, the different types, how they work and some common uses.
History of the transistor
1947 was the year when the transistor was invented at Bell Laboratories, USA. Greatly impacting the electronics and technology industry, these new components were much smaller than previous counterparts and could be used across multiple applications, unlike the heavier, more costly and inefficient options from before.
Like many electronic devices, transistors have continued to reduce in size, making them easier to apply to a range of computer microprocessors and other electronic items.
What parts make up a transistor?
A transistor is essentially a number of diodes placed together. It's made of different types of silicone and contains three terminals. In a BJT transistor, these are named the emitter (or negative lead), the base (which enables the transistor to function) and the collector (or positive lead). The small current at one terminal is used to generate a larger current at the remaining terminals. In an FET transistor, these terminals are named drain, gate and source.
A transistor also contains two PN junctions. In the case of a BJT transistor, these consist of a collector-base junction that is reverse-biased and an emitter-base junction that is forward-biased.
They can be either NPN or PNP and the emitter and collector will always feature the same charge, with the base acting as a conveyor between the two. However, the direction of the arrow is changed, depending on the configuration (whether it's an NPN transistor or PNP transistor).
An FET transistor is configured slightly differently, as explained further down, and as shown below in this transistor symbol:
What does a transistor do?
As mentioned previously, transistors are mainly employed for switching and amplification purposes. They can switch the flow of electricity in a circuit both off and on, depending on the application.
Transistors can also be used to limit or otherwise control the voltage, which is useful in a range of functions where too much current into a transistor emitter, without control, would cause a device to blow up.
When used for amplification, a transistor is configured to be partly switched on.
How does a transistor work?
To answer this, we need to take a look at the types of transistors. The two main ones are:
- BJT (Bipolar Junction Transistors)
- FET (Field-Effect Transistors)
- These can then be further divided into two categories per type as shown in the diagram below:
The workings of a transistor is directly related to the movement of electrons and holes but, to explain how they function, we'll take a look in detail at each type.
How does a bipolar junction transistor work?
These types of transistors can either be NPN or PNP and both use electrons and holes to create the flow of current.
NPN BJT transistor
This type of bipolar junction transistor features electrons as the main charge carriers while holes are the minority carriers. When voltage is applied at the base terminal, it allows the current to flow from collector to emitter terminal.
In an NPN transistor, when the transistor is in an OFF state and there is no current at the base side, the holes at the base terminal act as a barrier and prevent the movement of electrons from emitter to collector terminal.
PNP BJT transistor
A PNP transistor is a type of BJT transistor where holes are the major charge carriers and electrons are the minority carriers. When voltage is applied at the base terminal, current starts to flow from the emitter to the collector terminal.
The electrons then combine with the holes available in the base pin. The surplus electrons that don’t combine with the holes will then enter the collector terminal and will turn ON the transistor. Hence, the small current at the base pin will introduce a large current at the emitter and collector terminals. This phenomenon is used for amplification purposes.
The diagram below shows the workings of BJT transistor circuit:
If you're wondering how a transistor can be used as a switch, it essentially comes down to the base current.
When there is no current at the base pin, there will be no current at the remaining terminals, allowing the transistor to remain in an OFF state. However, little current at the base pin will result in a big current flow. Hence the base current is responsible for turning ON and OFF the transistor and making it work as a switch.
How does a field-effect transistor work?
FET transistors are voltage-controlled devices that are further divided into two main types .
Junction field-effect transistors (JFETs)
A junction-gate field-effect transistor is a semiconductor device mainly used to make amplifiers and electrically controlled switches. It is a voltage-controlled device because it doesn’t require a biasing current to start the transistor action.
Containing three terminals, drain, gate and source, a positive voltage can be attached to the gate, enabling electrons to travel through the source and to the gain. Junction field-effect transistors are further divided into two types - N-Channel JFET and P-Channel JFET.
The following figure shows the construction of N-Channel JFET where the gate terminal is made up of P-type material, which then forms the reverse-biased PN-junction. This junction produces the depletion region around the gate terminal in the absence of external voltage.
When there is no external voltage (VG = 0) at the gate terminal and a small voltage (VDS) applied across the source and drain terminals, the maximum saturation current (IDSS) can flow from the drain pin to the source pin.
If you apply a small negative voltage to the gate pin, it results in increasing the size of the depletion region and therefore reduces the channel area and the flow of current.
A larger amount of negative voltage at the gate pin will increase the depletion region. This will further reduce the channel width to the point when there is no more current flow between the source and drain terminals.
The point at which there is no current between source and drain terminals is called 'Pinched-Off' voltage.
H4: Metal-oxide-semiconductor field-effect transistors (MOSFETs)
The MOSFET transistor is a type of FET that is made from controlled oxidation of a semiconductor, where the gate terminal controls the flow of current. This transistor contains the same terminals as a JFET but features an additional body terminal.
The main advantage of a MOSFET transistor is that it can operate in both depletion and enhancement, unlike the standard field-effect transistor. While it can be more easily damaged, the MOSFET is generally suited to a wider range of applications.
Other types of transistors
There are few other transistor types which don't fall under the above categories and which serve specific purposes.
- Schottky transistor - a bipolar junction transistor with a Schottky diode, this transistor features a fast switching response.
- Thin-film transistor - Containing glass, instead of silicone, these types of transistors are found in devices such as LCD monitors.
You might also hear some types of transistors being referred to by other terms such as Darlington transistors, photo transistors and high-frequency transistors. These are categorised as such due to their functionality.
Transistors can be characterised by their I-V curve. Also known as a characteristic curve, this can be seen visually on a plotted graph and represents the relation between electric voltage and the electric current of the device.
Based on the configuration of the circuit, transistor characteristics curves are categorised into three main types .
The input-characteristic curve demonstrates any changes that happen in the input electric current due to the variation of input electric voltage in the presence of constant output voltage.
The output-characteristic curve is shown by a plot between output voltage on the y-axis and output current on the x-axis, while keeping the input current constant.
Current Transfer Characteristic Curve
This curve represents the change of the output current due to the input current while keeping the output voltage constant.
It's often important to know and understand the characteristic curve of a transistor to determine a device's performance and predict how components will behave within a circuit.
How to test a transistor
It can be helpful to test a transistor if you want to know that it's functioning correctly.
As transistors can be thought of as diodes, the leads can be tested individually by using a multimeter, set to the diode-testing function. The readings that you'll get will depend on whether you're testing an NPN transistor or a PNP transistor.
You'll need to connect the positive and negative multimeter leads to each terminal, in turn, to check the voltage.
Testing a bipolar junction transistor requires a slightly different method. To do this, you'll need to connect the red multimeter lead red to the source terminal and the black lead to the drain terminal. If the multimeter fails to display a reading, it's likely that the transistor is faulty.
The leads should then be reversed so that you're testing the opposites. The multimeter should display zero.
If any readings suggest an issue with a transistor, it’s important to carry out the test again and, if needed, replace the component.
Common applications of transistors
As explained, transistors have a multitude of uses, both in day-to-day applications and across specialised electrical equipment. In recent years, transistors have become a common component of many devices.
Transistors are frequently used for switching and amplification purposes across an extensive range of applications but more specific uses include:
- In the making of integrated circuits that are further used for the development of processors
- Logic gates and logarithmic converters
- Radio transmission and signal processing
- Computer memory chips for data storing
- Mobile phones, hearing aids, pacemakers and other similar devices
- Audio amplification such as in stereos
MOSFET transistors are commonly used in applications that require high frequencies such as diagnostic equipment and microwaves. They're also integral to the functioning of most modern and digital electronics such as smart devices. For this reason, these types of transistors are in high demand and are therefore produced on mass.
The future of transistors
According to Moore's Law, as suggested in 1965 by co-founder of Intel, Gordon Moore, the size of these components will decrease to the point that the number of transistors that can fit into any given space will double every two years. The law also correctly assumes that, along with the reduction in size, these tiny components will also become much more affordable.
It is, in part, due to this accurate prediction that technology and digital electronics have advanced at such a great rate over the past decades. While the development of transistors over the years has fulfilled - and even surpassed - this law, though, there inevitably becomes a point where it's impossible to continue at the same rate.
Technological advances and innovations naturally slow down and, as we look to other systems such as artificial intelligence and the Internet of Things, future developments could be centred around specialist conductors and materials as well as high-performing computer programming models.