Introduction
Transistors are devices that control the electric current or the electron movement, and therefore electricity. They can be compared to how a water faucet works, transistors are able to start and stop the flow of a current and at the same time they can also control the amount of current that flows. Transistor is considered to be a switch or can amplify electronic signals and allows precision control over the current flowing in the circuit.
Field Effect Transistor, FET is an active semiconductor device for the electronics industry. FET is used in many circuits constructed from individual components in areas from RF technology to power control and electronic switching common amplifiers. The main application for the field-effect transistor, however, lies within the FET integrated circuits. In this application, FET circuits can use only a small amount of power, and this enables large-scale integrated circuits. If bipolar technology is used, the power consumption will be higher orders of magnitude, and the power generated will be too large to accommodate a single integrated circuit. The concept of the field-effect transistor is based on the idea that charging on a nearby object will attract charges within a semiconductor channel. It works using an electric field effect.
Field Effect transistors used in the presented experiment result.
Before FETs were launched and adapted in the electronic product industry, FETs has been established for many years. There have been many challenges in designing the FETS. In a paper published in 1926 by Lilienfeld and in 1935 by Heil, they presented the early ideas and concepts for the field-effect transistor. The preceding state was established in the 1940s at Bell Laboratories, where the semiconductor research group was founded. This group looked at several topics related to semiconductors and semiconductor technologies, including a system that could amplify the current flowing through a semiconductor channel by inserting an electric field near it.
The developers had been unable to render the concept work during these early prototypes and trials. They moved devised other concepts that lead to the invention of the bipolar junction transistor. Following this, much of the semiconductor development was devoted to refining the bipolar transistor, but the field-effect transistor was not fully explored for a long time. FETs are now commonly used in integrated circuits, serving as the primary active feature. Electronic technologies would be somewhat different today if these electronic components were not present.
The FET consists of a semiconductor channel, which is referred to as a drain and source at both ends with electrodes. A control electrode called the gate is placed too close to the channel so that its electric charge can affect the channel. In this way, the gate of the FET restricts the flow of carriers (electrons or holes) out of the source. It does this by limiting the size and shape of the conductive channel. The current flow semiconductor channel can be either P-type or N-type. This leads to two types or types of FETs, called P-channel and N-channel FETs. Apart from this, there are two further sections. Increasing the voltage at the gate can reduce or increase the number of charge carriers available in the channel. The result is an expansion mode FET and a reduction mode FET.
Since it is the electric field that controls the current in the channel, the device is said to be driven by voltage and has a high input impedance, usually several megaohms. This may be a distinct advantage over the current operated and bipolar transistor with the lowest input impedance. Furthermore, specifications such as allowable currents across the maximum voltage and capacitance are included in the FET, which play an important role in determining which FET transistors are suitable or compatible for any circuit or application.
Types of FET
I–V characteristics and output plot of an n-channel JFET. (Wikimedia)
There are different types of Field Effect Transistor but it has two major categories like Junction FET(JFET) and Metal Oxide Silicon FET (MOSFET). Junction FET(JFET) is used as a reverse-biased junction that provides a gate connection, which only reverse current can flow. It is the most common basic type of FET which provides excellent service in electronic applications. The Junction FET transistor is available in two polarities which are the N-Channel JFET and P-Channel JFET. N-Channel JFET consists of an n-type bar at the side of which two p-type layers are doped. On the other hand, P-Channel JFET consists of P-type material, at two sides of which n-type layers are doped. But, the N- Channel JFET has a greater conductivity than the P-Channel JFET which means that the N-Channel JFET is a more efficient conductor compared to P channel JFET.
Simulation result for right side: formation of inversion channel (electron density) and left side: current-gate voltage curve (transfer characteristics) in an n-channel nanowire MOSFET. (Wikimedia)
In MOSFET, it is used for switching or amplifying signals where the voltage determines the conductivity of the device. It is a four-terminal device that consists of a source, gate, drain, and body terminals. MOSFET can function in two ways: depletion mode and enhancement mode. Depletion mode is when there is no voltage in the gate, then the channel shows its maximum conductance. When the voltage on the gate is either positive or negative, the channel conductivity decreases. While on Enhancement mode, there is no voltage on the gate the device does not conduct. When there is more voltage on the gate, the better the device can conduct.
Basic Circuit Configuration (with Experiment Results)
There are three distinct modes of operation based on their configuration; Common source configuration, common gate configuration, and common drain configuration. In common source, the AC voltage is applied through the input gate and the measured AC voltage output is taken from the drain section. It is typically used due to good voltage amplification.
Common Source circuit used in the experiment
Input signal recorded in the oscilloscope
Output signal recorded in the oscilloscope
The tabulated measured values for input, output and voltage gain from the experiment.
The second mode of operation is the Common Gate Configuration wherein the level of impedance between the input and output is inversely proportional. It result to having a lower input impedance and higher output impedance. It can be applied to circuits that have a higher frequency.
Common Gate circuit used in the experiment
Input signal recorded in the oscilloscope
Output signal recorded in the oscilloscope
The tabulated measured values for input, output and voltage gain from the experiment.
The last method of operation is common drain, which is the reciprocal of the common gate configuration. It has a high input impedance leads to low output impedance. It detects that no signal is received at the drain connection, and there is no voltage.
Common Drain circuit used in the experiment
Input signal recorded in the oscilloscope
Output signal recorded in the oscilloscope
The tabulated measured values for input, output and voltage gain from the experiment.
Testing the experiment board prior to experiment.
Difference between BJT and FET
BJTs and FETs are two separate transistor types and also called active semiconductor devices. The BJT acronym is the Bipolar Junction Transistor, and FET stands for the Transistor Field Effect. BJTS and FETS are available in a variety of operating frequency, current, voltage and power ratings packages. These types of devices allow for greater control over their functioning. The BJTS and FETs can be used in electrical and electronic circuits as switches and amplifiers. The major difference between BJT and FET is that only majority charge carries flows in a field effect transistor, whereas most and minority charge carriers flow in BJT.
BJT stands for transistor with bipolar junction. It consists of one p-type and both n-types, known as n-p-n or n-type and both p-types known as p-n-p. Generally, the theory behind this is that two diodes can be paired with the p-n junction in such a way as to form a bipolar junction. There are three terminals of a BJT, namely: Base, Emitter, and Collector. While, FET belongs to the transistor group, in which the electric field regulates the current flow. The service here is one-carrier based. There are two types of charging carriers that exist though they are electrons and holes. But in the FETs the conduction of the operation is either based on holes or electrons. Thus, these transistors are also known as unipolar transistors. Its three terminals are the Gate, Drain, and Source.
They could be the same, both are transistors but there are major differences between the two. Here’s some difference, BJT is referred to be as a transistor with bipolar junction and FET is a transistor with unijunction. BJT operation is dependent on both the charge carriers, while FET operation performed is due to the majority of the carriers it may be either electrons or due to holes. BJT has more gain than the FET counterpart. BJT are applicable for low current applications while FET is for low voltage since BJT are current controlled and FET are voltage controlled. In BJT, operation is dependent on current carriers that are both minority and majority. So, this device is known as Bipolar.
In FET, the process depends solely on the movement of majority carriers. Holes for P-Channel FET and N-Channel FET electrons. Hence, they're called as Unipolar units. BJT are Less impedance on inputs, the BJT input circuit is biased in forward. So, BJT has a low impedance on inputs. On the other hand, high input impedance; FET's input circuit is reverse biased, so FET exhibits much higher input impedance (> 100Mohms) and lower output impedance. FET acts as a buffer amplifier because they have a high degree of insulation between input and output.
References
- Boylestad and Nachelsky, Electronic Devices and Circuit Theory
- Floyd, Electronics Fundamentals. Circuits, Devices, and Applications
- Schultz, Grob’s Basic Electronics
Note: All images are from the author except with separate citation.