The basic classifications of FETs include JFETs, MOSFETs, and IGFETs.
MOSFETs are voltage-controlled devices, contrary to current-controlled bipolar junction transistors (BJTs).
The removal of the oxide layer in MOSFETs leads to the formation of MESFETs.
Transistors are the building blocks of several semiconductor devices. By using field effect transistor (FET) technology, various devices have been developed for power electronic applications.
Power electronics enthusiasts are probably familiar with metal oxide semiconductor field effect transistors, otherwise known as MOSFETs. MOSFETs are widely-used solid-state devices in switching circuits, amplification circuits, and power conversion systems. Similarly, there is another device, called a metal-semiconductor FET (MESFET), which is derived from the FET. The absence of the oxide layer is what differentiates MESFETs from MOSFETs, but there are also differences in the construction, device characteristics, frequency of operation, and applications.
General Classifications of Field Effect Transistors
FET technology is often used in several semiconductor devices. There are two main types of classification for FETs:
Metal-oxide semiconductor field effect transistors (MOSFETs), also called insulated gate field effect transistors (IGFETs).
Both JFETs and MOSFETs work on a similar principle, where an electric field takes charge of controlling the current through the device. However, the methods by which control is established in the devices vary. The variation in the control element introduces differences in the device characteristics, and changes are made in the circuit design.
When using FET-based devices, it is important to understand their characteristics. In the upcoming sections, we will compare the characteristics of MOSFETs vs. MESFETs.
The use of field effect transistors (FETs) is limited by disadvantages such as high drain resistance, slow operation, and moderate input impedance. MOSFETs were developed as a solid-state solution to overcome these disadvantages. Since then, MOSFETs have ruled the semiconductor industry.
MOSFETs are employed in any electronic circuit where we either switch, convert, or amplify the voltages. A MOSFET is a voltage-controlled device, contrary to current-controlled bipolar junction transistors (BJTs). However, like transistors, a MOSFET is also a three-terminal device.
Terminals of a MOSFET
The terminals of a MOSFET are:
Gate - The gate terminal is brought out from the thin metallic plate placed on top of the silicon dioxide layer coated over the base substrate of p-type semiconductors.
Source - On the base substrate of a p-type semiconductor, two regions are heavily doped with n-type (n+) impurity. The source terminal originates from one of the n+ regions.
Drain - In a MOSFET, the current flows from drain to source. The terminal taken from the other n+ region forms the drain.
Types of MOSFETs
Based on the construction, MOSFETs can be classified into:
Depletion mode MOSFETs - In a depletion mode MOSFET, maximum conductance is exhibited by the channel when there is no voltage across the gate terminal. There is a reduction in the channel conductivity, as the gate terminal is biased.
Enhancement mode MOSFETs - In enhancement mode MOSFETs, there is no device conduction when the gate terminal is unbiased. The conductivity is enhanced when the voltage across the gate terminal is maximum.
In both depletion and enhancement mode MOSFETs, there can be n-channel and p-channel types, making up a total of 4 MOSFET types.
The surface of a MOSFET structure is insulated with a thin silicon dioxide layer. Even though the purpose of the oxide layer is to insulate the channel from the gate and form a protective coating on the device surface, it probes some instabilities in the device. The contamination of the oxide layer due to sodium ions in the ambient atmosphere leads to long-term instability and changes the characteristics of the device. In MOSFETs, the nitride layer is used for eliminating contamination issues.
If the oxide layer is completely removed from the MOSFET structure, the formation of another semiconductor device, called a metal-semiconductor field effect transistor (MESFET), can occur. In MESFETs, there is no oxide layer separating the gate and substrate, and a Schottky junction replaces the oxide layer, which is responsible for controlling the electron flow from source to drain.
Similar to MOSFETs, p-channel and N-channel types of MESFETs are available. Within the two channel types, MESFETs can be either in depletion mode or enhancement mode.
Let’s compare MESFETs vs. MOSFETs.
MESFETs vs. MOSFETs
MESFETs and MOSFETs differ in their power losses, device stress levels, and integration capabilities. By comparing these transistors, engineers can choose the appropriate one for a given application.
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