Difference between JFET and MOSFET

A huge part of electronics is made using transistors. Transistors are versatile in nature. They can be used for many purposes such as switching, rectification of AC, amplification of signals, etc. Transistors can also be operated to act as other components such as diode, resistor, etc. The invention of the transistor caused the rapid improvement of computers. 3rd generation computers were made using transistors. Later the invention of Small Scale Integration, Medium Scale Integration, Large Scale Integration, and Very Large Scale Integration technology made the size of the device smaller and increased the speed of the operation.

Field Effect Transistor or FET is a category of transistors. There are many types of FET such as Junction Field Effect Transistor (JFET), Metal Semiconductor Field Effect Transistor (MESFET), Metal Oxide Semiconductor Field Effect Transistors (MOSFET), etc. In this article, we are going to discuss the difference between JFET and MOSFET.

Difference between JFET and MOSFET in tabular form

In the following table, we have pointed out some differences between JFET and MOSFET.

JFET MOSFET
JFET stands for Junction Field Effect Transistor. MOSFET stands for Metal Oxide Field Effect Transistor.
JFET has three terminals named – Source, Drain, and Gate. MOSFET has four terminals, named – Source, Gate, Drain, and Substrate.
The gate leakage current of JFET is of the order of few nano Amperes. The gate leakage current of MOSFET is of the order of few pico Amperes.
The input impedance of JFET is lower than that of the MOSFET The input impedance of MOSFET is higher than that of the JFET.
JFET can only be operated in depletion mode. A MOSFET has both depletion mode and enhancement mode of operation.
JFETs are suitable for low noise applications MOSFET can be used for high noise applications.
As JFET is easier to fabricate, it is relatively cheaper than the MOSFET. MOSFETs have a relatively complex fabricating process. That is why MOSFET is more expensive than JFET.

What is JFET?

The acronym JFET stands for Junction Field Effect Transistor. It is a three-terminal device. The terminals are called Source, Drain, and Gate. JFETs are of two types that are – N-channel JFET and P-channel JFET. Here we will consider N-channel JFET. A JFET has an n-type semiconductor that acts as a channel for carriers. The channel is connected to the Source and Drain terminals. The channel is sandwiched between two p-type semiconductors.  The p-type semiconductors are shorted and connected to the Gate terminal. This sums up the basic structure of a JFET (N-channel).

Working Principle of JFET

The JFET can only be operated in depletion mode. We apply a voltage across the source and drain. The drain terminal is kept at a higher potential. Now as the drain terminal is more reverse biased, the depletion layer width near the drain is higher than the depletion layer width at the source. So, as we increase the voltage, overall depletion layer width and depletion layer width at the drain end also increases, and the rate of increase of current through the channel decreases. At a certain voltage, the current saturates. This drain-to-source voltage is called pinch-off voltage.

In the former case, we considered that no voltage is applied at the gate terminal. If we apply a negative potential at the gate with respect to the source then the same situation will occur but the values of parameters like pinch-off voltage, saturation current will be different.

Applications of JFET

JFET has a wide range of applications. It has applications such as voltage variable resistor, amplifier, digital switch, speed controller, and many more.

What is MOSFET?

Metal Oxide Semiconductor Field Effect Transistor or MOSFET is a four-terminal device that has the terminals named – source, gate, drain, and substrate. There are two types of MOSFET – n-type MOSFET and p-type MOSFET. Here we will consider the structure of n-type MOSFET. It has a drain and a source terminal connected with highly conducting n+-type semiconductors which are separated by a p-type substrate. The substrate pin is connected to the p-type semiconductor. The gate is made using metal and covers the area between the drain and the source. But an oxide layer separates the gate from the semiconductors. This sums up the structure of an n-type MOSFET.

Working Principle of MOSFET

A voltage is applied across the drain and source terminal. The drain is kept at a higher potential. So, the PN junction at the drain is reverse biased and the PN junction at the source is forward bias. So, eventually, no channel is formed and no current flows. But if we also apply a positive voltage at the gate terminal, then due to electrostatic attraction electrons will gather near the gate terminal but could not flow into it because of the oxide layer. So, they will form a channel between drain and source, and a current will flow.

Applications of MOSFET

MOSFET has even more applications than JFET. It is used in IC design for its low-power performance. It can be converted into passive components. It offers high-speed switching.  It is used as amplifiers for high-noise applications, motor speed controllers, etc. There are many more applications of MOSFET.

Conclusion

Both JFET and MOSFET are widely used in electronics. Both are voltage-controlled devices and have their advantages and disadvantages. One of these components is needed to be chosen depending on the need of the project.

Author
Subhrajyoti Choudhury

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