JFET vs MOSFET Transistors 


               


JFET vs MOSFET (Transistors)

JFET vs MOSFET



In this article, we compare and contrast junction field effect transistors (JFETs) and metal oxide semiconductor field effect transistors (MOSFETs).

Though both are field effect transistors and and achieve similar functions, they're fundamentally different in composition. Thus, there are several key differences between the 2 transistors.

The table below gives a comparison between JFETs and MOSFETs.

JFETs vs MOSFETs
How it operates JFETs MOSFETs
Voltage controlled Voltage controlled.
Gain
(Transconductance)
Low transconductance (gain) Low transconductance (gain)
Input Impedance JFETs are depletion type transistors only. MOSFETs can be depletion type or enhancement type.
Input Impedance JFETs offer less input impedance than MOSFETs. JFETs typically offer about 109 Ω of impedance. MOSFETs offer greater input impedance. MOSFETs typically offer about 1014 Ω of impedance, sometimes greater.
Cost JFETs are somewhat cheaper to manufacture than MOSFETs. They have a less sophisticated manufacturing process. MOSFETs are slightly more expensive to manufacture than JFETs.
Susceptibility to Damage JFETs are less susceptible to damage from ESD because they have greater input capacitance than MOSFETs. MOSFETs are more susceptible to damage from ESD because the metal oxide insulator that insulates the gate from the drain-source channel lowers the capacitance of the gate. This makes high voltage more able to break through and destroy the transistor.
Popularity JFETs are less popular than MOSFETs. MOSFETs are more popular and widely used today than JFETs.



So the above table is a good, brief explanation of some of the differences between junction field effect transistors (JFETs) and metal oxide semiconductor field effect transistors (MOSFETs). Below we'll go over the table in more depth, so that you can get a better in-detailed explanation, if you feel that above lacked. We'll go in order.

On the similarities side, MOSFETs and JFETs are both voltage-controlled transistors. A voltage at the gate terminal of the transistor either turns the transistor on or off. They are unlike BJTs, which are current-controlled.

MOSFETs and JFETs also both have small transconductance (gain) values when compared to bipolar junction transistors. Transconductance is defined as the milliamp per volt ratio of the small change in the current output from an electronic device to the small change of voltage input. In other words, it is the gain of the transistor circuit. In terms of amplifier applications, this can lead to decreased gain values. For this reason, neither MOSFETs nor JFETs are used often in simple amplifier circuits. Instead, BJTs are preferred. The only exception if there is a need for very high input impedance and low current draw.

Going now to the differences, one of the differences between JFETs and MOSFETs is that JFETs only comes in depletion type. MOSFETs can either be depletion type or enhancement type. We'll explain in clarity what this all means. When a transistor is of depletion type, this means that the transistor is on fully and fully conducting when there is 0V at its control pin, which for FETs is the gate. Thus, JFETs all operate as depletion type transistors. When 0V is fed into the gate of a JFET along with proper biasing to the source and drain terminals, the JFET operates at full conduction. Applying voltage to the gate terminal of JFET makes it more resistive and less current flows. Once the voltage reaches a certain threshold, all current flow from the source-drain terminal ceases. This is why JFETs are referred to as 'normally on' transistors. Without any voltage to the control pin, JFETs conduct current across the source-drain region. MOSFETs, on the other hand, can either be of depletion type or enhancement type. As explained, depletion type is when a transistor conducts current across the drain-source terminal in the absence of voltage to the gate terminal. Enhancement type transistors are transistors that conduct current across the source-drain region only if voltage is applied to the gate terminal. In the absence of voltage to the gate terminal in an enhancement type transistor, the transistor will not conduct current across the drain-source region. Only if sufficient voltage is applied to the gate terminal of a transistor for an enhancement type transistor will it conduct current across the drain-source region. So again, JFETs are only of depletion type, while MOSFETs can either be either depletion type or enhancement type.

Another difference between JFETs and MOSFETs is that MOSFETs offer much higher input impedance than JFETs. JFETs typically have input impedances around 109 Ω. MOSFETs, on the other hand, have much larger gate lead input impedance, normally greater than 1014 Ω. This makes MOSFETs, on average, about 100,000 times more resistive than JFETs at the gate terminal. This means that MOSFETs draw almost no gate current at all. How MOSFETs achieve this very high input impedance is by placing a metal oxide insulator between the gate and drain and source channel. This insulates the gate terminal from the source and drain channel. With higher input impedance, the MOSFET draws in less input current than a JFET; thus, it doesn't load the circuit powering it barely at all. It allows for very good isolation being the circuit powering it and the load that the MOSFET is powering.

One drawback of MOSFETs that makes it disadvantageous to JFETs is that MOSFETs are more fragile and easier to destroy than JFETs. We said above that MOSFETs offer much higher input impedance than JFETs. This is achieved because MOSFETs have a metal oxide insulator placed between the gate and the source and drain channel. This supplies additional insulation and thus higher impedance, but there's a disadvantage to doing this. By placing in this metal oxide insulator layer, a very low gate-to-channel capacitance is formed. The capacitance between the gate and channel (source-drain channel) becomes very low, just a few picofarads. So if too much static electricity builds up on the gate of certain types of MOSFETs, the accumulated static charge may break through the gate and destroy the MOSFET. Some MOSFETs offer extra protection against this low input capacitance but not all do. Therefore, MOSFETs, though they offer greater input impedance, are more susceptible to damage than JFETs.

Another disadvantage is that MOSFETs are also more expensive than JFETs. JFETs are relatively simple to build. Building MOSFETs requires a more complicated, difficult process. This is because MOSFETs require an additional metal oxide insulator placed on it. Since this makes the MOSFET more susceptible to damage from electrostatic discharge, many times protection circuits are added so that it is not as susceptible to ESD. This brings up the cost. JFETs require a more straightforward manufacturing process; thus, they're cheaper.

Overall, MOSFETs are by far the more popular and widely used of the FETs. This is because they draw the least amount of input current due to the very high input impedance, use very little power, and still are not very difficult or expensive to manufacture in bulk, as in digital integrated circuits. If you consider a company like Intel which produces chips for many different electronic devices, they practically use all MOSFETs to produce digital circuits. So they're powering millions of devices with practically just MOSFETs. This shows the popularity of MOSFETs today for commercial consumer electronic products. MOSFETs surpass BJT and JFET use commercially by a large margin.

Thus, this is an overview of JFETs and MOSFETs.


Related Resources

BJT vs FET (Transistors)

Types of Transistors

Difference between an NPN and a PNP Transistor

Transistor Schematic Symbols

HTML Comment Box is loading comments...