JFETs vs. MOSFETs

As a PCB beginner or hobbyist, some of you may be not so familiar with JFETs and MOSFETs. We here will give a brief introducion.

On the similarities side, JFETs and MOSFETs are both voltage-controlled transistors; unlike bipolar junction transistors, which are current-controlled transistors. Besides, they both have small transconductance values when compared to bipolar junction transistors. In simple words, transconductance is the gain of the transistor circuit. In terms of amplifier applications, this can lead to decreased transconductance values. That’s why neither JFETs nor MOSFETs are used often in simple amplifier circuits.

As to the differences, firstly, JFETs are depletion type transistors only while MOSFETs can either be depletion type or enhancement type. Secondly, JFETs offer less input impedance than MOSFETs. JFETs typically offer about 10⁹ Ω of impedance, while MOSFETs typically are able to offer about 10¹⁴ Ω of impedance, sometimes even much greater. Thirdly, JFETs are less susceptible to damage from ESD because they have greater input capacitance than MOSFETs. That means 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. Finally, though JFETs are somewhat cheaper to fabricate than MOSFETs, MOSFETs are still more popular and widely used today than JFETs.