#FET TRANSISTOR SERIES#

This is what causes the diode to no longer conduct, and the size of the depletion zone depends upon the size of the electric field that exists across it. If however you give the gate a negative bias compared to the source, the diode becomes reverse-biased, and no current to speak of flows in the gate.Ī characteristic of a reverse-biased diode is that it has a depletion zone between anode and cathode, an area in which there are no electrons.

Were you to bias an n-channel JFET as you would a bipolar transistor with a positive bias on its gate, the diode between gate and source would conduct, and the transistor would remain a diode with two cathode terminals. There are equivalent P-channel devices, just as there are PNP and NPN bipolar transistors. This is referred to as an N-channel junction FET or JFET, because the channel through which current flows is N-type semiconductor, and because a diode junction exists between gate and channel. The simplest FET for demonstration purposes has a piece of N-type semiconductor with source and drain connections at opposite ends, and a zone of P-type semiconductor deposited in its middle. A closer similarity exists between a FET and a triode tube, if that helps. It is dangerous to stretch the analogy between bipolar transistors and FETs too far, though, because of their different mode of operation. Thus the three basic bipolar transistor circuit configurations have equivalents with a FET common-emitter becomes common-source, common-base becomes common-gate, and an emitter follower becomes a source follower. These are analogous to the terminals on a bipolar transistor, in that the source fulfills a similar role to the emitter, the gate to the base, and the drain to the collector. Chtaube,( CC BY-SA 2.0 DE)Ī basic FET has three terminals, a source (the source of electrons), a gate (the control terminal), and a drain (where electrons leave the device). As the negative gate voltage on the p-type silicon decreases in the lower diagram, its electric field restricts the area through which electrons can flow in the n-type channel. FET basics A diagram of an n-channel JFET. Join us as we take an in-depth look at their operation, and how and where you might use one. That said, the FET is a fascinating device. You’d use a FET if you needed a high-impedance audio preamp or a low-noise RF amplifier, and FETs are a good choice for high-current switching applications, but sadly you will probably never have a pile of general-purpose FETs in the way you will their bipolar equivalents. Every CMOS device you deal with relies on FETs for its operation and every high-quality op-amp you throw a signal at will do so through a FET input, but these FETs are buried inside the chip and you’d be hard-pressed to know they were there if we hadn’t told you.

In a way it’s less pertinent to look at FETs in the way we did bipolar transistors, because while they are very interesting devices that power much of what you will do with electronics, you will encounter them as discrete components surprisingly rarely. There is another family of transistors which have analogous circuit configurations to their bipolar cousins but work in a completely different way: the Field Effect Transistors, or FETs.

#FET TRANSISTOR SERIES#

It would however be improper to round off the series without also admitting that bipolar transistors are only part of the story. In a series whose impetus came from a friend musing upon his students arriving with highly developed knowledge of microcontrollers but little of basic electronic circuitry, we’ve examined the bipolar transistor in all its configurations. Over the recent weeks here at Hackaday, we’ve been taking a look at the humble transistor. The 2N3819 is the archetypal general-purpose N-channel FET.

Fet transistor