How FETs Work - The Learning Circuit

the following program is brought to you by element14 the electronic design community where you can connect and collaborate with top engineers from around the world join now at element14.com hi and welcome back to the learning circuit today we're going to learn about field effect transistors or FETs there are two basic types of FETs the junction field effect transistor or J FET and the metal oxide semiconductor field effect transistor or MOSFET previously we learned about bipolar Junction transistors bjts they have a collector emitter and base faites instead have a source drain and gate the source can be compared to the collector the drain like the emitter and the gate performs similarly to the base while B GTS have a linear structure with two PN junctions and are either NPN or PNP J FETs and MOSFETs have PN junctions configured in a way that gives them their unique characteristics and functions let's start by looking at J feds here are two different visualizations commonly used to represent the inside of a jfet notice the different layout of the n-type and p-type regions one type creates a channel directly connecting the source and drain while the other type sort of surrounds it and connects to the gate J feds are named for which type makes up the channel p channel J FETs and end channel J feds the current flowing through the channel can be controlled by varying the voltage at the gate we'll start with an N channel J FET we know from our diodes lesson that connecting a PN Junction in reverse bias will cause its depletion region to grow the depletion region acts as an insulator restricting the flow of current by increasing or decreasing the depletion zone at the PN junctions we can control the flow of current through the channel as you can see the source and drain are connected by a single region when a voltage is applied across them current flows freely between them as if the two leads were shorted together the J FET is essentially on by default now if we apply a sec voltage across the gate and source terminals but in reverse bias a depletion layer will form where the p-type and n-type regions meet in an n-channel jfet this requires the gate source voltage to be less than the drain source voltage the more bias the gate becomes the more the depletion region grows the narrower the channel becomes constricting the flow of current through the channel the channel can continue to narrow until the current is cut off completely the voltage difference at this point is known as the cutoff voltage to increase the bias the gate source voltage has to be lower than the drain source voltage to do this you can either increase the drain source voltage or decrease the gate source voltage the depletion region grows or shrinks relative to the difference between the voltages the higher the difference in voltage the more the depletion region grows the more current is restricted or decreased the less the difference in voltage the smaller the depletion region is the more current flows it increases n-channel jfet are used far more commonly than p-channel Chaifetz but they effectively work the same way simply with a reverse polarity in a p-channel J FET the gate would be connected to positive rather than negative this also means that the gate source voltage has to be higher relative to the drain source voltage rather than lower okay we know that the gate has to be reverse bias for the J FET to function but what happens if we forward bias the gate well sadly the gate Junction is not really designed to handle any significant amount of current and if current were to flow through the gate and forward bias it would probably destroy the component J fats are known as depletion mode devices since they rely on the growing and shrinking of the depletion zone but that's not the only way a FET can function [Music] while Jay fats are strictly depletion mode devices MOSFETs can be either depletion mode or enhancement mode devices the main difference is in their construction depletion mode or D MOSFETs have a physical channel connecting the source and the drain terminals in enhancement mode or emos FETs the source and drain are not connected and rely on the gate voltage to form a channel between them J FETs our junction field effect transistors using their PN junctions to affect the fields within the device MOSFETs have a metal oxide semiconductor the name comes from the metal conductor of the gate which connects to a silicon dioxide insulating layer that is between the gate terminal and the semi conductor that makes up the substrate in the rest of the component the foundation of the MOSFET is called the substrate and channel MOSFETs have ap material substrate p-channel MOSFETs have an N material substrate the moss acts like a capacitor due to the insulating oxide layer the conductive layers don't touch and therefore current can't flow between the gate and the rest of the FET but if the metal conductor at the gate becomes charged it will cause an equal and opposite charge in the semiconductor substrate just like in a capacitor let's look at an N channel D MOSFET like in the J Fed the drain and source are connected by a channel no connection or voltage at the gate is required for current to flow between the source and drain the MOSFET by default is effectively on when there is a voltage at the gate and the gate is reverse bias the D MOSFET is operating in depletion mode it behaves like a J FET as the bias at the gate grows stronger the depletion zone also grows proportionally the channel is narrowed increasing the resistance and restricting the flow of current through the channel between the source and drain now here is where the dimas FET is different when it operates in enhancement mode the gate is connected to a voltage and forward bias polarity at the gate draws charge carriers from the substrate towards the channel effectively widening the channe the channel grows beyond its default size this allows more current to flow through the channel than is usually possible Saudi MOSFETs can work with zero bias reverse bias or forward bias at the gate Amos FETs conduct only in enhancement mode in depletion mode the reverse bias of the gate in air is the channel restricting flow as you can see there is no channel between the source and drain so there's nothing that can be narrowed for current to flow between the source and drain the MOSFET has to be in enhancement mode in order to create a channel for the gate of this end channel ammas FET to be connected in forward bias the gate is connected to positive power the positive gate voltage attracts negative charge carriers joining the n-type regions of the source and drain creating a channel between them the more voltage applied at the gates the wider the channel gets if the voltage is lowered back down the channel will narrow the minimum amount of voltage required at the gate to create a channel is called the threshold voltage to summarize we have J FETs that only work in depletion mode they are always on unless acted upon by a voltage at the gate which can decrease the flow of current through its channel emos FETs only work in enhancement mode they are by default off requiring a voltage at the gate to create a channel between the source and drain and D MOSFETs that can work in depletion or enhancement mode like J feds they are also on by default but a voltage at the gate can cause it to decrease or increase the current flowing through its channel there's a lot more information to learn to be able to use FETs effectively in a circuit lots of voltage and current mass but hopefully with an understanding of how each fad works you've got a leg up on learning that next step if you have any information you'd like to add to today's lesson or questions you'd like to ask please post those on the element14 community on element14.com forward slash the learning circuit happy learning [Music]

Channel: element14 presents

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Keywords: electronics, hardware, gaming, hacking, mods, weekly, element14, tbhs, benheck, madison, wisconsin, maker, engineering

Id: wNiXUZIHQLw

Length: 8min 29sec (509 seconds)

Published: Wed Nov 21 2018