MOSFETs - High-Level Overview - Simply Put

MOSFETs are incredibly important they are the reason we have powerful multiprocessor computing systems that we can hold in the palm of our hand instead of just store in our basement but to someone like me who's trying to teach himself using nothing more than Wikipedia Stack Exchange in Google they are also incredibly difficult to understand fortunately just like BJ cheese there is a deceptively easy to understand and easy to use high-level model that is somewhat divorced from the physical reality but in 99.9 percent of cases works just fine and only the crazy high end devices like you know the nanometer big transistor processors put out by Intel and AMD and so forth need to worry about the bigger details that's why they employ actual physicists for you and me the high level models work great and now I have gone through the trouble of sifting through the Internet's terrible explanations where they leave all kinds of bits out they don't think they need to explain assuming your knowledge where they use all kinds of crazy analogies that make no sense whatsoever and overall it's just a mess but I'm here to give you the clean version and put it very simply in the future when I understand the actual physics going on I'll try to do another video for now don't worry about them I'm going to be giving a somewhat hand-wavy explanation but like I said it's a model just like we model weather to understand weather when we can't track individual molecules of air it's a model of a MOSFET that works just fine you might look at bjts J FET and MOSFETs as step one two and three along the semiconductor transistor time line if you will evolving from one another J FETs really aren't in common use although they have some interesting uses generally you're going to see BJ T's or MOSFETs MOSFETs are more useful for computing devices because they're low-power and you can put them in configurations that are even more low power whereas BJ tees are better at analog applications and use more power I'm going to be comparing BJ T's and MOSFETs because the high-level models are much closer between those two basically MOSFETs are closer to J fats but just worry about be JT's and MOSFETs right now so whereas you have your NPN and PNP bjts your p-type in your end type in your standard everyday configuration nothing fancy going on you look at your emitter on each as your power input if you will the p-type connects emitter to a high voltage the n-type connects emitter to a low voltage base is your control knob and then the collector is roughly the output because you split from emitter to base and emitter to collector so you might view the collector in a logic gate you would put the resistor here or the resistor here for example or put the load there you know about open collector designs so that's basically what you have a p-type and an n-type connect P too high and too low the outputs in the middle and the base you apply the opposite to turn it on similarly for your MOSFETs you have an n-type and a p-type that you hook up very similarly this circuit symbol is the most elaborate I have found it basically contains clues in the symbol to help you remember just like PNP and NPN you can figure out which pin is which and which type of transistor it is from what the arrow is doing same thing here you'll commonly see it without this over here and there's even simpler versions you'll see but this is the one that kind of has all the the hints so you have your p-type and your n type MOSFET you hook the p-type s for source so your source is roughly equivalent to your emitter the source of the p-type and the source of the n-type connect to high or low voltage your gate the G these are the same words as for the J FET the gate G is roughly equivalent to the base to turn on the p-type MOSFET you apply a low voltage to the gate for an n-type you apply a high voltage to the gate and the output is roughly the D the drain which is in the middle there is a direct correlation here in terms of how we draw them and that is mostly where the similarity ends you've heard of BJT s being described as current driven and MOSFETs are a voltage driven from a physical standpoint that's nonsense that's hand waving away 1800s little physical things that aren't that but from the high level remember we're only talking about the model where we use them forget about what the atoms are doing this is how we use them and the way we use them bjts are current driven you bias them to turn them on but then you turn up and down the current to make them do something MOSFETs are purely voltage driven the gate to source connection which is like the base to emitter connection the gate to source connection or the gate and source because it goes the other way in P does have a current just like a capacitor has a current but it's miniscule like I said in a capacitor like a fully charged capacitor one that's not getting any more charge into it one that's just sitting there there's a trickle same thing here there's just a beer trickle because electrons are flying everywhere but effectively no current no practical current flows into or out of the gate all of it is through the source to drain that's the open channel and this is one way that the MOSFETs are more power conserving when they're just on or off either one when they're just on or off there's no current being drawn no appreciable you know Piko amps maybe micro amps in the worst case whereas for bjts if they're on they're always drawing current at least through the base if nothing else whatever current is going through the collector and the drain is dependent upon the source if you have a chain of MOSFETs then they're not drawing current from each other so only the end of the line could possibly be drawing any appreciable current although they do draw current through the gate at one point when they're switching when they're sitting there they're not when they're switching they are drawing current hold that thought I'll explain in a minute so before I explain how the switching works and all that let me explain the symbol I'm using these diodes that's not actually diodes it's diode behavior sort of like how n PN is not actually two diodes but sometimes it's drawn is two diodes cuz that's the model they use same thing here there's something called a body diode in discrete MOSFETs as in ones you can hold in your hand that you might put in a breadboard or solder to a PCB or put in a car somewhere inside an integrated circuit it's a whole lot more complex and they're using semiconductor materials directly and doing funky stuff but on a MOSFET you just hold in your hand there's actually a fourth pin called the substrate do you see how there's three lines here you've got gate drain and source so gate is the one on the Left drain is the one up here and and source is the one down here the middle one all right the middle one is the substrate but in a discreet scimitar it's just connected to the source do you see this line here between source and the middle one and it's up here cuz sources on top that's indicating that the source and the substrate are shorted together they're just directly connected which makes it a three pin device instead of a four they do make discrete MOSFETs that have four exposed pins but they're incredibly hard to find here I don't even know where to get one but what happens because of that because of the physical properties which I only partially understand there's something called a body diode there is diode like behavior in the MOSFET if you hook up your you know your output your load whatever to the drain of an n-type and the source to ground just like this this is the the standard the standard arrangement so your load would be here and so the n-type you've got the source connected to zero essentially or a lower voltage it doesn't have to be 0 but 0 is common especially if digital logic is involved and then you connect your load to the drain you have to have higher voltage from drain to source the current has to go this way you'll notice it opposes the diode the idea of a transistor is it has to oppose the current or not oppose the current based on what the gate is doing this body diode is indicating that this is the way it can block current so you could hook up a load and when the n-type is off it cannot access ground and when the n-type is on so when gate has a high voltage the ground is through source and drain and then any current is going out the source into ground height of the voltages where the current goes conventional current if you hook it up the other way if you try to put it so that positive is on the source like this do you see now how you have a forward bias diode you have a voltage on the back side of the diode and then a lower voltage on the other side there's a voltage drop in this direction and this symbol is indicating the body diode causes the diode effect this will not block current it'll act roughly with weird characteristics but roughly as a diode so unlike bjts BJT you can run in Reverse and they will have different characteristics and they'll be goofy but it'll basically work it's called reverse active mode but for a MOSFET a discrete MOSFET with a shorted substrate to source connection you can't so this diode is indicating the direction it is able to block current if you hook it up so the diode the body diode the fake diode is forward biased then it's just gonna let current through so you have to connect the source of an end to a low voltage the source of a P to a high voltage so they have to be operated this way so now back to the gate so it's voltage controlled not current but it draws current when switching here's a trick think of a capacitor a capacitor when it's discharged is doing nothing when you apply a high voltage to a discharged capacitor that capacitor will charge here let's use a capacitor so we have a capacitor and nothing's connected and let's say we have both sides connected to negative so it's if it's not discharged it will be shortly but it's doing nothing there's no current flowing if I instead connect it positive then there is going to be current effectively going through it this way conventional current from positive to negative the current is not technically going through the capacitor what you've got is electrons are coming out of the negative and accumulating on this end they're grouping up which is pushing electrons away from this end and the positive is pulling on those electrons so you've got electrons entering and sticking to this end and leaving this end so there is current flows to the rest of the circuit and we just view it conventionally going the other way of course electron flows this way conventional is this way we view it as current going through the capacitor that's a high-level model that's not what the electrons are doing but that's what our understanding of the current is doing but when it charges when this side of the capacitor reaches this voltage or in other words the voltage difference between these two points is the same as the voltage difference between these two points the capacitor stops there's no more effective current through it and it's quiescent again even though it's charged it's not drawing or creating current at all the gate of a MOSFET in a high-level model is a capacitor MOSFETs have capacitance on their gates when you switch the voltage if you initially have your low voltage on gate and you connect at the high to turn the MOSFET on the gate just like a capacitor effectively charges from that high voltage and the transistor opens the MOSFET opens but then the gate will be charged and if you leave that high voltage there it'll just sit there then if you take it to half voltage it'll discharge to that and sit there and every time you change the voltage on the gate it'll charge a discharge through the gate source connection and therefore through the substrate as well since they're shorted but only until the voltage is equalized and then it won't anymore so a BJT if you turning it on and off and on and off and on and off is going to be using current when it's on not when it's off half when it's half or whatever and you can calculate how much a circuit made of BJT s is going to use in current by averages how much current does a BJT draw when it's on about how often is it on and you just calculate the average it doesn't matter how fast you turn them on and off it's just what percentage of the time they're on is the percentage of the current possible draw that they do draw MOSFETs use current to charge and discharge the gate so the higher frequency they switch the more often they switch not how often they're on how often they switch determines the power usage if you have a high frequency application and they're going on off on off on off really fast it will draw a ton of power through these MOSFETs that are designed to be low power the lowest power of MOSFETs is when only a few of them are switching at a time or they're switching slowly that's why there's no capacitor in the gate just like there's no diode between drain and source but this is how the physical construction ends up behaving in a high level model think of the gate as a Khepera so first of all this is why MOSFETs in your hand are considered more delicate and in addition MOSFETs in a chip you know how you're supposed to be careful with chips you know you got your graphics card in your hand you're supposed to have you little grounding strap on the thing about MOSFETs is the gate cannot be disconnected if you have an NPN and you connect your NPN positive and your negative of course here the NPN is on it doesn't matter whether anything's connected to collector or not you've got current going through base to emitter and it's open and it's ready in block if you connect negative to your base the transistor is off you know because you've got zero voltage across this but what if you have nothing if you have absolutely nothing it's just open to the air well that's called a floating input we've gone over that before in the video about pull-up and pulldown resistors but an NPN doesn't here there will be little you know static from the air static from your hand wires will act as antennas and pick up radio waves and all this other nonsense going on but the NPN while it may you know for nanoseconds at a time conduct generally it's just not going to so let's say you had your your PNP driving it right so you got your collector out of your PNP connected to the base here so when this PNP is on current is going out the collector into the base transistors on when the PNP is off you're not getting a zero voltage at a collector here you're getting nothing at a collector it's as if it were a floating input there's no voltage exposed to the collector so the wire between the collector and the base will act as an antenna and pick up stray stuff and you could touch it and it'll go there and whatever it's not grounded or anything if you have it just connected directly rather than with pull-up and pulldown resistors like logic gates for an NPN that's fine it doesn't care as long as the circuit can survive those transient inputs you know you're not going to do this in the life support machine but in a radio who cares nobody will ever notice for a MOSFET that's not the case because it's got the so capacitor here and it's not a big capacitor the ones on the microchips we're talking about femto farad's here that's a prefix you don't see much you get a nice D Ram chip nowadays you know your good old g.skill high-powered gaming memory you get little capacitor that are femto in size the gates are small you know bigger MOSFETs are gonna have bigger capacitors there's there's big ones out there but it's not going to conduct and not conduct when there's active current these require active current so as long as the shock from your finger lasts that our conduct and then stop immediately but that conducting from the shock of your fingers can charge up that gate capacitor and can charge it up too much a capacitor can blow but it'll also open up everything else and it could open it backwards and basically it'll grab the current and keep it and then probably fry itself that is why MOSFETs are more delicate because of the gate acting as if it were a capacitor so that tells you why you're not supposed to touch it or at least touch it carefully but it also tells you why you can't just do this let's say you have your NPN and you've got your collector just like we were saying before so you connect your collector here to the gate here well I mean the PNP would be giving the high voltage so we'll do this so the collector of the PNP when it's open will be giving a high voltage to the gate there's a low voltage and source so the transistor operates openly but if you turn off the PNP the gate is not getting zero volts it's not getting negative it's getting nothing it's floating if you just have a wire which means since it's not properly grounded since it's not connected directly to a voltage source then it's going to pick up whatever stray is in the air when you're connected to an actual voltage source the stray crap in the air including from your fingers is way too small I mean if you you know actively try to build up a charge and zap it you can probably still zap it but normal just random nonsense is so minuscule in comparison to an actual voltage source that when it's actively connected it doesn't care the magnitude of drift is miniscule plus you might have your decoupling capacitors to smooth it out even more but when that PNP is closed and there's no current through the collector there's no voltage to the collector then there's a big deviation from nothing to the shock of your fingers the radio waves echoing from other parts of the circuit or whatever so that's why you cannot operate an NPN as an input to a gate or a physical switch or anything floating you have to use either a configuration of logic of transistors we'll always feed it a higher low or whatever so it has to always be fed or you have to use pull up a pull down so like if you have a physical switch a button connected to a gate of a MOSFET you use a pull-up or pulldown resistor so it always has a voltage and it won't be drawing current remember it won't be drawing current when it's just sitting there it only draws current when it switches and your pull-up and pulldown resistor won't be drawing current either because that only draws current when the connection is closed so you might have your positive and resistor for a pull-up resistor and normally it's got positive and resistor going in here and nothing's gonna go in because it's you know the gates charged and then it stops and then you press your button and it connects the ground so you've got positive resistor and ground so while you're pushing the button there's current through the resistor and then the gate uses current to switch to low and then you let go of the button and the gate uses current to switch to high again from the pull-up resistor but overall it's drawing nothing but you need that pull-up resistor because you don't want it loose to absorb voltage from the ear to observe current from the air I should say so that's what this symbol is which you usually will not see in the symbol the body diode just remember that n goes towards ground and P goes up or whatever you just have to remember it this arrow you usually will see because that's how you can tell the difference between N and P type this arrow here is between the substrate and source it's like it's another some people say it looks like another diode I'm it makes no sense don't worry about what other people say it's just an arrow and unfortunately it's backwards NPN and PNP transistors the arrow indicates which way the base emitter current is going this indicates the opposite so you've basically got an arrow effectively because substrate and source are shorted you've got an arrow going from substrate and source out to the gate in an n-type but that's the opposite so if you put a high voltage on gate into the low voltage on source when you turn it on the current through the capacitor out the source when you turn it off it goes the other way see the arrow indicates the reverse direction of applying voltage so apply the higher voltage to the head of the arrow and a lower voltage to the tail so you a high voltage see the arrows coming out and going up the source so higher voltage where the arrows pointing I guess you could think of it that way the arrow points to where you're supposed to put higher voltage if that works for you great but that's I guess that's essentially what it is the arrow this little arrow not the diode but the little arrow you'll see is pointing towards which part gets the higher voltage and you see these this line because you've got the firm the solid line on the left and the dashed line on the right representing the three pins essentially the source the substrate in the drain there this is indicating an enhancement type of MOSFET you might remember the J FETs the J FETs by default our depletion type normally they're on and you bias them to turn them off whereas a BJT is an enhancement type normally they're off you apply a current to turn them on MOSFETs come in both types enhancement and depletion enhancement means if you apply no voltage across gate to source right not floating but like zero voltage difference so is their voltage then it's off and drain will not conduct it'll be a floating output so whatever drain is connected to had better handle it alright when I do see Mo's you looks you'll see how logic is built to avoid this issue but hold on a second so if there's no voltage across gate to source gate end source in an enhancement type the transistors off in a depletion type if there's no voltage across gate the source then the transistor is on it's just reverse of each other depletion type MOSFET you're generally not going to see they exist you can get them some people use them but very commonly only enhancement types are used because you don't really need both an enhancement type is easier to understand easier to wire up and so forth much easier to wire up compared to J FETs you saw the issues we had with that this is not as difficult but it's just easier with enhancement type another thing you'll see if you look around is people commonly don't use P type MOSFETs they commonly only use n type now we see that with your bjts it's always n type isn't it your NP NS your n type jfet it's your end type MOSFETs just because those are the ones that are kind of active we commonly think of hi as being active high on the base high on the gate whatever is turning it on but the physical reason is n-type tends to conduct better than p-type so p-type MOSFETs aren't is good letting current through because of the whole electrons versus holes thing I'll in the future I'll try and do a video on that but basically p-type don't work as well now when you have integrated circuits actually a CMOS chip which I'll get to in another video this isn't the same thing because of the way it's hooked up because again you're not just wiring to get the transistors in an integrated circuit you're actually layering on your substances and etching them and doing fancy things with them this is a thing with discrete MOSFETs so to keep in mind if you're trying to use N and P just in your breadboard or whatever the P won't work as well it'll work fine for low frequency hobby applications like I do you're never gonna notice the difference between you know 500 milliamps and 498 milliamps by default or whatever it ends up being for your specific device it's gonna be rounding error but generally people use only n-type whenever they need MOSFETs when you see P and n-type use together it's a CMOS logic gate arrangement once again that's my next video so just a recap of basically how you use these transistors emitter and source are roughly equivalent collector and drain are roughly equivalent base and Gator roughly equivalent p-type and n-type are roughly equivalent you hook up your p-type input source or emitter to high-voltage n-type emitter or source to low voltage you give your gate the opposite your base the opposite to turn it on by default it's off the MOSFETs take current when they are switching states bjts take current when they're on at all based on how much they're on MOSFETs cannot be connected to floating inputs cannot have their inputs left floating because the gates act as a capacitor and can grab that loose current whereas bjts will conduct partially or fully well that loose current is going through them and then stop and the drain and collector are roughly equivalent that's where you connect your load both be JT's and MOSFETs will have floating outputs when they're closed when then pians off the collector is not putting out a voltage or current when the n-type gate is discharged to match the source then the drain is not letting through voltage or current so it'll be floating outputs in both if you see this little symbol a diode inside the circle that's talking about the body diode which is not a real diode it's just a consequence of the construction where you have the substrate and source shorted together it indicates how the MOSFET is able to block current it's able to block if your current is connected if your voltage is connected across the diode backwards if you connect the current the voltage in the direction the body diode indicates then your MOSFET doesn't work right and the little arrow essentially indicates the arrow is pointing in the direction that you want to apply your higher voltage higher on the gate in an n-type higher on the source in a p-type to turn it on and that's your high-level overview you can use MOSFETs the same way you use bjts that's not how they're commonly used in chips they're used in a CMOS arrangement next video discretely they can be used like this you can make the same gates remember the and and rather than NAND and nor all those logic gates with the pull-up and pulldown resistors you can do the same thing with the MOSFETs you can do exactly the same thing and the bonus is the feedback there's no base to emitter current remember the problems we had base to emitter current if you hook them up with feedback you got not your nice clean high and low with MOSFETs you would so if you use MOSFETs you'll get your nice high five volts low zero volts every single time but I don't use MOSFETs that way because again I'm rough with them they're delicate things you know I'm throwing them around a breadboard so I don't want to fry my MOSFETs so this is why I continue to worry about the goofiness of having to deal with this but integrated circuits don't have to worry about that and if you're careful and make sure to always hook these up right with your pull-up and pulldown resistors then MOSFETs would make for easier if more delicate wiring so next time we'll discuss c MOS using MOSFETs and integrated circuits to do actual logic in the most power efficient way of all until then I'll be seeing you

Channel: Simply Put

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Keywords: simply put, simply, put, circuit, circuits, electric, electrical, electronic, electronics, electricity, mosfet, mosfets, igfet, igfets, transistor, transistors, cmos, nmos, pmos, overview, guide, tutorial, fet, fets

Id: psKY89neSQ4

Length: 26min 33sec (1593 seconds)

Published: Sun Apr 07 2019