Why different terminologies for FET vs Transistor - Page 1
Forum section: electronics / beginners

author: Stonent
author_rank: Super Contributor
timestamp: July 04, 2013, 11:19:34 pm
content: I've always wondered why does Transistor nomenclature use Base, Emitter and Collector but when you're talking about FETs you use Gate, Source and Drain, when essentially they are the same thing? Or is there really a subtle difference that would make it technically incorrect to call them the same?

author: IanB
author_rank: Super Contributor
timestamp: July 04, 2013, 11:25:03 pm
content: Quote from: Stonent on July 04, 2013, 11:19:34 pmI think they are really not the same thing. If you look at the physical construction of field effect and bipolar junction transistors the internal structure is different. For many purposes the Source and Drain are interchangeable and if you flip an FET it will still mostly work. Or not1. In a BJT the Collector and Emitter are different and if you flip a BJT it will work poorly, if at all.It also helps to avoid confusion. If someone talks about the "base" you know they must mean a BJT and you know some context right away. If the base of a BJT was called the gate you would lose some information.(In simple terms a field effect transistor has no actual PN junctions and is not a polar device, whereas a bipolar junction transistor does have junctions and does have polarity.) OK, so that statement was a bit of vague hand waving that doesn't tell very much. An FET is constructed as a four terminal device, where the fourth terminal is the base or body on which the device is constructed. In most packages the body terminal is internally tied to the source terminal and this makes the source different from the drain. There is as a result an intrinsic body diode between source and drain that is reverse biased and not conducting when the transistor is connected the normal way round. If the transistor were connected the other way round the intrinsic diode would be forward biased and would conduct current around the transistor independently of the gate. So in practical terms the source and drain are different and not interchangeable.

author: Stonent
author_rank: Super Contributor
timestamp: July 04, 2013, 11:35:51 pm
content: Yeah, I suspected it had something to do with the Field Effect part and the completely different looking symbols.

author: IanB
author_rank: Super Contributor
timestamp: July 04, 2013, 11:39:44 pm
content: Another thing is that the whole family of three terminal semiconductor amplifying devices are "transistors". An FET is a transistor too!

author: Unknown
author_rank: Unknown
timestamp: July 04, 2013, 11:47:11 pm
content: I think the idea here (to rephrase the above posters) is that they all belong to the same class of component called "transistors". Inside that class there are different species... FET, BJT, etc. Those have different terminal nomenclatures because they're built using completely different techniques.I'm wondering about that comment above... if you switch an FET around, it'll still *mostly* work. Why mostly? What will happen exactly? Will it roll its eyes like a teenager you ask to take out the garbage and only take it outside and put it next to the door?

author: Stonent
author_rank: Super Contributor
timestamp: July 04, 2013, 11:48:21 pm
content: Quote from: IanB on July 04, 2013, 11:39:44 pmYeah I knew that. I just always remembered (and I'm sure there are exceptions) that FETs were transistors that looked like voltage regulators.However as I have gotten more into electronics again this year, I've started realizing that the form factor doesn't always tell you what it is, such as a temperature sensor that looks just like a bipolar transistor.

author: free_electron
author_rank: Super Contributor
timestamp: July 05, 2013, 01:27:18 am
content: Whoa.... Some people need a iostory and a transistor physics lesson.The base emittor collector denomination comes from the original bipolar transistor design.The base is simply that : the base structure on which the transistor was built. The emittor is the electrode that provides electrons into the transistor while the collector is the electrode that collects those electrons permittedto flow by modulating the base current.I have done a very extensive post here on the forum on how bipolar transistors were invented, built and how they really work. Go read it.Now, onto fets. Field effect transistors work differently. Lets look at junction fets first (j-fet)A fet is essentially a diode. It is a construction made from an n-doped and p-doped material. In The area where the materials make comtact an electron exchange takes place. This is called the recombination zone. This zone is non-conductive as there are no free electroms available.If you look at a diode : if you forward polarize a diode you provide enough electrons to remove this recombination zone and electron flow is possible again. If you reverse polarize the diode you actually widen the recombination zone. You essentially create a wider band where no conduction of electrons is possible.Code:Reverse.| Anode ||recombination|| Cathode |Now, if you were to attach two electrodes to one side you will be abble to run a current between them. Since we are still dealing with a semiconductor this conduction is not ideal... The resistance of the doped material is in the order of a few hundred ohms. 1 can send a current from electrode 1 to 2 or vice versa. The direction of the flow does not matter as it crosses no junction. We call this path the channel.Code:Reverse.1-| Anode |-2|recombination|| Cathode |Now, if i apply a positive voltage to the cathode i will push the recombination layer into the physical area of the anode. If i push hard enough i can make the entire anode devoid of free electrons. At that point no further consuction is possible.Compare it to a water hose. The water hose diameter, in rest, determines the max flow rate. Which way water flows is irrelevant. If i squeeze the hose i change the diameter, ie make it smaller so i pinch off the flow.So jfets are essentially a diode through which there is a lateral electron flow in one of the electrodes. They conduct by default and can be pinched off. The gate controls how far the recombination zone extends.The source supplies electrons while the drain lets them flow out of the fet.Mosfets employ a different technique. They work on the principle of charge attraction.In a jfet there is an actual junction. A mosfet (in its raw form) has no junction at all. The gate is fully isolated from the channel. Enhancement mosfets work on the principle of attracting charge from the source. The gate is polarized in such a way that electrons are attracted towards the gate plate. Think of it as a capacitor charging.These electrons are free electrons. When the channel is saturated with free electrons then electron flow is possible through the channel. Under normal circumstances the channel is devoid of free electrons.A depeltion mosfet has plenty of free electrons. Applying charge on the gate pushes these free electrons away (like charges repel each other). The channel is now devoid of carriers and nor conduction is possible.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 05, 2013, 08:27:36 am
content: Quote from: IanB on July 04, 2013, 11:25:03 pmreally? can you post some example FET? all i know is nFET and pFET which i dont believe work the way you've stated. the 2 way transistor that i know but never used is triac?

author: Dave
author_rank: Super Contributor
timestamp: July 05, 2013, 08:38:17 am
content: Quote from: Mechatrommer on July 05, 2013, 08:27:36 amA reversed MOSFET will act as a diode. That's exactly why you often see a reversed diode drawn into the symbol of a MOSFET as a reminder of that PN junction.But if you are talking about JFETs, it really doesn't matter how you flip it.A triac is basically a two-way thyristor (aka SCR - silicon-controlled rectifier), not transistor.

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 09:04:32 am
content: Quote from: Mechatrommer on July 05, 2013, 08:27:36 amSee the clarifying note I added to the post above, as also pointed out by Dave just here.

author: peter.mitchell
author_rank: Super Contributor
timestamp: July 05, 2013, 09:05:40 am
content: Where do IGBTs fit into this shomozzle?

author: jpb
author_rank: Super Contributor
timestamp: July 05, 2013, 09:17:38 am
content: FETs are unipolar devices, all the current is carried by electrons which leave the source and flow to the drain, the amount of current being controlled by the field from the gate which depletes the channel (i.e. makes it smaller or larger).A transistor is really a bipolar transistor and both holes and electrons carry current. Current flows from the emitter into the base region where, because the base region is very thin, most of it over-shoots and ends up in the collector region, with just a portion of it going out the base terminal.A key difference is that a FET is very simple, current just flows from one ohmic contact (the source) to another (the drain) whilst a bipolar transistor involves current flow through two pn junctions. But I guess the real reason for the different terminologies is historic but people wanted to emphasise that they are very different devices.On the topic of switching Source and Drain, with a Schottky barrier GaAs FET (the sort I used to work on), you could switch source and drain but it would behave very differently. The physical spacing between gate and drain is generally much larger which allows the depletion region to extend and increases the breakdown, if you reversed source and drain the device would saturate (the current go flat) and would breakdown at a lower voltage. (Breakdown depends on field and very crudely speaking for a given voltage the peak field will go up as the inverse of the distance the voltage is dropped across.)

author: free_electron
author_rank: Super Contributor
timestamp: July 05, 2013, 11:29:15 am
content: Not only because the base is thin , but because the base is doped differently. You cant cram that much electrons in there.In an integrated circuit there is no difference between drain and source of a mosfet. The fet is symmetrical. There is also not this reverse diode. this diode is formed between the bulk and the channel. we connect those bulks in such a way they don't bug us.For io structures the fets are assymetrical.

author: madires
author_rank: Super Contributor
timestamp: July 05, 2013, 01:32:10 pm
content: Quote from: Mechatrommer on July 05, 2013, 08:27:36 amThat's true for JFETs. n and p is the channel type, so you got a n-channel JFET and a p-channel JFET, similar to NPN and PNP for BJTs. JFETs are also depletion-mode FETs by default. Commonly used JFETs are 2N3819, 2N3820 or BF256 for example.

author: JackOfVA
author_rank: Unknown
timestamp: July 05, 2013, 01:33:13 pm
content: As IanB said, many JFET devices are symmetrical and the source/drain may be interchanged with no performance change.Sometimes the data sheet will indicate S/D interchangeability but not always.(Not the case for MOSFET because of the inbuilt diode.)It's also possible to flip the emitter and collector of a bipolar transistor (the so-called "inverted mode") and find it works after a fashion. Beta is much lower, but you can get some amplification. In ye olde days of germanium transistors, devices started out as physically symmetrical and inverted mode worked almost as well as normal configuration. That's not the case with modern silicon BJTs, but many will still show some DC gain in inverted configuration.Like many folks, I found this out the hard way when I installed a 2N3904 backwards due to an error I made in a PCB layout. The circuit sort of worked, but not anywhere like it should. After an hour or two of parts replacement and head scratching, I checked the data sheet and discovered my silk screen error.

author: madires
author_rank: Super Contributor
timestamp: July 05, 2013, 01:50:03 pm
content: Quote from: JackOfVA on July 05, 2013, 01:33:13 pmGermanium transistors got a high leakage current around 0.1-0.5mA while a silicon one is in the nA range.

author: A Hellene
author_rank: Frequent Contributor
timestamp: July 05, 2013, 02:22:47 pm
content: Quote from: Mechatrommer on July 05, 2013, 08:27:36 amThis is right, Shafri!For most of the J-FETs, the channel construction is exactly the same for its Source and Drain ends.For example, see the BFJ309 (an N-Ch UHF RF amplifier JFET in SOT-23 package) with the marking '6U' (theMMBFJ309LT1actually) the DS1000 has in the front-end as a mixer/amplifier of the DC and the AC components of the under-test signal.Though the pins of that JFET are 1=Drain, 2=Source and 3=Gate, in theDS1000 schematics I have postedit can clearly be seen that, though in the Ch1 (and Ch2) analog front-end schematic sheet it is biased "correctly" (with pin 2 as Source and pin 1 as Drain), in the Trigger Input Front-End it is NOT(!), with pin 2 acting as the Drain and pin 1 as the Source. Yet, the device works fine!And this is not a schematic's typo, since that strange biasing of the specific component can be confirmed in any of the high resolution PCB pictures available of the DS1000.Actually, just read the first feature of that component, in the On-Semiconductor datasheet posted above in page #1, clearly stating that:"Drain and Sourceare Interchangeable"EDIT:The same exactly states Fairchild (as a fourth feature) for their version ofBFJ309!-George

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 05, 2013, 02:43:30 pm
content: yup interesting info its like dream comes true. i just havent found such interchangeable S-D JFET device around. all i have seen is the FET with arrow symbol in schematics. i'll study when i have time thanks all.

author: c4757p
author_rank: Super Contributor
timestamp: July 05, 2013, 02:46:24 pm
content: The arrow symbol doesn't mean they aren't interchangeable...Ideally, if the arrow is off-center it's by the source, and it should be centered for symmetric FETs. But it's misused all the time.J113is symmetric, for instance, and I've got a pile of them and can affirm that.

author: lewis
author_rank: Frequent Contributor
timestamp: July 05, 2013, 04:21:16 pm
content: Quote from: Mechatrommer on July 05, 2013, 08:27:36 amA MOSFET* will conduct current in both directions through the channel. I've used this circuit below many times in high power AC switching applications where the dissipation or distortion of a Triac would be too great. It looks a bit weird, there are two MOSFETS in series with the source terminals connected.At first glance it looks like on, say, the positive half-cycle of the AC waveform the top MOSFET conducts as normal and the bottom MOSFET cuts off with the load current flowing through the bottom FET's body diode. But this is not so. The bottom fet conducts 'backwards' - the load current flows through BOTH fet channels, not the body diodes.It's possible to verify this in two ways. If the load current was flowing alternately through the body diodes, at high currents you would expect significant dissipation out of the fets due to the body diode Vf. Second, if the body diodes were conducting you would see distortion at your load when the AC voltage falls below Vf. Neither phenomenon is present.This circuit is great for all sorts of AC applications. I've used it for mains power switching and loudspeaker switching many times. It also works at DC, and has very high bandwidth.Vg must be isolated from the AC supply. There are photovoltaic optocouplers specifically designed for this application where the receiver is a solar cell. Like this one:http://www.farnell.com/datasheets/358233.pdfand these:http://uk.farnell.com/jsp/search/browse.jsp?N=2031+203598&Ntk=gensearch&Ntt=photovoltaic&Ntx=mode+matchallpartial*my cat is called Mosfet...

author: A Hellene
author_rank: Frequent Contributor
timestamp: July 05, 2013, 04:23:51 pm
content: Quote from: c4757p on July 05, 2013, 02:46:24 pmExactly! Thank you for the clarification.My misuse of the FET arrow positioning in the trigger input front-end sheet above was intentional, for readability reasons; this is the reason why I did not forget to add the part's pin numbering, in order for the schematic to also agree with the PCB layout.___For anyone interested, please read Vincent's multi-part essay on the transistors,beginning from this post(and going on for pages!); this is one of the best pieces I have ever read on that subject matter.Additionally, for those (like me), who have a hard time accepting that the BJT is acurrent-drivendevice instead of avoltage-drivenone, feel free to readWilliam Beaty'sfine essay on how do the transistors really work (part 1andpart 2), if not to explore his whole site to readabout electricityand lots ofcommon misconceptions.-George

author: enz
author_rank: Regular Contributor
timestamp: July 05, 2013, 04:33:14 pm
content: Quote from: A Hellene on July 05, 2013, 04:23:51 pmYou are totally right.The BJT is indeed a voltage driven device. The myth of current driving comes from the rule of thumb that ICis roughly equal to beta times IB.But as soon as you look at the Ebers-Moll Equation it is clear that tha BJT is a voltage controlled device.It doesn't matter that one mostly puts a resistor in series to the base to make the control of the BJT more convenient.The BJT itself only reacts to the Base-Emitter voltage.Martin

author: A Hellene
author_rank: Frequent Contributor
timestamp: July 05, 2013, 04:46:07 pm
content: ...and the current drawn by the BE junction is merelya healthyside effectof a forward biased P-N (or N-P) junction, andNOT the primary causeof the transistor conduction.The VBEpotential (orvoltage) is the cause of the retraction of the depletion area which, in turn, allows the ICEcurrent.Thank you, Martin!-George

author: edavid
author_rank: Super Contributor
timestamp: July 05, 2013, 06:25:20 pm
content: Quote from: Dave on July 05, 2013, 08:38:17 amThis is only true for power MOSFETs. Small signal MOSFETs like the BF998, or the devices inside logic ICs, don't have body diodes.

author: c4757p
author_rank: Super Contributor
timestamp: July 05, 2013, 06:26:24 pm
content: Oh yes they do.The arrow in the MOSFET symbol represents the junction between substrate and channel. That's always a diode whether you like it or not. It'snot always tied to the source, but it's always there.Quote from: IanB on July 05, 2013, 06:29:51 pmNo, sorry.

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 06:29:51 pm
content: Oh no they don't!(You might not get the British cultural reference here.)

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 06:33:52 pm
content: Reference:Quote from: free_electron on July 05, 2013, 11:29:15 am(The arrow on the circuit symbol does not represent the body diode, it represents the source of charge carriers. The body diode is formed by a connection between the bulk substrate--the fourth terminal--and the source. If this connection between the bulk and the source is not made inside the device there is no body diode. What you have in fact is two diodes back to back so that one of them is always reverse biased and forward conduction never occurs.)

author: c4757p
author_rank: Super Contributor
timestamp: July 05, 2013, 06:37:23 pm
content: Quote from: free_electron on July 05, 2013, 11:29:15 amI think we might be arguing over a difference in terminology. I'm not saying there's always a diode from source to drain, but unless I've been horribly mistaken for a long time, thereisa diode from the substrate to the channel (or reverse, in the case of a P-channel FET). As f_e said, the substrate in an IC is usually connected to whichever power rail is most convenient to keep the diode always reverse-biased.I'm fairly certain, though, that the substrate-channel diode is always present.

author: A Hellene
author_rank: Frequent Contributor
timestamp: July 05, 2013, 06:39:22 pm
content: Allthe MOSFETs are constructed asfour-terminaldevices...You can read morehereor, even better,here.-George

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 06:52:16 pm
content: Quote from: c4757p on July 05, 2013, 06:37:23 pmThere are two diodes, not one. Take a look at this diagram from theWikipedia article:There are two islands of N-type material sitting on a substrate of P-type material. Each of the N-P interfaces forms a diode junction, so between source and drain you have an N-P-N sandwich making two diodes back to back. If you connect the body terminal to the source terminal as is commonly done, then one of the diodes is shorted out and the other diode remains as the body diode, as there is now a single P-N junction between source and drain.On an IC, or in some other applications, the body terminal might be connected to some other equipotential point apart from the source and then there is no parasitic diode. The FET becomes symmetrical and source and drain become equivalent.

author: c4757p
author_rank: Super Contributor
timestamp: July 05, 2013, 06:59:28 pm
content: Right. I was mistakenly picturing the channel as one conductive region, as if we were talking about depletion-mode FETs.. Still,two diodes- but my point stands, that there is always a diode there. The substrate can't be left floating, so it has to be connected somewhere, and then you have one diode each from the substrate to source and drain. It's not right to say that you can avoid the diode entirely by building it inside an IC, or that small MOSFETs like BF998 don't have one, even if it istechnicallyincorrect to call it one diode.In other words, regardless of how you explain it or how the MOSFET is constructed, a diode tester will always be triggered when it's connected from the substrate to either end of the channel. Always, no matter what kind of MOSFET it is.

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 07:09:33 pm
content: There are certain circumstances where apparently you can avoid the diode entirely by building the transistor inside an IC, when using silicon on sapphire technology. In this case the semiconductor substrate is replaced by an insulating sapphire substrate and then no diode junctions exist.

author: c4757p
author_rank: Super Contributor
timestamp: July 05, 2013, 07:13:42 pm
content: That is pretty cool, I'll admit.Still, I don't think most of us will ever have to worry about what happens when our substrates are made of sapphire.OK, replace "all MOSFETs" by "all silicon-only MOSFETs".

author: SeanB
author_rank: Super Contributor
timestamp: July 05, 2013, 07:15:17 pm
content: SOI still has the diode, just it is connected to the one terminal so it will still have an effect when voltage is high enough to cause leakage current to flow. The substrate connection will just be a small isolated area on the bottom of the device, but it still will have the diode as part of the intrinsic construction. The only advantage of SOI is a high tolerance to radiation SEU and a low leakage current.

author: wbeaty
author_rank: Contributor
timestamp: July 05, 2013, 08:03:06 pm
content: Heh. What if we're looking at 3-terminal active components in general?If it's got a BASE, then BJTIf it's got a GRID, then vacuum tube.If the input is the GATE, then obviously it's ...an SCR or Triac! Or FET.Once I saw a brief mention of a very cool 3-terminal electronic device. It was a vacuum tube with a wide, fan-shaped electron beam. (More of a cathode-ray tube, I guess.) A beam of microwaves was aimed at the electrons. Free electrons are a conductor, so a small part of the microwave beam was reflected. A third electrode caused the electron beam to sweep back and forth.Not very efficient, since electrons in vacuum are so sparse. But any time you need to scan a microwave beam back and forth at tens of megahertz, they've got you covered.

author: madires
author_rank: Super Contributor
timestamp: July 05, 2013, 08:19:52 pm
content: Quote from: wbeaty on July 05, 2013, 08:03:06 pmCould be also an IGBT :-)

author: free_electron
author_rank: Super Contributor
timestamp: July 11, 2013, 03:05:37 pm
content: Quote from: IanB on July 05, 2013, 06:52:16 pmIt's worse than that. That shit is actually a transistor in many cases , and it has some gain !For large geometry fets you can consider it two diodes. The smaller we become the narrower the gate and the thinner that base layer becomes. There is a point where that stuff begins to work as a bipolar transistor !Somebody mentioned silicon on sapphire. You can also construct on depleted silicon.( that is a perfect isolator)

author: nukie
author_rank: Frequent Contributor
timestamp: July 12, 2013, 03:59:18 am
content: Please, just watch this guy's videos then all will be clear.https://www.youtube.com/user/booksofscience/videos

author: megajocke
author_rank: Regular Contributor
timestamp: July 13, 2013, 08:24:15 am
content: Quote from: free_electron on July 11, 2013, 03:05:37 pmI think I had this bite me in a circuit where a 16-to-1 MUX selected the input to an ADC. One of the sources had a problem, sending 5 V through a 10 kohm resistor to one input of the MUX which was on a 3.3 V supply. Many sources were high impedance, and the current being injected into the MUX output through the parasitic bipolar transistor on the faulty channel gave rise to an offset of about 1/3 full scale on all channels...

author: free_electron
author_rank: Super Contributor
timestamp: July 13, 2013, 12:47:31 pm
content: Could be. The analog mux chips frequently have no protection structures on the analog signals... So if you pull the input higher or lower than the rails by 0.7 volts you may trigger the transistor... Since you had 10k there you did not fry it but were 'steering' it ...That is another case of the old rule : never, ever apply a signal beyond the rails hoping the internal protection structure will clip it... You simply dont know how it is built and what it can handle.Only if it is explicitly stated by the manufacturer , and is listed as a marketing feature, can you do that...

author: SeanB
author_rank: Super Contributor
timestamp: July 13, 2013, 02:16:54 pm
content: Even if it is do not rely on it though, add the diodes. you never know just how that junction will degrade with time and if it will still behave the same with a change in the die during a redesign or process change. Unless it is a part of the design and is given specs and typical and min/max values in the datasheet and is tested in production it is only a feature and not a designed in spec, subject to change without notice*.* often undocumented or buried deep in small print in a small errata note in a datasheet for another device that coincidentally uses the part as a unit in a reference design.

author: A Hellene
author_rank: Frequent Contributor
timestamp: July 13, 2013, 02:51:08 pm
content: ... and this problem exists,especiallyfor theSilicon On Insulator(SOI) technology devices, because the electrical potential of thepseudo-floating(see:'insulated')bulk material(the fourth terminal of those four-terminal MOS devices) can easily cause localchannellatchupsunder the specific condition of an I/O exceeding the absolute maximum Vdd/Vss thresholds.In the modern semiconductors (devices ofa few tens of atoms thickfor each transistor(!) --see the modern PC RAM modules) evencosmic radiationcan cause random reversals of an I/O state, oftenly resulting in computer systems uncalled crashes...Life can be a real bitch; and it seems that this is not only for us, the humans!-George