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How can I tell (by reading the datasheet) if a specific MOSFET can be turned fully on (with the minimum drop) with simply 5V on the gate? (assuming 5V or 12V potential between the drain and source).

What specs am I looking for (preferably ones that are not on graphs, ew) to determine if a MOSFET is suitable, apart from the fact that it must be N-Channel?

64bit_twitchyliquid
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    Vth or Vgsth is a useful starting point BUT the graphs are MUCH more useful and are your great friends. Learn to use them and appreciate what they tell you. A single parameter is relevant at one point only and is usually a typical value. Rdson (= the slope of the Vds/Ids curve at some specified point) is often given for pulsed operation at 25C. To properly understand what happens you need to look at the graphs. DO NOT be scared of them - once you learn to use them they are vastly more useful than any single figure can be. – Russell McMahon Dec 30 '14 at 14:24
  • Leave the dark side - use the force. (1) Burn fig 2. Use fig 1 (2) BS270 is also not a very nice FET. There are much better ones. Note the date on that data sheet. – Russell McMahon Dec 30 '14 at 14:28
  • But its 6c each! – 64bit_twitchyliquid Dec 30 '14 at 14:34
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    While this may be a bit pedantic, a 5V TTL would not actually have a 5V output. I'm not sure that you're using *actual* TTL parts - if you are that would change the answers a bit so that the FET is on at 2.8V instead of 5.0V. – W5VO Dec 30 '14 at 15:31
  • If you want a TL:DR approach (or a quick sort parameter) look for 'logic level gate drive' somewhere on the front page. – Adam Lawrence Dec 30 '14 at 17:06
  • @twitchyliquid - The cheapest & most nasty horrible ugly mishappen FET in the Digikey quiver is the [2N7002P](http://www.digikey.com/product-detail/en/2N7002P,215/568-5818-1-ND/2531105) ar 14 13 8 3 cents in 1 10 100 1000 quanity. | Compare it to the [IRLM6402](http://www.digikey.com/product-detail/en/IRLML6402TRPBF/IRLML6402PBFCT-ND/812500) [Datasheet](http://www.irf.com/product-info/datasheets/data/irlml6402pbf.pdf) at 40 22 13 10 in same volumes (ie 2 to 3 x as dear) AND P Channel and see the differences !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Vds 0.25V at 1A at Vgs = 2.25V, .... – Russell McMahon Dec 31 '14 at 02:59
  • .... Vds =~ 0.12V at 2A at Vgs = 7V. 25A at 1V Vds just before it vaporises at 7V Vgs (ie for super short pulses it will work miracles). | Utterly no comparison. | Whatever :-) – Russell McMahon Dec 31 '14 at 03:02

2 Answers2

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Turn on is governed by the threshold voltage.
Vth or Vgs_th or similar.
This is specified at a stated current - usually small - 10 uA or 100 uA or sometimes 1 mA.
Useful turn on voltage is a volt or more above this value and fully enhanced operation needs typically 2 or 3 times as much voltage.

Most data sheets (all good ones) contain graphs that show the current that can be passed at various combinations of Vgs and Vds. These are often given at two temperatures - often 25C and 125 C

Below are two sets of curves for a BSS138 MOSFET. I do not recommend this MOSFET as there are many many .. better ones available BUT it is a popular hobbyist choice.

Fig 6 shows Vds versus Ids (or Id) for various values of Vgs.
When the curves rise relatively vertically the FET is in a near resistive mode - the resistance at any point is given by R = V/I = Vds/Ids at that point. When they begin to curve to the right the FET is entering an ~= constant current mode and resistance risews with increasing Vds wit minimal increase in current.

For Vgs = 2V the FET is != fully on up to about 0.1A. It is close to CC (constant current mode by Id=0.2A.

At Vgs = 2.5V it is in resistive mode up to about 0.5A when Vds is about 0.9 V.

At Vgs = 3.5V it is in resistive mode for all Ids shown on the graph.

Fig 7 is unusual - not many FETs have this graph shown.
This shows Id for values of Vgs below what would be considered to be Vth usually. Also it shows min, typical and max curves - a very unusual set of data to be given but very informative. It can be seen from Fig 7 that to get 1 uA Id you need about 0.55 / 0.8 / 1.15V Vgs min/typ/max.
To get 1 mA you need about 0.8 / 1.0 / 1.6V min/typ max Vgs.

SO in this case - the FET can be said to be "starting to turn on " with about 1.1V worst case on the gate - when you get about 1 mA Ids. But to get say 500 mA you'd want 3.5V or better for Vgs.

As a general rule, the higher Vgs the better as long as Vgsmax is never exceeded. Exceeding Vgs by a modest amount (say 10%) MAY lead to gate-source oxide insulation breakdown and destruction of the FET.

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ADDED:

Vth or Vgsth is a useful starting point BUT the graphs are MUCH more useful and are your great friends. Learn to use them and appreciate what they tell you. A single parameter is relevant at one point only and is usually a typical value. Rdson (= the slope of the Vds/Ids curve at some specified point) is often given for pulsed operation at 25C. To properly understand what happens you need to look at the graphs. DO NOT be scared of them - once you learn to use them they are vastly more useful than any single figure can be.

Russell McMahon
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    Great explanation thanks. To clarify, a good way to look at it for a noob like me would be to: 1. determine the current draw and voltage for my load. 2. Lookup the datasheets for the drain current vs gate voltage graphs. 3. Find chips that can deliver my given current for those voltages, at my gate voltage. – 64bit_twitchyliquid Dec 30 '14 at 14:28
  • @64bit_twitchyliquid Yes and no. I'm assuming operation effectively as an on/off switch. Idsmax should be ample for your need and and Vdsmax needs to be at least higher than the highest voltage it will ever see for any reason during operation. Then you look at how much power loss and voltage drop you can tolerate when on. Power loss is I^2 x Rdson plus switching losses. Voltage drop is I x Rdson. I is I instantaneous and whether you use I mean or I peak or Ixxx but you usually know if you can accept 1 Watt or 10 or 100 losses (usually yes/perhaps/no way but this can vary) .... – Russell McMahon Dec 31 '14 at 19:04
  • .... and you know if 0.1 Volt or 1 Volt or 10 Volts drop is OK. eg in a 3V system at 10A 0.1 V MAY be tolerable and 1V very seldom would be. So Rdson of < R = V/I = 0.1/10 = 10 milliOhms would be a starting point. Power loss resistive then = I^2 x R = 10^2 x 0.01 = 1 Watt and total power here = V x I = 3 x 10 = 30 W so loss is 1/30 ~= 3% so maybe OK. A 5 or 2 or 1 milliOhm FET would be better but dearer so you would look at specs and cost etc and decide. THEN there are switching losses and gate capacitance (affecting switching speed etc) and ... to consider. – Russell McMahon Dec 31 '14 at 19:09
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By checking the \$R_{ds}\$ to \$V_{gs}\$ graph.

A MOSFET is on as long as \$V_{gs} \ge V_{gs(on)}\$. Usually you determine if the FET is turned on adequately at the given voltage.

For example, 2N7000 datasheet specifies a \$R_{ds(on)}\$ at \$V_{gs}=10 \mathrm V\$ but it is turned on enough with \$V_{gs}=4.5 \mathrm V\$ so it is useable in logic.

Maxthon Chan
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  • Um, I think I understand. So the Rds --> Vgs graph shows what resistance will be there between Drain and Source for a given Gate voltage? will this be (mostly) irrespective of the Drain-Source voltage? I cannot find such a graph on the BS270 datasheet. http://www.farnell.com/datasheets/104927.pdf Instead, there is a graph with Rds and drain current. Can I use that instead? – 64bit_twitchyliquid Dec 30 '14 at 14:19
  • \$R_{ds}\$ is also a function of \$I_d\$ so definitely watch that one out. On page 3 figure 2 the graph is \$R_{ds}\$ to \$I_d\$ at various \$V_{gs}\$ and there is a plot line for 5V so use that. Also why using `BS270`? That part is almost an order of magnitude worse th – Maxthon Chan Dec 30 '14 at 14:32
  • ... worse than `2N7000` and they cost about the same. I have very positive experiences interfacing MCUs with 2N7000 directly, using both 5V and 3.3V logic. – Maxthon Chan Dec 30 '14 at 14:39
  • I have been looking at the BS270 because it is only $0.06AUD each and it is in a T-92 case. I am trying to build up a set of general purpose components in my parts drawers at a cheap price, and I need to find something for 200-300ma at 5V. I have already found FETs for high current stuff and NPNs for lower current stuff. The reason I asked _how_ to read the datasheets in this way is because I want to learn to be able to compare based on these attributes and price, rather than post my requirements and people suggest individual components. – 64bit_twitchyliquid Dec 30 '14 at 14:41
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    I bought a stock of 50 `2N7000`s, the same TO-92 package, at less than half a US cent each from Shenzhen and they claimed that it is from Fairchild Semiconductor. Those jellybean parts are mass produced (try billions a week) and dirt cheap. When looking for generic parts search terms like "generic n-channnel mosfet" in Google and it will help a lot. Those high current FETs from IR (e.g. `IRF540` and `IRF4905`) are also mass produced in a similar fashion. The stock of `BS250` you bought will probably be just relabeled `2N7000` sold at a higher price. – Maxthon Chan Dec 30 '14 at 14:48
  • [This dude](http://item.taobao.com/item.htm?spm=a230r.1.14.36.Ivr9Mf&id=27407120226&ns=1&abbucket=12#detail) sells packs of 100 `2N7000`s at CNY 35, roughly half a US cent each. And [this dude](http://detail.tmall.com/item.htm?spm=a230r.1.14.125.AqJT4e&id=42049082724&abbucket=12) offers `2N7002`s at about 1 US cent each. – Maxthon Chan Dec 30 '14 at 14:53
  • Does TeoBao only ship inside China? Ive soldiered on bravely with google translate but I cannot find options for international shipping. – 64bit_twitchyliquid Dec 30 '14 at 15:05
  • Some sellers do. Or I can handle the international shipping for you but that will cost you US$ 5 extra. – Maxthon Chan Dec 30 '14 at 15:24
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    @64bit_twitchyliquid Maxthon sounds worth knowing :-) ! - but there is also Alibaba and its many sellers - some who offer free international postage on even very small orders. Or about $30US for an eg DHL courier if time matters. I've had good results with Baite electronics (10 x Arduino mini pro? for $30 free shipping, work well) & almost only seller of CN3058E when I wanted some for prototyping (many non -E suffix sellers). Also many ebay sellers operate ex HK or China with advertised rates or "free" with order. – Russell McMahon Dec 31 '14 at 19:15