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Basic Electronics » Getting my head around MOSFET's

March 08, 2010
by nickolai
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I'd like to firm up my understanding of MOSFET's

A couple things that have thrown me off.

I'm confused on the little arrow that points towards or away from the gate (page 10 in the Nerdkits Guide). Does it represent the flow of negative current or positive current or something else?

Also, does current always flow a certain direction? like from the negative to the positive?

Any insight is appreciated, Nick

March 08, 2010
by BobaMosfet
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nickolai-

Don't let the arrow confuse you. Just like with a BJT, the arrow tells you which terminal is positive. If it points TOWARDS the transistor center, it is a positive terminal/connector. If it points away, it is negative. Which of course, with any type of transistor, tells you what the other terminals are. From this you may infer current direction, and the channel type.

BM

March 08, 2010
by bretm
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Arrows in transistors and diodes work the same way. It shows the direction of "forward bias". The pointy end of the arrow is the more negative voltage end.

But MOSFETs are different than BJTs in one important way: current doesn't actually flow through the gate (except for some leakage) so the arrow is showing you the voltage polarity, not necessarily the current flow. In BJTs, the gate-emitter current controls the collector-emitter current. In MOSFETs the gate-source voltage controls the drain-source current.

This is a P-channel MOSFET, so it switches on when the voltage at the gate (G) is more positive than at the source (S).

Public domain image from http://en.wikipedia.org/wiki/File:IGFET_P-Ch_Enh_Labelled.svg

This is an N-channel MOSFET, so it switches on when the gate voltage is more negative than the source.

Public domain image from http://en.wikipedia.org/wiki/File:IGFET_N-Ch_Enh_Labelled.svg

Another thing that's different between BJTs and MOSFETs is that in BJTs the arrow is also in the way of the collector-emitter current flow, so the arrow also tells you the polarity of the collector relative to the emitter. In MOSFETs the arrow is between the gate and the source but it's not between the drain and source. Current can flow in either direction between drain and source.

Another thing to note is the "dashed line" between the drain and source. This indicates that current flow is normally interrupted, and a gate voltage will "close the gaps". That means the diagrams above are for "enhancement-mode" MOSFETs. A depletion-mode MOSFET looks like this:

Public domain image from http://en.wikipedia.org/wiki/File:IGFET_P-Ch_Dep_Labelled.svg

In this case the line between drain and source is solid, so current flows between them relatively unimpeded until a gate voltage is applied to reduce the drain-source current flow.

March 08, 2010
by nickolai
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Thank you guys for your replies. They've helped.

The pictures of n and p channel MOSFET's on page 10 of the Nerd Guide looks backwards to me. Perhaps, it's just a different way of drawing the circuit diagram then Wikipedia's example? http://en.wikipedia.org/wiki/MOSFET

My follow up question is how do you control the gate via all the digital 0's and 1's? Note: I understand that a high voltage between the gate and source connects the source and drain. I guess I'm trying to bring it all together but it might not be that simple.

Thanks, Nick

March 08, 2010
by bretm
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You know what, I think I explained it backwards. I totally agree, those arrows are confusing. Some styles show arrow pointing toward the gate and some show it away from the gate in n-channel mosfets. And my description was completely backwards.

March 08, 2010
by bretm
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The 2N7000's that come with the nerdkit are n-channel. If you set the source to ground level and then connect a pin directly to the gate, the pin will switch the drain-to-source connection. You don't need a current-limiting resistor for the gate connection because the gate is very high impedance already. You have to limit the drain current, though.

May 20, 2010
by DavidinLA
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Hello all, new to the forum.

If you connect a 12V power source to the drain, will the 2N700 produce a 12V output at the source? (assuming you connect the gate to one of the 5V pins)

May 20, 2010
by DavidinLA
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Also ... which wire is drain, gate, source? Looking at the flat side.

May 21, 2010
by hevans
(NerdKits Staff)

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Hi DavidinLA,

I think the answer to your first question is no. In the configuration we often put it in you can think of the mosfet as a voltage controlled switch. When the the voltage between the gate and the source is large enough (greater than Vth of the transistor), then current will flow from the drain to the source. Exactly how much current is dependent on the voltage difference between the drain and the source.

Remember that for an n-mosfet to act this way the voltage Vgs must be grater than the threshold voltage (about 2V). If what you said was true, and with the mosfet on we got 12V at the source, then Vgs would be 5-12=-7. Since that is less than 2V, the mosfet would turn off.

Looking at the flat side the pins are source, gate, drain from left to right. Here is the 2n7000 datasheet.

Humberto

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