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Basic Electronics » Transistor vs mosfet

April 29, 2011
by Noter
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I posted this schematic -

Ralph said -

"And what is the n-channel (?) MOSFET doing? There doesn't appear to be much of a load couldn't/wouldn't a NPN transistor work just as well?"

Bretm said -

"Because of that 10k base resistor I'm guessing it used to be a BJT, but he just swapped in a mosfet (which wouldn't need the resistor) because he had one handy."

I said -

"Bretm, I didn't know the resistor wasn't necessary. I can program ok but the electronics side is my weakness, I would appreciate more explaination on why a transistor needs the resistor but a mosfet doesn't."

So here we are on a new thread Bretm, please continue.

April 29, 2011
by bretm
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BJTs use the base-emitter current to control the collector-emitter current. MOSFETs use the gate-source voltage to control the drain-source current.

The gate is insulated from the body by an oxide layer so that very little current enters through the gate. The DC resistance is in the mega-ohm range. This this layer acts as a dialetric, so the gate actually behaves like a small capacitor, about 20-50pF for a 2N7000.

The circuit above is a DC circuit--the gate voltage doesn't change except for a few nanoseconds while the programmer switches the state of the reset pin--so the gate essentially lets zero current through. Adding 10k to the gate resistance doesn't affect the circuit.

The MOSFET gate will conduct current if the gate is switched at high frequencies. That's the reason that the MCU consumes more power at higher clock frequencies, for example.

April 29, 2011
by mongo
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Right... The 10K isn't necessary but it doesn't hurt either. That's because the gate of a MOSFET does not actually make contact inside the device. It simply excites an electric field that triggers conduction.

A BJT (BiJunction Transistor) does make physical contact and the resistor is to limit the current to the base. If you put a hard voltage to it, the voltage can cause damage to both the transistor and the MCU via feedback through the transistor.

April 29, 2011
by mongo
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Echo...Echo...

April 29, 2011
by Noter
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Ok, I think I understand now - thanks!

April 29, 2011
by bretm
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An interesting thing about discrete MOSFETs is that they only have three terminals. A MOSFET is actually a four-terminal device. The drain-source current is controlled by the gate-body voltage, and the drain and source are interchangeable--there is no preferred direction for current to flow except in specific applications where the doping is intentionally asymmetrical.

What makes the drain and source behave differently in discrete MOSFETs is that the body connection (the back of the gate) is connected directly to the source. That causes the internal diode effect (indicated in the schematic symbol), makes it an asymmetrical device, and makes the gate-source voltage special compared to the gate-drain voltage. You can see this body-source connection in the schematic symbol.

In integrated circuits the transistors are often not connected this way. For example they may all share a common substrate where the body connections are all connected together. In integrated circuits a simplified version of the symbol is used. The diode isn't shown (because it often doesn't exist) and the body connection is omitted altogether.

The symmetric capability of the MOSFET is used in the CMOS transmission gate for example.

April 30, 2011
by Ralphxyz
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Excellant, thanks Paul for posting this and bretm and mongo for your explanations.

Ralph

April 30, 2011
by Noter
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I like being able to leave out the base resistor but otherwise why would a mosfet be chosen instead of a transistor for digital circuits?

April 30, 2011
by mongo
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Static discharge...

The Achile's heel for anything MOS.

New stuff is better protected but some times the tiniest bump can generate a static impulse and take out a component.

April 30, 2011
by Ralphxyz
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So the MOSFET is protected from static discharge because there is no physical connection through the Gate?

Ralph

April 30, 2011
by Noter
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I think mongo is saying it's the other way around, static is rough on mosfet's. ?

May 01, 2011
by Ralphxyz
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Noter asked:

[quote]"why would a mosfet be chosen instead of a transistor for digital circuits?"[/quote]

mongo answered:

[quote]"Static discharge...
           The Achiles' heel for anything MOS."[/quote]

I am still not sure, the Achilles' heel part implies you are correct Paul but ...

Ralph

May 01, 2011
by Noter
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Hey Ralph, This page clears it up - static electricty can destroy a mosfet.

I guess the best reason to choose a mosfet over a BJT is the low power consumption. If your circuit is going to run on batteries that would be most important.

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