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DPRG: Mosfets or Transistors

Subject: DPRG: Mosfets or Transistors
From: Jim Brown jgbrown at spd.dsccc.com
Date: Tue Jan 20 16:25:23 CST 1998

<snip>
> Also could someone give me a simple reason and
> explain why one would use a mosfet or a transistor.  I'm trying to
> understand the application of both.  Thanks and keep up the good work.
> 
> Erick Guzowsky


Probably someone with more knowledge should be answering, but hey, I'll
give it a shot and put it in my myopic-electrical-knowledge terminology
to work:

A transistor is sort of like two diodes hooked back to back.  Where a
diode would be made of N & P material, a Transisitor is made of N & P & N
material or P & N & P material.  When voltage is applied to the base material,
the collector and emitter sides semi-conduct linearly.  Since it's doing
some sort of semi-conducting, it acts like a resistor and generates heat.
Since heat is generated, it's a good bet that power is being lost in the process.
What's worse it the base is not isolated at all, so some current conducts
through the base back to the controlling device.  A resistor is usually a
good idea between the transistor and the controlling device to help isolate
them.  Transistors are very sturdy devices, and are not very susceptible
to electrostatic shock.

A mosfet, on the otherhand is more of a digital switch.  Instead of a
base, it has a gate, and instead of a collector and emitter it has
drain a source.  When voltage is applied to the gate, an electrical field
is generated which sort of creates a connection between source and drain.
Since the gate is not really connected to source and drain, it's more
isolated so a resistor back to the controlling device is not as necessary.
Also, because of the way it conducts, it generates much less heat in the
process and can usually handle many more amps than a transistor.  Since
less heat is generated, less power is lost so they are more efficient. There
are two types of Mosfets "P-channel" and "N-channel"  The p-channel mosfets
will source power, and n-channel mosfets will sink power.  Mosfets are
very sensitive devices and are very susceptible to electrostatic shock.
Care should be taken to ground yourself before handling these devices
or you may damage them.

Both transistors do essentially the same thing in that voltage applied
on one pin cause the other two pins to conduct.  It would probably be
best if mosfets could be used for everything, but their high price forbids that;
transistors are much cheaper.  Since mosfets can handle a lot of amps (30amps
is common), they are usually used for driving motors or in power supplies.
Transistors are usually found in low voltage low amperage circuits (something
like +5 @ 500ma).  Other than that, I suppose that there are other tricks
you could do with transistor circuits and other tricks you could do with
mosfet circuits.


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