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Tuesday, July 10, 2012


Recently I needed to better understand MOSFTETs so I have been experimenting a little.

MOSFET (Metal Oxyde Semiconductor Field Effect Transistors) are extremely interesting devices and in the end quite simple to understand.
They are transistors, but work slightly in a different way from traditional transistors, plus they can drive easily significative currents and voltages.
Basically they are used to control electronically the current flow from the SOURCE to the DRAIN by generating a "channel" between them.
This channel is created by attracting electrical charges (electrons or "holes", depending on the type of MOSFET) via a metallic plate connected to the GATE.

This metallic plate is insulated and works similarly to a capacitor plate, storing charges that polarize the near region in the semiconductor substrate, which generates a "conenction" between source and drain, allowing electrons to flow.

Finally it works like a valve that we can open by supplying charges to the gate o close by discharging it.
The interesting part is that a rather small amount of charges are needed to operate the gate, so, while we can easily have on a  rather small device a 20Amps current with 100V potential between Source and Drain, we can simply operate the gate with a couple of volts between source and gate.

There is not much energy depletion in charging the gate, because once charges are provided, they tend to stay there, untill we remove them.

There are two types of MOSFETS, depending of the semicoductor substrate used, they are called P-Channel and N-Channel Mosfets.

The two symbols represent a P-Channel device (top) and a N-Channel One (bottom)

Source and drain are connected to semiconductor regions of the same kind (P/N) and of the opposite type of the substrate between them.

In the picture above, a N-Channel device is represented.
In this kind of device the gate is charged positively to recall electrons in the upper part of the P semiconductor substrate, generating a thin channel with negative charges (N) that connects Source and drain.

On a P channel device the gate would be charged negatively and this would "push away" electrons from the upper part of the N Substrate, creating a positive channel between S and D (which would also be  connected to P type regions).

I have seen some papers on this subject stating that on a P-Channel you will connect the Drain to the ground and Source to Vdd and then in order to open the channel the Gate needs to be pulled to ground.
Others would instead keep the same connections as for N-Channel ones and assume the Vgs is negative.
It is technically the exact same thing and I do prefer the second approach as it is easier for me to understand and remember.
Once you understand that he gate needs to be charged negatively for P channel ones and positively for N channel ones, you should be all set.

While theory is cool, I tend to understand better with a little practice, plus it's much more fun, so I got myself a couple of P-channel mosftes from ebay (IRF9540) for less than one dollar each, shipment included all the way from Mighty China to Europe.

Hey, what's better than something that costs less than a coffee and even makes me less nervous than caffeine?
This is my little experiment and -incredibly- practice confirmed the theory.
Something I did not expect was that the gate tends to stay charged for quite some time, good to know.
This means that if you need it to go off quickly you definitely need a pull-down resistor between Gate and Source.

Notice in the video I also state that I would use a negative voltage from Source to Drain (not only to charge the Gate).
Technically, in a MOSFET, once the channel is open, is open in both directions, so we could have electrons flowing in either way (with positive or negative currents from S to D).
However these devices normally have an internal diode between Source and Drain, therefore they can stop the current in one single direction, in the other one they will always conduct.

So, cool tiny devices, now what?
Since we can drive these devices with small currents and small voltages, they can quite easily be interfaced with digital outputs of microcontrollers.
Moreover the gate can be charged and discharged extremely quickly allowing the implementation of PWM techniques to control electric motors, switching power supplies, inverters etc.
And that's my target, I need a step down "buck" converter driven by a microcontroller, these circuits are  commonly used in battery chargers and other power devices.

I might tell a bit more about that after some experiments.
Do you like physics, electronics and technology? Get yourself a couple of mosftes, you can have a lot of fun with them!

Note : while these devices can handle high voltages and amps between source and drain (check the datasheet of each specific device you plan to use for that), they normally tolerate much lower voltages on the gate (usually <12V, again check the datasheet. Apply more than that and they can go "KABOOOM").


santhosh.T.K. said...

good work Francesco Agosti. No textbooks would have taught me that..

Michael Pound said...

Try to add more Unipolar transistor characteristics for now