# Field effect transistor

Discussion in 'Electronics Homework Help' started by anni, Apr 1, 2020.

1. ### anni

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Apr 1, 2020
From basic electrostatic considerations derive the output and transfer equations for field-effect transistor. Include in the derivation the short channel case.

current vs drain-source voltage at constant gate-source voltage, for the output;

current vs gate-source voltage at constant drain-source voltage, for the transfer

2. ### Nanren888

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Nov 8, 2015
How far have you got so far?
Any relevant relationships?

3. ### anni

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Apr 1, 2020
I don't understand what you say?

4. ### Nanren888

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Nov 8, 2015
The problem was Derive .... things
Which part do you need help with?
Do you have that "basic electrostatic considerations" reference sorted out?
Do you have an idea of what an FET looks like, semiconductor construction-wise?

5. ### anni

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Apr 1, 2020
I find some relations for MOSFET n-channel, but I'm not sure that in short channel the equations is different or not. and it is transfer and output equation?

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6. ### anni

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Apr 1, 2020
Thank you for your reply. I read about the structure of FET, but I don't know about know how these conditions change equation. Are the equation that I find is correct or no?

7. ### Nanren888

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Nov 8, 2015
So, is it only the "short-channel" case that's giving you trouble?

For the normal case, I guess both cases, I guess I'd start by looking for the correct answer, so I know what it is that I'm trying to derive. Is that where you're up to?

8. ### anni

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Apr 1, 2020
I'm not sure if I understand your point

9. ### Nanren888

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Nov 8, 2015
It appears that someone who set this homework thought it was a good idea for students to derive the relationship themselves. I do remember this as an effective way of understanding, myself.
Policy here, which I agree with, is to not cheat students of the learning opportunity that goes with doing that sort of thing.
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There are many here who are very knowledgeable over many areas and may be happy to help with parts that are causing you trouble, if you tell them which parts are causing you trouble.
My approach for problems of "derive ...." I think is to collect the starting points, the knowns and the hints, and look for the known relationships covering those things that might, when combined, substituted, lead to the relationship needed.
From memory, an that's awhile ago, JFETs are all about a conduction channel and its width/area. But maybe you have a better model for the physics of what's going on.

Harald Kapp likes this.
10. ### Harald KappModeratorModerator

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Nov 17, 2011
First thing to consider: Junction FET or MOSFET? Probably MOSFEt as JFETs are not that common anymore as they used to be.

My interpretation of the task description is that from an electrostatic model of the MOSFET operation (using the gate-channel capacitor and the charge distribution within the channel) you are to derive the equations describing the behavior.
I found this description online. It looks as if this is a comprehensive description of the MOSFET, how it operates, how it can be modeled, how it can be described mathematically. In my opinion you'll find the answers you are looking for if you study this text thoroughly.
If you get stuck at any point feel free to ask for help so we may be able to get you going.

hevans1944 likes this.