The Learning Circuit 31: How FETs Function

A few years back I did some 'experimenting with MOSFETs' blogs. They weren't intended as STEM teaching material, but might be of some interest to you.

/members-area/b/blog/posts/experimenting-with-mosfets-total-gate-charge

/members-area/b/blog/posts/experimenting-with-mosfets-transfer-characteristic

/members-area/b/blog/posts/experimenting-with-mosfets-threshold-voltage

/members-area/b/blog/posts/experimenting-with-mosfets-output-characteristics

A list of my other blogs can be found here (it includes experimenting with the characteristics of JFETs and bipolar transistors, if you happen to be interested in those too)

/members-area/b/blog/posts/jc2048-blog-index-new-version

I'm not sure if you noticed that there is an ebook available from the community. /learn/publications/ebooks/?ICID=homepage-rotational%20banner I'm not aware of lab material. Our very own bald engineering has a tutorial /challenges-projects/element14-presents/vcp-program/w/documents/3705/a-bald-engineer-video-mosfet-tutorial . I found some useful information in the material.

Do any lab experiments exist somewhere to better understand mosfets?

Thank-you so much! I think I understand the parts of the gfet a lot better now!

So, (I'm speculating a little, but I"m pretty confident in my understanding) the Si is the conductive substance the gate signal connects to, allowing the voltage at the gate to affect the charge of that layer. The SiO2 layer is the non-conductive "buffer" layer that keeps the Si layer separate from the rest of the FET.

Think of it like the Drain and Source being the In and Out and the graphene being the pipe allowing flow between. The ionic solution(negatively charged) and the Si layer(positively charged) act as a control valve for how much can pass through the graphene pipe.

The only conductive path between the source and drain pins through the FET is through the graphene layer. It's like a bridge between them surrounded by non-conductive material. There is conductive material inside the FET that the external source pin connects to. And a second piece of conductive material inside that the drain pin connects to. Internally, the only connection between that allows electricity to flow from one to the other, is the graphene layer. So it's considered a channel that connects the two.

The signal at the gate pin controls the amount of positive charge in the Si layer. The more charged it is, the more it attracts the negative particles  in the ionic solution. The two sets of particles attract and "squeeze" or cut off the flow through the graphene layer.

Charged particles are always drawn to the nearest, strongest opposing charge. If the charge/voltage at the gate is strong enough, say strong than that at the drain, particles trying to flow between source and drain are instead attracted to Si layer at the Gate. They will flow less or not at all depending on the strength of that charge/voltage at the Gate.

This is a bit oversimplified, but I hope it gives you a general idea of the concept. I always think of charged particles like magnets. If you put a piece of metal between two magnets, it'll be attracted to the stronger one. If you can make the second magnet strong, it could make the metal detach from the first and attract to the second.

I might have this backwards in that you actually want more charge at the gate to attract the negative particles of the ionic solution and that increases flow instead of limits it, but I would need to look into the structure more to figure that out.

It's called a FET because the amount of flow is controlled by the electromagnetic field generated at the gate pin. It functions like a (T) transistor, but is controlled by the electromagnetic (F) field (E) effect of the gate charge acting as a valve allowing more or less flow between the drain and source.

I wrote this a bit choppy, so sorry if the flow of thought is a bit disjointed.

Thanks so much for the response!

What I'm confused about is what the purpose of SiO2 and the Silicon Substrate in these diagrams is:

From my understanding, the substrate is doped with something and becomes the "back gate" which can change the sensitivity of the graphene to something, while the SiO2 I have no idea why it's there other than to somehow help the graphene "channel" work.

Would the source and drain be doped with anything?

And what exactly do people mean when they say that the graphene is a "channel"?

Why is a GFET called a FET?

I'm not sure what you mean by "making a FET". Given the complexities of construction, it's usually an off the shelf part. Some logic devices you can make with discrete components like transistors, but I don't believe that's possible with FETs, so I'm not sure what you mean.

I had never heard of GFETs before, but I found this and it's very interesting:

"Graphene field-effect transistors (GFETs) take the typical FET device and insert a graphene channel tens of microns in size between the source and drain. Being graphene, a lattice of carbon atoms that is only one atom thick, the channels in GFETs have unprecedented sensitivity, which can be exploited on a wide variety of applications such as photosensing, magnetic sensing and biosensing."

It's from this article which seems to explain the difference quite well. https://www.graphenea.com/pages/what-are-graphene-field-effect-transistors-gfets

If you're still confused after reading the article, come back and ask questions and I'll see if I can help.

Thank you for making this tutorial! The whole n and p stuff makes more sense now!

Can I ask what people mean when they say they're making a FET though? What are the components people think about? Specifically, I want to understand how a GFET works in general (for a paper for school) but I can't seem to find out what the graphene/substrate/dielectric is supposed to do.

https://www.onsemi.com/pub/Collateral/AN-9010.pdf.pdf

If you look at Figure 4 it shows the different structures for power MOSFETs. The V and U ones have the gate as a trench that has been etched into the semiconductor.

Is a mosfet using trench technology just another name for one of the mosfets described in the video?

Sean