Transistor (MOFSET) confusion

[Denver Dang]

Homework Statement

I currently reading about transistors in a solid state course. And the transistor is a MOFSET, which I have to describe. I can't really find a nice figure of the one I have in my book, so I will just try to explain my confusion.

It says that you make a MOFSET by having a p-doped substrate in which you have two n-doped regions (source and drain), which are connected by an oxide layer, which again is connected to a metal electrode (gate).

Now, when you "open" the gate, you repel the holes underneath the oxide layer, making "room" for electrons and attracting them, right ? And first when you apply a voltage, there is enough energy to make the electrons move from source to drain, or maybe I have misunderstood something ?

So, my main question is: Since the p-substrate has a lot of holes, why are the electrons from the two n-type regions not attracted to them, even when there is no gate or voltage applied ? Why do they first start to be attracted when the gate is opened, and making the oxide layer positive ?

Don't know if I have explained it in a good way, but I hope you understand.

Homework Equations

EDIT:
I actually have the figure from my book right here:

http://i.imgur.com/Qda9msK.png

The Attempt at a Solution

 

[phyzguy]

So, my main question is: Since the p-substrate has a lot of holes, why are the electrons from the two n-type regions not attracted to them, even when there is no gate or voltage applied ?

The positive charge of the holes in the p-substrate is balanced by the negative charge of the acceptor atoms in the p-substrate. So the p-substrate is charge-neutral and there is no net charge to attract electrons from the n-type source and drain. It is only when a positive voltage (larger than the threshold voltage) is applied to the gate that an n-type channel forms between the source and drain. Does this answer your question?

 

[Denver Dang]

Indeed it did :)

Although, the threshold voltage, how is that defined ?

 

[phyzguy]

Indeed it did :)

Although, the threshold voltage, how is that defined ?

Try this:

http://en.wikipedia.org/wiki/Threshold_Voltage

 

[HalfLostAndSearching]

Transistors are semiconductor devices that are used to amplify or switch electronic signals. A MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of transistor that uses a metal gate and an oxide layer to control the flow of electrons between the source and drain regions.

In a MOSFET, the p-doped substrate acts as a channel for current flow. When a voltage is applied to the gate, it creates an electric field that attracts electrons from the source region towards the drain region. This allows current to flow from the source to the drain.

In the absence of a gate voltage, the p-substrate is positively charged due to the presence of holes. However, these holes are not attracted to the n-doped regions because they are already filled with electrons. The electrons in the n-doped regions are also not attracted to the p-substrate because they are repelled by the positively charged holes.

When a gate voltage is applied, it creates an electric field that repels the holes in the p-substrate, creating a depletion region. This depletion region allows the electrons in the n-doped regions to flow towards the p-substrate, creating a conductive channel between the source and drain regions.

In summary, the gate voltage is necessary to create a conductive channel between the source and drain regions in a MOSFET. Without it, there is no path for the electrons to flow from the source to the drain. I hope this explanation helps to clarify your confusion.