hai dudes,
i am sathish kumar doing my engg in e.c.e.
i have a doubt remained in my soul long time
the doubt is,

how gate attracts or repels charge carriers from substrate? the gate is isolated from the substrate or drain or source or channel by means of thin insulating layer.

if thin insulating layer is present then how the electron charge is transfererd to the substrate?

 

It's a _Field Effect_ device. The voltage attracts or repels electrons. If you repel enough of the free electrons in an intrinsically conductive material it becomes an insulator. If you attract enough free electrons into a mildly doped material it conducts. These are depletion and enhancement modes respectively. The electrons. or holes in the opposite majority carrier type, material come from the vast supply available from the power supply.

If that blows your mind, you can think of a capacitor. It too, has an insulator between the electrodes and they seem to move charge around just fine. And you needn't worry too much about it, it happens anyway, whether you believe or not.

Regards
cww

 

Responding to Sathish’s question… here is a simplified description of the MOSFET principle;

Don’t think of the material between the gate and the substrate as an insulator. Instead, think of it as a capacitor. One side is the gate-metal while the substrate material in proximity to the gate, the other side. Then, the application of a positive voltage to the gate will induce negative charges in the substrate material below the insulation layer. This results in a depletion layer in the substrate. As voltage increases, more negative charges accumulate until a channel is formed between source and drain terminals, producing a drain current.

Regards,
Phil Corso ([email protected])

 

Sorry sir, I can't understand your explanation. My question is:

There is an insulation layer between the gate and substrate. Then how will the application of a positive voltage to the gate induce negative charges in the substrate material below the insulation layer?

Sorry sir, I also can't understand the working principle of capacitor for the same reason.

Sir, please help me to understand the mosfet topic, it basically needed because I am more interested in VLSI design.

 

Hello,

Sorry sir, can you explain further about "If you repel enough of the free electrons in an intrinsically conductive material it becomes an insulator. If you attract enough free electrons into a mildly doped material it conducts"?

 

Certainly.

Materials vary from insulators to conductors by the number of free electrons available in their outer shell. Semiconductors reside in the middle range. By introducing small amounts of impurities into their crystal lattice, called doping, they can become conductive with an excess of carriers available, either electrons or spots for electrons called holes.

The classic FET has a narrow channel of semiconductor in the physical position to be affected by the field of the voltage on a gate electrode. Taking the case of electrons, if the channel is non-conductive by virtue of having insufficient free electrons and the polarity and magnitude of the voltage on the gate is sufficient to attract free electrons into the channel, it will conduct. And if the channel is conductive by virtue of being heavily doped, and if the magnitude and polarity of the voltage on the gate terminal is sufficient to repel the free electrons from the channel, it will become less conductive or insulate. The gate is insulated by a thin layer of oxide, hence the Metal, Oxide, Semiconductor designation. Pictures would help.

They work somewhat akin to vacuum tubes in that a voltage field controls the flow of current. All semiconductor magic occurs by fields moving carriers around, even the simplest diode in which
a negative bias creates a depletion region where the carriers are pushed away so the material acts as an insulator. I hope this helps, I've tried very hard when teaching to come up with a way of
explaining this that makes sense without getting too far into semiconductor physics. Please excuse any over-simplifications. We can talk about charge carrier lifetime another day.

Regards

cww

 

Sir,

I am very thankful to you to explain the concept. I understood the 1st part of your answer.

> It's a _Field Effect_ device. The voltage attracts or repels electrons. If you repel enough of the free electrons in an intrinsically conductive material it becomes an insulator. If you attract enough free electrons into a mildly doped material it conducts. These are depletion and enhancement modes respectively. The electrons. or holes in the opposite majority carrier type, material come from the vast supply available from the power supply. <

But I have not understood the concept you explained in the second part, i.e.

> If that blows your mind, you can think of a capacitor. It too, has an insulator between the electrodes and they seem to move charge around just fine. <

I want to know how they have insulator in between and can move charge well??

 

Sir,

I am asking this question to my professor. He replied to me:

The silicon and sio2 are making bond in crystalline structure due to imperfection in the material. There is some number of bonds still vacant (i.e. it has some random sodium atom around them). Due to this only the +ve gate voltage attracts the electron.

Sir, I want to know whether his answer about my question is correct or not? If not, what do you feel is the reason?

 

Yes, if there is sodium in the chemistry. I haven't kept up on the dopants since I left the IC fab end of the business. But that is characteristic of a doped silicon. The silicon is of much higher quality these days. We had yield problem with 40 mil square devices. You would have zero yield with todays devices built on that Si. Millions were spent on the research for dislocation free Si and oxide techniques. The refinement is truly astounding and very much taken for granted.

Regards
cww

 

It's all in the fields. Attraction and repulsion.
Holds atom together and moves electrons around.
In space or in solids.

Regards
cww

 

Sir,

I am very thankful to you to provide such reply to me. I want a web site address which provides information like my requirement is:
"When deposting photoresitive material over sio2, how the structure of "si+sio2" bonding changed?"

Any web site providing information like this, send it to me sir.

 

hai dudes,
i am sathish kumar doing my engg in e.c.e.
i have a doubt remained in my soul long time
the doubt is,

how gate attracts or repels charge carriers from substrate? the gate is isolated from the substrate or drain or source or channel by means of thin insulating layer.

if thin insulating layer is present then how the electron charge is transfererd to the substrate?

for the movement of charges there is 3D animation that show in detail you can watch it:

 

MOSFET (Metal oxide field affect Transistor).
it has two major types Enhancement type and depletion type. further has two types N-Channel and P-Channel.
Enhancemet mean there is no channel between Drai and source if there is no gate voltage, we need to creat or enhance the channel by applying gate voltage to make the connection between drai and source. Enhancement type also called normally open.
Depletion type has the channel without applying any gate voltage, we need to apply gate voltage to deplete/remove the channel if we want to make it open thats why its called depletion type, you can watch this video for better understanding of MOSFET 3D simulation working principle.

 

for the movement of charges there is 3D animation that show in detail you can watch it:

Hi, there is electric field that each charge contain, charge can not pass from gate to source if you whatc the video it will show you the movement of charges. charges onlly attact the other chatges in substrate and repell the the charges in substrate.