Questions about enhancement type MOS from Razavis book

[kidman]

mos question

I am just trying to transfer from digital design to analog or mix.

I am start reading Razavi's book. But there are several points I am not clear about mosfet:
1. It seems the enhancement type of MOS is used as example, then why there is a depletion region created when Vgs increase from 0 to Vth? I though depletion region only exists in depletion type MOS. And when Vgs>Vth, an inverted layer is created, right?

2. When Vds > Vgs-Vth, MOS enters saturation. What does "pinch-off" mean in such case? What is the actual relation between Vds and "pinch-off" phenomenon?

Thanks!

 

[mazelk]

Re: mos question

Hi...

1) While Vgs increases toward Vth, the P semiconductor gets away from gate, because the positive charges are accumulated over the top plate of gate capacitor, in consecuence, negative charges are extracted from P substrate, and the positive charges are spread away from there. In this way, can be possible a conduction channel betwen drain and source. The depletion region increases, but it didn´t exist before Vgs.

Other hand, depletion region in deployment MOS has already exist even before Vgs.

2) Pinch-off occurs when the channel is strangulated toward drain. One of its consequences is channel modulation (LAMBDA), which multiplies Vds in saturation current modeling.

 

[andy2049]

Re: mos question

hi mazelk

What makes the deployment MOS different from the enhancement MOS?
heavy doped Source and Drain? light doped substrate? or very short channel?

Thanks.

 

[mazelk]

Re: mos question

Hi...

Deployment MOS has channel built at Vgs = 0. Enhancement MOS doesn´t have channel built at Vgs = 0. In deployment MOS, the channel deep, by consequence its conductivity, can be controlled in same way that enhancement MOS one. Unlike enhancement MOS, you must apply negative Vgs to spread away electrons, and control the conductance.

 

[andy2049]

Re: mos question

Hi mazelk,

I guess you misunderstand my question. How to build the deployment MOS to make a channel when Vgs=0? Thanks.

 

[Edward_2288]

mos question

the channel is made by ion implantation. that's what we call enhancement type.

 

[yjkwon57]

Re: mos question

Hi.

1. The depletion layer under the gate for 0 < Vgs < Vt is formed due to the work function differences of the gate material and the substrate material.

2. The pinch-off occurs when Vds > Vgs - Vt as you know. At this Vds, the channel disappears at the drain side. Since the accumulated carriers disappear, the conductivity becomes very very large. Thus, the saturation in the drain current occurs.

Bye~~~

 

[visualart]

Re: mos question

I am just trying to transfer from digital design to analog or mix.

I am start reading Razavi's book. But there are several points I am not clear about mosfet:
1. It seems the enhancement type of MOS is used as example, then why there is a depletion region created when Vgs increase from 0 to Vth? I though depletion region only exists in depletion type MOS. And when Vgs>Vth, an inverted layer is created, right?

2. When Vds > Vgs-Vth, MOS enters saturation. What does "pinch-off" mean in such case? What is the actual relation between Vds and "pinch-off" phenomenon?

Thanks!

u have the Razavi's book(ebook)? can u share it ? Tia

 

[hoangthanhtung]

Re: mos question

I am just trying to transfer from digital design to analog or mix.

I am start reading Razavi's book. But there are several points I am not clear about mosfet:
1. It seems the enhancement type of MOS is used as example, then why there is a depletion region created when Vgs increase from 0 to Vth? I though depletion region only exists in depletion type MOS. And when Vgs>Vth, an inverted layer is created, right?

2. When Vds > Vgs-Vth, MOS enters saturation. What does "pinch-off" mean in such case? What is the actual relation between Vds and "pinch-off" phenomenon?

Thanks!

Read this document very carefully. It contain answer for your question. Best regards.

 

[hoangthanhtung]

Re: mos question

u have the Razavi's book(ebook)? can u share it ? Tia[/quote]

This is a link to download razavi's book for you.
**broken link removed**

About the solution of razavi's book. You can download in mcu.cz site
Bye

 

[srivatsan]

Re: mos question

i dont understand the language at mcu.cz site. could you provide the url where i can find the solutions for razavi problems? thanks a lot.

 

[whizking]

Re: mos question

hye this pdf should clear the doubts..

 

[whizking]

Re: mos question

sorry that one was a worng pdf....i deleted the original