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Modeling >> Semiconductor Devices >> MOS weak inversion=sub threshold?
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Message started by toseii on Oct 24^th, 2007, 7:56pm

Title: MOS weak inversion=sub threshold?
Post by toseii on Oct 24^th, 2007, 7:56pm

Hi all,

I barely dealt with MOS devices in weak inversion condition before, and now I'm facing such problem. I have a basic question concerning the differences (if any) between weak inversion condition and sub-threshold. :-/

From what I know, weak inversion could be achieved under the following conditions:

1) if VG < VTH (subthreshold)
2) if VG > VTH and both VS and VD are higher than the pinch-off voltage

From my standpoint, scenario #2 is different than #1 since the channel was previously well stablished (strong inversion) and afterwards the pinch-off condition is reached. On the other hand in #1 there are some free minority carriers which comprise the sub-threshold current, which is mainly a diffusion current.

I also know both conditions are well modeled by ID being exponentially dependent on the gate voltage, but not sure if the parameters are the same (I would expect they are different).

Could anyone enlighten me about the differences about these two operating conditions for a MOS device?

Many thanks
tosei

Title: Re: MOS weak inversion=sub threshold?
Post by vivkr on Oct 25^th, 2007, 12:54am

Hi Tosei,

I don't see any difference between the 2 situations. I see that you are using the bulk terminal as your reference.

The only difference between (1) & (2) is that the VTH (referred to source) is now higher. There is no channel
in either case. Once VS & VD both have crossed the pinchoff point, the channel ceases to exist.

Regards
Vivek

Title: Re: MOS weak inversion=sub threshold?
Post by toseii on Oct 25^th, 2007, 6:49pm

Hi Vivek,

Thanks much for your answer. However, I'm still a little bit confused:
In which case you are saying that VTH is higher? I understand that there is no channel in both cases; so I guess I'm getting confused with the references I'm using.

Thanks & Regards
Tosei

Title: Re: MOS weak inversion=sub threshold?
Post by fran2k5 on Jan 12^th, 2008, 8:12am

Hi,

weak inversion and sub-threshold mean the same thing. The first term is mostly used by the analogue design community, the second term by the digital design community.

Regards,

fran2k5