vivkr
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Hi,
I have read that NQS models are needed to accurately predict some effects, such as charge-injection, or at very high frequencies. I have some doubts on this subject, particularly with the case of charge-injection.
The basic idea is to use an NQS model if the carrier distribution in the channel does not change fast enough so as to keep pace with the changes in the terminal voltages (gate voltage). There are 2 scenarios:
1. An ideal very fast edge square wave turns a MOS switch ON/OFF. The ideal approximation may be achieved by using say a modern fast signal generator with a MOS transistor which is say 100um-200um long and the process very old (say a 2um CMOS).
2. In a real scenario, the gate of a MOS is probably being driven by another MOS device on-chip. As charge-injection is an issue in switches, and switches are always made with min. channel length, it is unlikely that the driving edge is so fast that it can change the gate potential of the controlled MOS switch faster than the rate at which the carrier distribution in the channel can change. This is because the controlling MOS driver itself will experience inertia in responding to its own input. Additionally, there are parasitics on-chip which further slow things down.
So, when do we really need to use NQS models? It would appear that for a typical system, the risetime of any controlling signal generated on-chip can seldom be fast enough to force the use of such a model, unless one were driving a device which is considerably slower (very large L as in 1.).
Regards Vivek
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