Re: MOS process variation

On Tue, 20 Sep 2005 14:22:50 -0700, Jim Thompson wrote:

> On Tue, 20 Sep 2005 11:03:18 -0400, Keith Williams <krw@xxxxxxxxxx>
> wrote:
>
>>In article <uq70j1hfj9kdu3h7clokfqgajmscdh1mqf@xxxxxxx>,
>>thegreatone@xxxxxxxxxxx says...
>>> On 20 Sep 2005 03:52:14 -0700, "daniel" <askvijay.mishra@xxxxxxxxx>
>>> wrote:
>>>
>>> >HI,
>>> >I would like to know what is the criteria of classifying a mos
>>> >transistor as slow and fast,eventhough they are being fabricated on the
>>> >same wafer?
>>> >
>>> >Thanks in advance
>>>
>>> "slow" or "fast" refers to the process, not individual devices.
>>
>>
>>Yes, "slow" and "fast", particularly WRT the subject line, refer to the
>>processing of the wafer. Wafers can be intentionally processed for
>>"fast" transistors, possibly at the cost of power, yield, or both.
>>
>>There are also "fast" and "slow" transistors within a wafer (low Vt vs.
>>high Vt, Thin vs. thick gate ox., for instance).
>
> With shitty processing.

Not at all. It is done quite intentionally.

> Wafers tend to be relatively uniform across a wafer, and quite uniform
> across a single die. Otherwise analog stuff would never work ;-)

It depends on the parameters of the transistor in question. If one needs
a *FAST* transistor, one selects a thin-ox with a low-Vt. If it
needs high voltage on the gate, thin-ox doesn't work so well. If it
doesn't need speed, one chooses a transistor with thick-ox and high-vt
(saves power). ...and all variations inbetween. That's one of the knobs
(powering of gates is another biggie) the timing folks twist.

Not everyone wants every transistor to be equal. There are good reasons
to tweak individual transistors.

--
Keith

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