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MOS process variation

Discussion in 'Electronic Design' started by daniel, Sep 20, 2005.

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  1. daniel

    daniel Guest

    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
  2. Jim Thompson

    Jim Thompson Guest

    "slow" or "fast" refers to the process, not individual devices.

    ...Jim Thompson

  3. 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).
  4. Jim Thompson

    Jim Thompson Guest

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

    ...Jim Thompson
  5. Rich Grise

    Rich Grise Guest

    But could transistors with different characteristics be _intentionally_
    put on a die? I think that's what Keith is getting at.

  6. keith

    keith Guest

    Not at all. It is done quite intentionally.
    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.
  7. keith

    keith Guest

    Sure. It's done _all_ the time in advanced processes. One selects the
    parameters needed for each transistor, if need be.
  8. Jim Thompson

    Jim Thompson Guest

    I've just not seen "fast" and "slow" as descriptors for such
    processing. I deal with multiple device TYPES all the time.

    ...Jim Thompson
  9. We don't really call them "fast" or "slow" either. Rather "High
    Vt"/"Low Vt" or "Thin-Ox"/"Thick-Ox" (or something inbetween).
    Depending on the application, either are used for fast/slow choices.
    ....or maybe "fast/not-fast" or "fast/who cares" are better choices of
    descriptors. ;-)
  10. Jim Thompson

    Jim Thompson Guest

    half-fast ?:)

    ...Jim Thompson
  11. Aw, Jim. You peeked at our spec! ;-)
  12. Of course. Analogue design is a *trade off* of various parameters. There
    is no "one size fits all". For example, right now I am designing a HV
    cmos amplifier using extended drain transisters to obtain the required
    50V voltage rating. Extended drain transisters require several times
    more area then a conventional device, so one only uses them when there
    is HV. In addition, a penalty of HV rating is lower speed, e.g you cant
    usually use the minium process gate length.

    Other variations are "natural devices". For an nmnos, a natural can have
    a Vt of around OV. This is useful in source followers. It can allow the
    output to be pulled much closer to the supply. Some devices are isolated
    in a well. This avoids the backgate bias increasing the threshold
    voltage as you can connect the backgate to the source, e.g. in diff
    pairs. These devices are larger then non isolated ones, hence again a
    trade of size verses performance.

    I could go on, but am sure one gets the picture...

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
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