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In implementation section of C-element at Wikipedia website: https://en.wikipedia.org/wiki/C-element there is a diagram that points to a weak transistor.

What is a weak transistor?

Below is the diagram that I am talking about:

Weak transistor

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A "weak" transistor has a lower transconductance, which could be done by making something longer, current starved or a threshold implant.

I would encourage you to avoid "weak" transistors for a few reasons:

  • we really cannot use them on aggressive processes due to doping spacing
  • you cannot just make "Long" devices because it moves the threshold toward the bulk, and you need to have a biased FET or resistor.
  • Significant power to overdrive the other FET.

You have Muller C-Elements there, and there are ways to make static Muller C-elements without weak transistors.

Edit

As mentioned in the comments, weak transistors are used in I/O, and I was focused on the datapath, but here's a feel for making these devices with resistors on a commercially available CMOS process that I'm allowed to talk about. As devices have gotten smaller, you find that you have greater space between implants, also, devices cannot be made "longer" without tiling standard devices in a chain, and this makes a mess of your datapath; however, I/O drivers are always the exception. The I/O drivers are huge, so you generally use resistors to make your weak devices as you have space.

resistors

There's a yellow box in the picture above for the "digital minimum" size, and the size for a driver is 2x that on this process. I use the n+ diffusion resistors for my FPGA reset pin because I also use it for a double bus fault notification. You bring the pin low to reset, OR I can wire logic up to it to bring the pin low when I have an unrecoverable timing issue.

If you want to compare this to the length of a FET, I spec my I/0 drivers for 20mA, which makes them 4-micrometers wide at a minimum length for I/O FETs, which gives me 25 "squares" of equivalent space. This would give me 8.7k of resistance if I ran a p+ poly up the side in parallel with my driver.

There's the high altitude view of where weak transistors are actually used, and we (well, me anyway) use resistors to make them instead of playing with threshold implants or length.

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  • \$\begingroup\$ Thank you Dr. Brian. So it is better to have 12 transistors than to have 4 transistor + 2 weak ones. \$\endgroup\$ Jun 12, 2016 at 2:21
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    \$\begingroup\$ Weak transistors are very common in MPU's where an I/O pin may have a tri-state or OC option. As pointed out in other answers, it is also a bus-holder if no I/O signal is present. They are internal to the IC, as opposed to pull-up / pull-down resistors. \$\endgroup\$
    – user105652
    Jun 12, 2016 at 3:58
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    \$\begingroup\$ @Ehsan You can use them, but there's some smart ways to not use them in C-elements. It depends on your power requirements. I do not use weak transistors because I cannot guarantee their performance as transistors. \$\endgroup\$
    – b degnan
    Jun 12, 2016 at 10:53
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    \$\begingroup\$ @Sparky256 That would be the exception. I'll update my answer to include that. \$\endgroup\$
    – b degnan
    Jun 12, 2016 at 10:54
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I'm pretty sure they mean a transitor that is designed to have a deliberately high on resistance. This allows other transistors to overpower it.

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  • \$\begingroup\$ They are also used to prevent a analog, I/O pin or internal connection from truly 'floating' at such a high impedance that its behavior is unpredictable. \$\endgroup\$
    – user105652
    Jun 11, 2016 at 22:38
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A weak transistor is one that has lower current \$ I_{ds,sat}\$ relative to others in the circuit. This is done primarily with a lower \$ \frac{W}{L} \$ although in most digital processes you don't change the L so much, so that means that the W must be smaller than those other transistors.

In this circuit these weak transistors are being used as bus holders and need to not cause as much conflict on the input to that last inverter.

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  • \$\begingroup\$ Yes it is used as a memory element or as you said "bus holder". \$\endgroup\$ Jun 12, 2016 at 2:22

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