Symbolism: Bulk-Source Difference Explained

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In summary, the conversation discusses a layout trick commonly used in MOS design where series transistors are implemented without popping back up to a metal layer. This saves space and reduces contact failure points. The conversation also explains how this layout technique can be seen in the schematic and how to handle it in a typical SPICE simulation.
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Turin231
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Turin231 said:
[IMG=http://img43.imageshack.us/img43/1997/symbolx.png][/PLAIN]

I was looking at a book with the same symbolism as above on the left and i assumed that the bulk was always connected to the source.Yet at some point i saw this...

http://img707.imageshack.us/img707/8231/whyj.png [Broken]

Why the difference here?if the default in the symbolism is for the bulk to be connected to the source why is there a difference here?

You can see it below (a NAND gate) - your example is a NOR gate but it's the same thing - a NOR would do what I'm describing for NAND but on the PMOS transistors instead (like your circuit):

There is little layout trick often used in MOS design: implementing series transistors without popping back up to a metal layer. It saves space and is two less contact failure points.

You see this in the NMOS devices (bottom) where the series NMOS transistors are implemented as shown. The green is source-drain implant mask. The red is poly mask for the gate and define the self-aligned source-drain channel boundaries.

Compare this to the parallel PMOS devices on top. Notice that the source of the top NMOS transistor is the drain of the bottom NMOS transistor. The implant for that source-drain node (imagine the rectangle defined by the edges of poly and implant) is floating, ergo, not connected to the substrate or floating like the circuit symbol. Seeing this in the schematic tells you that this layout technique was used.[PLAIN]http://www1bpt.bridgeport.edu/~matanya/images/ict2.gif [Broken]
 
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Thanks for the reply.This is actually part of an analog circuit.So to simulate the general behavior o the circuit in a typical schematic cad tool how do I handle this one?
 
  • #4


The simplest answer is the typical SPICE way: either leave the terminal unconnected if the model allows it or put a large resistor from the node to source or macromodel as a resistor and reverse biased diode.
 

1. What is the significance of symbolisms in bulk-source difference?

The use of symbolism in bulk-source difference refers to the representation of abstract concepts or ideas through concrete objects or actions. It helps to convey deeper meaning and adds layers of complexity to the overall message.

2. How do symbols enhance the understanding of bulk-source difference?

Symbols can help to make the concept of bulk-source difference more relatable and easier to understand. They can also add emotional depth and create a more vivid image in the reader's mind, making the concept more memorable.

3. Can symbols have different meanings in bulk-source difference?

Yes, symbols can have multiple meanings and interpretations in bulk-source difference. The meaning of a symbol can vary based on the context and the perspective of the reader. This adds to the richness and complexity of the concept.

4. How do authors use symbolism in bulk-source difference?

Authors may use various literary devices such as metaphors, similes, and allegories to incorporate symbolism into bulk-source difference. They may also use specific objects, colors, or actions to represent abstract concepts and add depth to the overall meaning.

5. What is the difference between symbolism and metaphor in bulk-source difference?

While both symbolism and metaphor involve the use of figurative language to represent abstract concepts, the main difference is that a metaphor directly compares two things, while symbolism uses an object or action to stand for something else. In bulk-source difference, both techniques can be used to convey deeper meaning and enhance the understanding of the concept.

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