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diode connected diode in cmos

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chang830

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In cmos design, we always use the diode connected NMOS and pMOS transistors. Why it is called diode conected? Is it because its I-V characteristics like a diode connected bipolar one?I can not see a diode structure from the view of process.


Another question is about the diode in standard cmos process. I need a voltage drop diode in my circuit, I used a diode connected PMOS transistor but I connected the bulk(NWELL) to its drain, then a P+/Nwell diode is developed. I want to know what's the disavantages in using this diode in my design?

Thanks in advance!!!
 

Fisrt, I think for diode connect NMOS Vds has to go above Vth to open the transistor, which is like what we do to make a diode conducted. This is where the diode connected name comes from.

As for your second question, I think there will be no detrimental results. But why don't you use a diode?
 
laglead said:
Fisrt, I think for diode connect NMOS Vds has to go above Vth to open the transistor, which is like what we do to make a diode conducted. This is where the diode connected name comes from.

As for your second question, I think there will be no detrimental results. But why don't you use a diode?
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thanks for the reply. it's helpful to me. As for why not use a diode, because I heared that there is no diode in standard process. The availble diode is to use a CMOS transistor connected as I said before, i.e., connect the bulk to its gain. But I found a diode model in my spice model. But I am not sure how this diode is developed in fabrication, maybe, P+diffusion/nwell. I will replce with this diode in design.
 

At least, I know in UMC process, there are P+/N-well and N+/P-sub diodes.
For low noise concern, P+/N-well is preferred.
 

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    chang830

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