Is it possible to use a resistor 1.8M ohms in TSMC 180nm?

[anhnha]

Hi.

I am wondering if it is possible to use a resistor 1.8M ohms in TSMC 180nm.
Is it too big?
Thanks.

 

[Dominik Przyborowski]

I'm not familiar with TSMC 0.18 but if You have hi-resistivity polysilicon or well resistors available with sheet resistance around 1-2k\[\Omega\]/square it still will be possible, but!

1. Assuming minimum width of resistor (e.g. 0.3um) and 2k\[\Omega\]/square You will need resistor with W/L=0.3um/270um with probably very bad matching or 1um/900um with better expected parameters.
2. Assuming 0.5fF/um^2 of parasitic capacitance to subtrate You get approximately 75fF for 0.3/270 and as high as 450fF of parasitic cap.

So the question is why do You need 1.8M\[\Omega\] resistor and why don't use a proper biased transistor?

 

[anhnha]

Thank you. I need the resistor for biasing circuit for my LDO.
Could you tell me how to make resistor from transistor?

 

[Dominik Przyborowski]

I assume that You want used this large resistor to set current in current mirror?
If yes, the better choice is to design some kind of widlar source or other biasing circuit which is immune on supply changes.