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MOS Bulk connection in analog and RF design

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borodenkov

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nmos bulk connection

There are 2 options for bulk connection in the MOS transisor:

1. connect it to Vss for NMOS and Vdd for PMOS
2. always connect it to the source of the transistor

What I've seen, in analog (low-freq) circuits, bulk is usually connected to the ground (or Vdd), but in RF circuits - to the source (in TSMC 0.18u RF transistor even does not have a bulk terminal).

Can someone clarify when which option is reasonable?
 

pmos bulk connection

For PMOS its ok to connect the substrate to the source but I never seen the substrate of NMOS connected to its source, unless its N-type process.
 

pmos bulk

I have checked TSMC's documentation: RF transistor has 3 terminals, bulk is always connected to source. Does it mean that the bulk of such NMOS transistor is layed out in a seperate p-Well?? Or using guard ring around it NMOS transistor will prevent substrate coupling?
 

mos bulk

Which TSMC process? 0.18?
 

mos bulk connection

TSMC0.18rf is a twin-well process. All the RF MOSFETs sit on their own wells for the sake of good isolation.
 

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    borodenkov

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bulk nmos connection

ok, that clarifies the question
 

what is cbs and cbd with anlog ic design

I am still in the black.
I am using TSMC 0.18um, PDK 1.3B. It provides RF transistors with 4 terminals (G, D, S, P).
Should I connect Bulk of RF transistor to GND (NMOS) or to Vdd (PMOS) (Vbs ≠0)?

Best regards
 

nmos bulk and source

Though question.. if the process allows it I would use the active well (e.g. well connected to any potential) everytime I need speed, as you minimize the Vth shift and you can eventually get rid of Cbs and Cbd (think about connecting the well to a common mode voltage)..

Hope this helps..

LucaS.
 

layout bulk connection

acutallly it depends on the process you can use.

if it just have one well, you will not have more choices. otherwise, It will depends on the circuit you want to use.
 

transistor mos bulk

Hello,

**broken link removed**

You can find there a lot of interesting things.
I recommand you:
Substrate Noise Coupling Analysis in Mixed-Signal ICs
Parasitic Coupling in RF Circuits: A Real-Life Experience
Do We Have RF Performance Degradation Due to Substrate Noise?
Substrate Effects in Smart-Power ICs

Read the first one, and you will find a lot of answers.

Enjoy!
 

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