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Design >> Analog Design >> how to implement a resistor of value around 450M ohms on chip
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Message started by jovial on May 30^th, 2014, 10:49pm

Title: how to implement a resistor of value around 450M ohms on chip
Post by jovial on May 30^th, 2014, 10:49pm

HI,
I want to how to implement a resistor of value around 450M ohms on chip. I am working on UMC 180nm technology.
Can anyone suggest me a simple MOS based circuit for the same?

Title: Re: how to implement a resistor of value around 450M ohms on chip
Post by loose-electron on May 31^st, 2014, 2:49pm

Simply put - redesign your circuit or method

Anything that uses a resistance value that large will not be reliable.

Title: Re: how to implement a resistor of value around 450M ohms on chip
Post by cchen on Jun 4^th, 2014, 1:23am

If available in your design, trim by digital code may be a choice.

Title: Re: how to implement a resistor of value around 450M ohms on chip
Post by sheldon on Jun 5^th, 2014, 3:25pm

CChen,

Title: Re: how to implement a resistor of value around 450M ohms on chip
Post by sheldon on Jun 5^th, 2014, 3:41pm

Jovial,

A value of 450 MegaOhms is a very large value to try to integrate.
Typical diffused and poly silicon resistors have relatively low sheet
resistances, which implies long, thin resistors. Not sure what value
of sheet resistance the process has. However, even assuming a very
high sheet resistance of 10kOhm/sq, means that the resistor would
require 45,000 squares to realize. In addition, to the resistor, you
will probably need guard rings around the resistor to prevent
leakage further increasing the area. BTW, the 10kOhm/sq assumes
the process has a pinched resistor which means that there is a very
limited voltage range of operation for the resistor. More practical
diffused and poly resistors have sheet resistances on the order of
~100 Ohms/sq. Which implies ~1M squares of resistance. Loose
Electron is correct, a design requiring a 450MOhm resistor implies
that the design is not appropriate for integration.

Best Regards,

Sheldon

Title: Re: how to implement a resistor of value around 450M ohms on chip
Post by rf-design on Jun 6^th, 2014, 1:32am

jovial wrote on May 30^th, 2014, 10:49pm:

HI,
I want to how to implement a resistor of value around 450M ohms on chip. I am working on UMC 180nm technology.
Can anyone suggest me a simple MOS based circuit for the same?

I suggest to change the implementation of the functional element which need the 450M resistor.

The current from a 0.18u max supply of 1.8V on this 450M resistor is 4nA. So this is not far away from leakage currents of the active devices. So the total accuracy comparing leakage and signal is not high. Furtheron the a 450M with 2k/sq high ohmic poly need 225000sq so that the parasitic cap of this resistor could create stability issues if used in an amplifier feedback configuration.

An alternative for the function "low current over signal voltage" could be using a much smaller resistor and an active current division. So the total function is a transconductive element with output stage current scaling.

Title: Re: how to implement a resistor of value around 450M ohms on chip
Post by loose-electron on Jun 6^th, 2014, 6:17pm

What is it that you are trying to do that requires such a device?
I am sure that there is a better way or method.