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Design >> RF Design >> diode in CMOS
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Message started by aaron_do on Sep 27^th, 2007, 7:05am

Title: diode in CMOS
Post by aaron_do on Sep 27^th, 2007, 7:05am

Hi all,

If i want to make a diode, i can use a MOS transistor and connect the gate to the drain. However, the diode turn on voltage will equal Vth. Is there a way to have a diode with a turn on voltage closer to zero in CMOS? thanks,
Aaron

Title: Re: diode in CMOS
Post by Stefan on Sep 27^th, 2007, 7:10am

In some CMOS processes there are special low_Vth Transistors (and therefore also diodes) available.
As far as I remember they maybe go down to 0.18 V in CMOS processes.

How low do you need to go ?

Title: Re: diode in CMOS
Post by ACWWong on Sep 27^th, 2007, 8:15am

yes using a low vt device if available...... and if your process has a Schottky Diode option thats even more cool.... (both though at the cost of the additional masks)

and of course you can size/bias a normal vth device to tweak its vth within reason (like utilsing short-channel effects or forward body biasing etc).

Title: Re: diode in CMOS
Post by aaron_do on Sep 27^th, 2007, 9:29am

thanks,

how about VPNP? I need it as close to zero as possible really. I'm trying to rectify an RF signal, but the amplitude is only around 500 mV. I'm not trying to detect the power so loss isn't the most important issue...

Aaron

Title: Re: diode in CMOS
Post by nxing on Sep 27^th, 2007, 9:53am

In some process, for example, TSMC. they provide native device which has the 0v Vth with no additional cost.
However, when you use MOS's diode connection, the I-V relationship is not as same as the real diode. for example, for a long-channel device, I=a*V^2 while in diode, I=e^(-v)....

Title: Re: diode in CMOS
Post by loose-electron on Sep 27^th, 2007, 12:45pm

think outside the box ---

Use a comparator and a switch.

I have made those work up to 200-300 MHz

also, a lot of the diode models on foundry are not accurate at high frequencies.

Jerry

Title: Re: diode in CMOS
Post by aaron_do on Sep 27^th, 2007, 8:48pm

Thanks for the advice.

Actually it would need to work up to 3 GHz...also, the comparator would need a lot of power. My idea was meant to save power :D

Anyway after thinking about it i realized the idea probably wouldn't work even if i had the diode so nvm

thanks,
Aaron