Full Member level 5
Where does the name diode-connected MOS come from?
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!!!
5th April 2006, 10:28
Full Member level 1
diode connected nmos
Diode connected mos always remains in saturation once it is powered but Vds is not equal to Vt, actually Vds depends on Id flowing thru it and Id depends on Vds thus we get a complex equation to solve for and another point here is that Vt also depends on Vs which is unknow.
Originally Posted by chang830
5th April 2006, 16:05
Advanced Member level 2
About P+/NWELL diode.
1. Actually it is not diode it is pnp transistor to substrate, emitter current will differ from base current. You should remember that.
2. The current to substrate can flow both trough substrate contact or trough any reverse biased junction and thus can cause unexpected behavior of your circuit.
6th April 2006, 04:32
Member level 1
It seems to me that a diode connected device has gets its name for two reasons:
1. It is carry over from the bjt world where there is actually a diode resulting from the similar connection (shorting base to collector).
2. It only conducts in only one direction. In many cases a MOSFET is a symetrical thing with the source and drain being interchangeble (ignoring body bias effects). When we diode connect a device, it can only conduct in one direction.
Just my interpretation...
This being said, the IV characteristics are as jiveshgovil describes.