How to determine the L of a design

[devop]

hi
most of the books tell us how to determine the W/L,How can I determine the L?what I should consider more about?For example: for a 0.35um process ,will the length of most of MOSFET be 0.35 for analog device?
thanks in advance

 

[aryajur]

L is determined considering factors like Transistor matching, output impedance needed, capacitance of the transistor.

 

[devop]

thanks aryajur
excluding the special factors ,such as high impedance result in a large L.......
for most of the MOSFET,is there a experiential rule?

 

[aryajur]

If you want good matching current mirrors, the L has to be large, if you want Vgs to match then make L small. For non matching applications make L at least 10 times the ΔL for the technology.

 

[devop]

thanks for your efficiently reply
but it gaves me an other question :
if you want Vgs to match then make L small,why?

 

[mists]

then what is mean of ΔL for the technology?

 

[aryajur]

For Vgs to match basically W/L should be large. This can be seen if you differentiate the current equation assuming current constant but Vgs and Vth differentiable variables. See the Appendix of Art of Analog Layout or Gray and Meyer.
ΔL for a technology is the difference in the drawn length and the actual length in silicon.

 

[Guest]

Hi
In fact, while techmology trends let us to have smaller channel length for transistors, this advance is not very nice for analog design since supply voltage must be reduced (for a good reliability); so analog circuits often designed no with minimum allowable channel length (i.e. 0.18u for 0.18um CMOS process) to avoid short channel effects!
but since for a specific amout of current output resistance is very dependent on bias Vds and L of transistor, so to have higher DC gain for high speed application (particularly in low-voltage applications) L must be large and to have a specified W/L ratio, parasitic caps. also is increased.
Here CADs are introduced to optimum design, but many years experience can help (analog) designers.
Hope to be useful, if you want please click on "Help me" button; it doesn't cost you!

Regards,
SAZ

 

[fazal_ece]

L is detemined by considering systematic errors and random errors also....

 

[eeliuliu]

use small L when you want to reduce the flick-noise