# Impedance of an analog switch (mosfet)

#### Geektronic

Joined Sep 6, 2019
4
Hi, I am looking for a switch to be designed in 150 nm process technology node that can carry a current if up to 4A. Hence, I need it to be of very low impedance. What is the lowest achievable impedance at that process node for a mosfet based switch?

#### dl324

Joined Mar 30, 2015
9,154
What is the lowest achievable impedance at that process node for a mosfet based switch?
It can be as low as you want it to be but you have to trade off area for low resistance.

Joined Mar 10, 2018
3,627

#### WBahn

Joined Mar 31, 2012
24,854
Why are you needing/wanting to do this? That's going to be a pretty expensive switch.

What problem are you actually trying to solve? A completely different approach may be better suited.

#### MisterBill2

Joined Jan 23, 2018
4,055
WB is correct. With enough information about the application and whatever limitations and voltages may be involved this extended group can provide you with useful answers. Otherwise all that you get is a bunch of guesses that may or not relate to your application at all. And a whole lot of time would be wasted chasing in wrong directions.
Details such as the voltage, the switching frequency, the switching speed requirements, and even the load impedance, may all come in to play in the selection of a device.

#### eetech00

Joined Jun 8, 2013
1,726
Hi, I am looking for a switch to be designed in 150 nm process technology node that can carry a current if up to 4A. Hence, I need it to be of very low impedance. What is the lowest achievable impedance at that process node for a mosfet based switch?
As long as the device fits the other requirements, why does the process used matter?

eT

#### Geektronic

Joined Sep 6, 2019
4
Thanks everyone. Some details about the application- the switch needs to carry current of up to 4A (SMPS output) at a voltage of 0.8V typical. The voltage drop acceptable is maximum of 10%. This means the impedance of the switch can be 20m ohm. Is it practical to have such a switch at 150 nm technology process node?

Regards,
G

#### dl324

Joined Mar 30, 2015
9,154
The voltage drop acceptable is maximum of 10%. This means the impedance of the switch can be 20m ohm. Is it practical to have such a switch at 150 nm technology process node?
I don't see why it wouldn't be. To get low on resistance and handle a largish current, you need a larger device. The process data will tell you how many microns of width you need for the current you want. Satisfying the current/micron requirement may give you the on resistance you want. If not, the device needs to be even larger. If you have the area, there's no problem.

#### MrSoftware

Joined Oct 29, 2013
1,576
There are a lot of off the shelf MOSFETs with lower Rds than that (n-channel, I didn't check p-channel), so the only question is size required. Here are some examples of existing commercial products:

https://www.digikey.com/products/en/discrete-semiconductor-products/transistors-fets-mosfets-single/278?k=&pkeyword=&sv=0&pv606=1&umin2068=&umax2068=&rfu2068=V&umin2262=&umax2262=&rfu2262=pC&umin2263=&umax2263=&rfu2263=pF&sf=0&FV=ffe00116&quantity=&ColumnSort=612&page=1&stock=1&nstock=1&pageSize=500

#### crutschow

Joined Mar 14, 2008
23,533
Suggest you try to find the layout of presently design power ICs with on-chip MOSFETs to see how they do it.

#### tindel

Joined Sep 16, 2012
671
As long as the device fits the other requirements, why does the process used matter?

eT
This reminds me of Jim Williams zoo circuit. The client was insistent that it used CMOS technology because "the low power requirement is nonnegotiable". Jim designed it with nearly all bipolar components (a single cmos inverter being the exception) and still met the requirements. I love his recollection - "I'm fairly certain the customer wouldn't mind if I had used 12AX7's as long as it met specifications. It runs well in production, and they make lots of them, which makes my boss and the stockholders happy."

Who cares what process you use if you meet the design requirements!