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L squared pi mos transistor 2sk941

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Jan 17, 2013
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Hi. I am building a circuit from a schematic. The circuits purpose is to provide isolated communication between USARTS. I don't think the CPU should supply the current for the LED on the optocoupler. I have a reference schematic that calls for a 2sk941 to drive the LED. the gate connects to the microprocessor, and the collector connects to the LED cathode.

My issue is this. What the heck is a 2sk941? the datasheet calls it a L²-Π-mos. the datasheet is mostly in mandarin. I know what a Π-mos is. its a 3d fet whose particular geometry gives it exceptional short channel effects and enhanced current driving capability. at least, that is what it says in this article about it

Multiple-gate SOI MOSFETs
Jean-Pierre Colinge *
Department of Electrical and Computer Engineering, University of California, Davis, CA 95616, USA
The review of this paper was arranged by Prof. S. Cristoloveanu

what is the L² part about then? the schematic symbol shows like a bidirectional breakdown diode or something between the gate and source. what is this transistor? What are its special properties? can I just substitute it with a 2N2222A and forget about it?

Most microcontrollers should be able to drive an optocoupler with an external transistor but with only a limited current. I would suggest replacing the 2sk941 with any general purpose MOSFET such as 2N7002. You could use the 2N2222 but with any bipolar you need to add a series base resistor.


will that work if my microprocessor outputs 3.3V? the threshold for the 2n7002 is 2.5V. only having a difference of .8V seems marginal.

2.5V threshold worst case. So, even in the worst case you will be good for >100mA at 3.3V gate voltage. If you are concerned, simply search for other MOSFET - there are plenty of low threshold logic devices out there.


... can I just substitute it with a 2N2222A and forget about it?
Yes, together with a base series resistor (say 10..100kΩ). Or practically any N-channel MOSFET.

And don't forget to limit the LED current by an appropriate series resistor (because the 2N2222A or MOSFET could supply too much current)!
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