I know LVS can combine multiple extracted transistor into one with multiple fingers, but can it also combine multiple transistors one after the other into one of longer length ?

@tnt device reduction in netgen is handled in the

sky130A_setup.tcl

file. Currently there are no commands for series reduction of mosfets. Also the netgen page here says

Note that for devices that connect in series, the terminals must be the first two pins of the device; any additional pins (e.g., substrate connection) must be connected to the same node on all instances of that device.

Since the first two terminals of mosfets are drain and gate, series reduction may not be possible with netgen. @Tim Edwards is that analysis correct?

Yes. Two transistors in series is not really the same as one transistor of twice the length, which is why that case is not normally handled as a series combination. But the setup file could be revised to make it work that way.

@Tim Edwards thanks for the response. Considering that the netgen documentation that says series reduction uses the first 2 terminals of the device (drain and gate in the case of mosfets), how is it possible to reduce series mosfet (source and drain) with just changes to the setup file?

After thinking about it, I think it's not necessarily a good idea. As @Tim Edwards pointed thought they will behave and simulate differently (as opposed to the

nf

which is literally what's expressed with several in parallel), so better express the intent in the schematic explicitly, it's probably rare enough that it doesn't matter.