@Stefan Schippers I was trying to do a sweep of the width of

sky130_fd_pr__nfet_01v8

with the following spice code:

.title MOS Characteristic

******************************
* Include model files
******************************
.lib /home/nelson/cad/share/pdk/sky130A/libs.tech/ngspice/sky130.lib.spice tt


******************************
* Circuit netlist
******************************
Vg gate  0 dc 1v
Vd drain 0 dc 1v
xm0 drain gate 0 0 sky130_fd_pr__nfet_01v8 l=0.15 w=1
.end


******************************
* Control section
******************************
.control

* sweep transistor width and print dc operating point (op) drain current
let w_start = 1
let w_stop = 5
let delta_w = 1

* initialize loop variable
let w_test = w_start

* loop 
while w_test le w_stop
 alter @xm0[w] = w_test
 op
 print w_test @xm0[id]
 let w_test = w_test + delta_w
end
* end loop

.endc


******************************
* End of file
******************************
.end

but when I run it I get this error: It's not recognizing the model and I don't know why that message is appearing, because I run correctly another file where only do a dc sweep of the drain voltage (fig 2) Could you help me with that?