Hi everyone. I'm trying to use some of the skywater high voltage transistors in a design and am seeing huge Vds voltage drops in my design across these. Are the threshold voltage/resistances of these in saturation really that substantial? I measured the threshold voltage to be closer to 700-800mV in a separate test using the linear region extrapolation method, so does this discrepancy just come from short channel effects? Here is the transistor in series with a 1 mega ohm resistor, and I still see 1.5V drop across it. I just want to make sure I am not missing something and that this result is expected. Thanks.

.lib "hspice/sky130.lib.spice" tt

Vvdd vdd 0 5V
Xtx vdd vdd A 0 sky130_fd_pr__nfet_g5v0d10v5 W=0.75 L=0.5
R1 A 0 1000k

.OPTION POST=2
.tran 0.01ns 10ns
.PROBE V(*)
.end

1 Meg resistor and 3.5 V. 3.5 uA. There is a noticeable body effect there also, as there is a VBS = -3.5 V. This must have increased the transistor VT by almost 1 V, I guess. A better testbench would be to replace the resistor with a current source. Maybe short the gate and drain to node A and short bulk and source terminals.

Thanks Luis. Using the topology you suggested I see a 960mV drop over the transistor which is more in line with what I was expecting. The high body effect makes sense as an explanation. My application uses the transistors as bidirectional pass transistors, with the nmos coupling a high voltage to a relatively high resistance load, so maybe I will have to use a full transmission gate (I was trying to save on area).