# LTSPICE P-Channel MOSFET

I'm learning LTSPICE and am very confused by the results coming from playing around with P-channel MOSFETs.

I created a very complete test of P-channel and N-channel MOSFETs, and all the N-channel results make sense to me, but can someone please explain what's going on with the P-channel?

M1 behaves as a forward biased diode with a voltage drop of about 0.105 V (seems small but believable).

M2, which according to my understanding of P-FETs should behave like a closed circuit, behaves virtually identically to M1.

M3 and M4 are also identical, and the results are:

M4, which is likely the most useful configuration, has a waveform that makes sense, but the numbers don't make any sense. Shouldn't this work as a switch, meaning that the voltage should go between roughly 0 and roughly 20, not 19.898 and 19.895? (R_ds,on = 0.035 ohms) And M3 is the same - is that okay?

Lool I was fighting with a more complicated simulation all day and finally decided to go back to the basics, and now I'm just more confused...

• The P=channel MOSFET are similar to PNP. Do you know how to use a PNP transistor? Only M2 and M4 are properelty connected. But in M4 you need to change the V2 voltage swing up to 20V. 5V is too small to Turn-OFF the P-MOS. In general P-MOS is a high side device. The source terminal must be connected to +Vsupply. And the be able to turn-on the P-MOS the gate terminal voltage must be lower than source voltage by a Vgs(th) voltage or more (0V). And to Turn_OFF the P-MOS the gate voltage must be equal to +Vspply or Vg > Vsupply - Vgs(th) And M2 and M3 conduct current due to the Body diode.
– G36
Dec 26, 2019 at 8:20
• Body diode electronics.stackexchange.com/questions/368171/… And datasheet aosmd.com/res/data_sheets/AON6407.pdf
– G36
Dec 26, 2019 at 8:20
• Thanks, I'm assuming in the last sentence you meant to say M1 and M3 conduct current due to the body diode.
– BenG
Dec 26, 2019 at 20:05

As it is, you are only testing the difference between the transistor being very strongly "on" ($$\V_{gs}=-20\ V\$$) and being just slightly less strongly on ($$\V_{gs}=-15\ V\$$).
To put the MOSFET into cut-off mode, you want $$\V_{gs} > -0.3\ V\$$ (based on the datasheet minimum $$\V_{gs}({\rm th})\$$).