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I am new at studying microelectronics and I really want to understand why the current in the drain goes to negative (see below). When I am decreasing the voltage to 0.1 V the transistor acts like high pass filter like on the left of the schematic (\$R_1\$, \$C_2\$, \$V_1\$) but when I increase the voltage the current drops to negative. enter image description here

enter image description here

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  • \$\begingroup\$ 1) I would like you to explain why you think that the current should not go negative. 2) does the current go negative in the circuit with C2 and R1? \$\endgroup\$
    – Bimpelrekkie
    Commented Feb 22, 2018 at 10:30
  • \$\begingroup\$ Please explain it to me. I really want to understand. I think because it acts like a forward bias diode, but I am not sure. When I replace the nmos with a resistor I get exactly the same picture as in the left part in the schematic. My semiconductor was acting as a high-pass filter for voltages 0 and 0.1 but when I increase the voltage to 1 V the current goes to negative. Thank you in advance. \$\endgroup\$
    – Vahram Voskerchyan
    Commented Feb 22, 2018 at 10:40

1 Answer 1

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Regular N channel MOSFETs have what is called a body diode inside them and this will cause the MOSFET to conduct when the drain voltage starts to go negative relative to the source: -

enter image description here

Internally the "body" is connected to the source in most MOSFETs so this means that an N channel MOSFET will conduct negative currents into the drain: -

![enter image description here

Picture taken and modified from here. If you used two back-to back MOSFETs you can make a kind of analogue switch (aka solid state relay) and this might work for you: -

enter image description here

Picture taken from here

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  • \$\begingroup\$ The body diode isn't needed to explain negative drain current in OP's circuit. They have the gate biased above the source (presumably above threshold), and they're applying an AC voltage to the drain. \$\endgroup\$
    – The Photon
    Commented Feb 22, 2018 at 16:52
  • \$\begingroup\$ @ThePhoton I assumed the OP was refering to the negative spike of current on one of his pictures. \$\endgroup\$
    – Andy aka
    Commented Feb 22, 2018 at 16:54
  • \$\begingroup\$ yes, I was referring to negative spike of the current. \$\endgroup\$
    – Vahram Voskerchyan
    Commented Feb 26, 2018 at 13:26
  • \$\begingroup\$ @VahramVoskerchyan that's good. Do you understand my answer then? \$\endgroup\$
    – Andy aka
    Commented Feb 26, 2018 at 13:29

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