The idealized BJT characteristic it's represented as:

enter image description here

Meaning that the transistor is able to conduct current from + to - when is turned on, and when is off it will block positive voltages. I found the same images in both books, Fundamentals of Power Electronics by Robert Erickson, and in Power Electronics by Need Mohan.

In the case of Mosfet transistors, Mohan's book shows this characteristic:

enter image description here

Same behavior as the BJT.

But in the book of Robert Erickson this image is shown:

enter image description here

Meaning that Mosfet is able to conduct current positive and negative currents. Drain to source and Source to Drain. Why this discrepancy in the behavior of the Mosfet?

And also why is not shown the behavior for negative voltages? Can't those transistors allow negative voltages when are off?

  • \$\begingroup\$ As for "Why this discrepancy" are you asking for how the physics work differently in the FET case, allowing for current in both directions (as opposed to PN diode like behaviour in the BJT case)? \$\endgroup\$
    – PlasmaHH
    May 25, 2016 at 19:47
  • \$\begingroup\$ Something is not right here. Reverse current usually damage bjt's and mosfets, but not jfets. Jfets conduct a small current even if gate is 'off '. Are these the only drawings of this type? \$\endgroup\$
    – user105652
    May 25, 2016 at 19:54
  • \$\begingroup\$ @PlasmaHH I mean between the books. \$\endgroup\$ May 25, 2016 at 19:59

2 Answers 2


The Power mosfet can block the voltage only in one direction. In the other direction, Source to Drain, it is a diode (as the center portion of the simbol suggests) if the gate is OFF. If the Gate is ON it conduces with the same resistance then the Drain to Source case. Considering this case Erickson's ideal representation is correct.


A mosfet conducts equally well in both directions so when a mosfet is activated it will have similar on-resistances in both directions. Typically, the solid state relay: -

enter image description here

When activated there is a reverse connected and a conventionally connected mosfet in series with the AC load. It's not the case that the parasitic diode inside the reverse connected mosfet is conducting and producing a volt drop of 0.7 volts - no, both MOSFETs are conducting fully when on and producing a low volt drop.


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