I've been trying to get into some low level electronics to explore to "Internet of Things" and still struggling a little with MOSFETS.

I'm trying to make a single push button act as a latching power switch for a microcontroller, then act as an input button when the mc has powered up (further details in my previous question).

Following the suggestion to use a BSS84, I discovered the mircocontroller I'm using (ESP8266 ESP-12) draws a current of around 300mA and, looking at the datasheet for the BSS84, I saw the continuous current draw is -0.13A?

This is where I get confused a little. I understand the continuous draw needs to be adequate (130mA is too low) but why is it +/-?

Does the current not flow from Drain to Source? If so how can there be +/- voltages? (I realise I may have a very poor understanding of FETS)


I'm aiming to use a logic level mosfet to provide at least 300mA when the gate is connected to ground. Would something like the IRLML5103TRPBF work or are there better options?

Thanks in advance for your help. Apologies if I've got entirely the wrong idea and sounds like a crazy person ha!


3 Answers 3


-0.13A is the continuous drain current, Id.

Whenever someone says "something current", they mean the current into that thing. You hear this frequently with transistors: "drain current", "collector current", "base current", and so on.

With N-channel FETs or NPN BJTs, the current usually goes in the drain/collector, so the current is positive.

With P-channel FETs or PNP BJTs, the current usually goes out the drain/collector, so the current is negative.


simulate this circuit – Schematic created using CircuitLab


The BSS84 is a P-channel transistor, in which the current flows from source to drain. So the indicated current carries a minus sign because it is measured relative to a drain-to-source convention.

For more information, see : In an NMOS, does current flow from source to drain or vice-versa? The answers are well detailed.

  • \$\begingroup\$ Taking current in to a pin as positive is not a special convention for MOSFETs, it's the passive sign convention, which is used for all kinds of devices. \$\endgroup\$
    – The Photon
    Commented Mar 21, 2016 at 16:15

A P-Channel MOSFET used as a switch is normally connected such that conventional current flow would be from source to drain when it is switched on (not drain to source), due to the fact that the MOSFET's internal diode would always conduct from drain to source.

The Drain Current spec (ID) in MOSFET datasheets is usually given as positive for current flowing from drain to source.

So for P-Channel MOSFETs you'll usually see ID given as a (-) negative value to indicate that the current is flowing in the opposite direction.

If you compare this to the datasheet for an N-Channel MOSFET, you'll see that the ID value there is (+) positive.


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