I am using a P-channel MOSFET as a switch in my design & I have connected the drain to the load. The load needs 0.8mA of current. How can I decide whether the MOSFET is conducting that much current if I connect the source of the MOSFET to 3V ? The MOSFET used is BSS84 from Fairchild & the link for BSS84 datasheet is given below.
I think this is the graph of your datasheet that gives all the information you need:
So assuming this circuit
V_GS is -3V which allows I_D to be up to ca. 0.5 A.
clabacchio is right pointing out that the graph is actually for V_DS = 5V.
Of course a graph for V_DS = 3V would be better, but there isn't one in the datasheet. So the question is: Will the resistance R_DSON be different for very low drain currents?
Looking at Figure 2 ("On-Resistance Variation with Drain Current and Gate Voltage") gives the answer:
it does change vs. I_DS, but only for currents above ca. 0.1 A and: resistance even gets lower (=better) for lower I_DS.