What kind of MOSFET use as a switch touch on, touch- off
Here's a very low voltage on-off latch circuit: -



Inverter U1 and U2 are wired in a circle and that forms the basic latch. When the on/off button is pressed, the voltage accumulated on C1 will be the inverse of the voltage on the input of U1 therefore the switch reverses the input by overriding the positive feedback from U2's output.

If I were building this circuit I'd probably replace Q1 with a FET for operating on supplies from 2.7V to 6V because the FET would likely turn on more powerfully and it can take a gate drive across the range of power voltages you need. The BJT is OK but may get a little warm Running below 2V will probably need a BJT because it will switch on at lower supply voltages better than a FET.

If you use a FET, look for one that has a low Vgs(threshold) voltage to ensure it works down at 2.7V. Also check the inverters (U1 and U2) to make sure they can run from 6V. If (say) 5.5V is the max limit for these parts you might want to power the inverters using a LDO low power regulator or resistor and zener diode.

Should you need a switching element in the positive rail to the motor then a suitable P channel FET should be looked for, again with low Vgs(threshold).

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