I think that you just talking about the 2N7000, that is a small current N channel MOSFET.
Circuit a:
1. D - in series a limiting resister and sink from your load.
2. S - connecting to GND.
3. G - in series a resistor to the input signal or connecting to the +V.
Circuit b:
1. D - connecting to +V.
2. S - in series a resistor and connecting to the Load.
3. G - in series a resistor and connecting to the +V.