The bipolar transistor is a current operated device. To drive a motor, you should supply 1/10 of the motor current to the base, or if the transistor is a darlington, 1/100 of the motor current. The mosfet is a voltage operated device. You should get the data sheet to see how much voltage is required to turn it on, but typically 10 volts will do it. Most mosfets are enhancement mode, that is, it takes a positive voltage to turn on an N type. There are a few depletion mode mosfets available, these are normally on and require a negative voltage to turn an N type off.