This pair is my favorite when driving small solenoids or motors. SOT-32 12 Watts..
BD139 – NPN, 80V and 1.5A, 40 beta min.
BD140 – PNP, 80V and 1.5A, 40 beta min.
This means any small solenoid, relay or DC motor can be controlled with this. I would use it upto 24V DC and max of 1A for applications of full on and full off.
In applications like a regulator or variable motor control, you have to keep in mind the current thru and voltage across the transistor which results in power dissipation. Let us say it is driving 0.5A thru a motor in speed control and the Voltage across is 30V then the dissipation could be 30 x 0.5 = 15 Watts.
That is the reason we use switched regulators or switched controllers. So if we chop the DC and do PWM you have only the Switching losses and also a bit of DC loss. Switched controllers are also green, they save power for the same work done, but generate EMI-RFI and care is required in sensitive uV circuits. The transistors for switching are also special as they have to be fast with less capacitance, hence the mosfets are preferred in those applications. – ananth (san)