6v switching large load
A MOSFET may* be more suitable - Particularly if your available drive current is limited. The current required to drive the gate is almost zero.

http://www.onsemi.com/PowerSolutions/product.do?id=ATP202  Is an example of a device with suitable Vth and RdsON, it will happily switch 3A with little to no heat-sinking. Note it is only 30V maximum, generally suitable for switching &lt; 24V at the output.

*I would recommend adding a 'free-wheel diode' for inductive loads.