Solid state relay (or equivalent/alternative) to drive 6V DC from a 3.3V micro
I have used a Photovoltaic relay for this in an Arduino driven application, the IR PVT312.

It is designed to work as a substitute for a Relay, and is rated to switch up to 250V at 190mA. It is suitable for AC and DC switching.

As you can see from the diagrams below, it's an opto-isolated MOSFET switch.