HI, have you checked out the data sheet of your transistor? or it maight be heatsinked? u need make your own H-bridge? If this is not a reqiurement, y dont you just try this L293D, it can produce you up to 36V, 1A, due to the power disipation, a simple iron or zinc plate stamp together with the IC to enlarge the area for heat disposal. And u can use your Atmel controller to control the enable pin at the IC, u can make it bidirectional. One IC can control two bidirectional motors.