In a half wave rectifier only a single diode is present. One end of the secondary wire of the transistor is connected to the p side of diode while the other to the load resistor. The n side is connected to the load resistor. When the diode is reverse biased no current passes through it. But current does pass through the other wire of the secondary which eventually meets the load transistor. So wouldn't the output in this case be an ac current?