Due to forward biasing during positive half cycle of input AC voltage, the resistance of junction is low. The current in the circuit is maximum. In this situation, a maximum potential difference will appear across resistance connected in series of circuit. This result into zero output voltage across junction. Similarly during negative half cycle of AC voltage, the junction is reverse biased. The resistance of junction becomes high which will be more than resistance in series. That is why, there will be voltage across junction with negative cycle in output.