The small currents in reverse biased condition of p-n diode are due to

In a p-n junction, when p-type is connected to negative terminal of battery and n-type is connected to positive terminal the current is reverse biased. A reverse bias strengthens the potential barrier and therefore obstructs the flow of charge carriers. Due to thermal energy a small number of minority charge Carriers produced in the depletion region and there will be a very low current flow.