When the polarity of the external voltage source is opposite to the above case i.e. when positive terminal is connected to n-side and negative terminal to p-side, then diode is said to be reverse biased as shown below:
When we reverse bias the diode, the majority carriers have again 3 types of forces acting on them as shown below but in this case both forces due to barrier electric field and the external voltage act in the same direction and hence pull the majority carriers away from the junction.
Hence the space charge region or say depletion region is enlarged and minority carriers are attracted to move to the other region crossing the barrier as shown:
But as number of minority carriers is very less and current is proportional to the charge carriers flowing across the barrier, hence current through the diode is less and measured in micro-amperes