Question

Why reverse bias is measure in microampere but forward bias in milliampere?

Answer 1

When forward bias is applied across the pn junction diode, it lowers the potential barrier and significant amount of current follows through the junction, however when we applied a reverse bias across the junction, width of the potential barrier increases due to which a very small amount of current follows through the junction.

Hence measured current in forward bias is large than the reversed bias.
Therefore forward based current is measured in mA while reverse bias current is measured in $\mu$A.