Question

what is H parameters nd why do we need them​

Answer 1

H-parameters are one system for characterizing bipolar transistors.That means it is the ratio of output to input in the common emitter configuration, which in turn means it is the ratio of collector current to base current, which is basically the gain of a bipolar transistor.
The h-parameters of a transistor will give you a good idea what it can do, how touse it effectively in a circuit, and whether it is appropriate for a particular circuit. In practise, only a few h-parameters are commonly used. The most common one if hfe, which stands for h-forward-emitter.