Dear Manoj,

Welcome!

Adding to Aparna, any transistor in active mode of operation acts as a current source at the output which is equal to  Io= gm Vi,where gm is the transconductance and Vi is the input signal.

At the input the transistor is equivalent to an input impedance Zi which is normally a parallel combination of an input capacitance Ci and Resistance ri.

Then the input current Ii = Vi/Zi. It follows that the current gain of the transistor Gi = Io/ii = gm Zi, For the gain magnitude mag Gi= gm xmagnitude of Zi.

Since Zi is a parallel combination of a resistor and a capacitor the magnitude of iys impedance  decreases with the increase in frequency leading to the decrease of the current gain of the transistor with frequency.

Every transistor is characterized by a figure of merit called the gain band width product Gx BW = gm/ 2 pi C, where C is total parasitic capacitance of the transistor.

For bipolar transistors C= Cbe + Cbc,

and for MOS transistor C= Cgs+ Cgd,

For more information please see the the course in the link: https://www.researchgate.net/publication/256346392_The_bipolar_transistors_theory_and_basic_applications

wish you success

Book The bipolar transistors, theory and basic applications

Aparna Murthy  and Abdelhalim Zekry both professors shed, light on your query.

What I assumed that during high frequency the dielectric media of parasitic capacitors (in MOS transistor C= Cgs+ Cgd) loose its polarization. Hence offers low impedance with decreasing in gain 

the performance of transistors degraded at high frequency due to internal capacitance. there are three main capacitance inside an FET

the oxide and gate capacitance

the body of the transistor and drain

the body (bulk) of the transistor and source