Internal Capacitances of the Device
Transistors have internal capacitances that affect their performance at high frequencies. These capacitances include the input capacitance (Cin), output capacitance (Cout), and transfer capacitance (Cμ). These capacitances are inherent in the construction and design of the transistor and cannot be eliminated.

Effect of Internal Capacitances
At high frequencies, the internal capacitances of the transistor start to have a significant impact on the amplifier's performance. These capacitances act as low-pass filters, allowing low-frequency signals to pass through while attenuating high-frequency signals. As a result, the gain of the amplifier decreases at high frequencies.

Input Capacitance (Cin)
The input capacitance of the transistor, Cin, is the capacitance between the input terminal (base) and the common terminal (emitter). This capacitance forms a low-pass filter with the input resistance (Rin) of the amplifier, reducing the gain at high frequencies. As the frequency increases, the reactance of the input capacitance decreases, causing a decrease in gain.

Output Capacitance (Cout)
The output capacitance of the transistor, Cout, is the capacitance between the output terminal (collector) and the common terminal (emitter). This capacitance forms a low-pass filter with the output resistance (Rout) of the amplifier, reducing the gain at high frequencies. As the frequency increases, the reactance of the output capacitance decreases, causing a decrease in gain.

Transfer Capacitance (Cμ)
The transfer capacitance of the transistor, Cμ, is the capacitance between the input terminal (base) and the output terminal (collector). This capacitance affects the coupling between the input and output of the amplifier. At high frequencies, the reactance of the transfer capacitance decreases, causing a decrease in gain.

Conclusion
In conclusion, the gain of a transistor amplifier falls at high frequencies due to the internal capacitances of the device. These capacitances, including the input capacitance, output capacitance, and transfer capacitance, act as low-pass filters, attenuating high-frequency signals and reducing the overall gain of the amplifier. It is important to consider and minimize the effects of these capacitances in transistor amplifier design to achieve desired performance at high frequencies.