Need help in an Ultrasonic circuit

Salam,

I intended to make an Ultrasonic based communication but there is something that i need help with.

The design is using SL100 transistor it is used for "input matching"
The datasheet is here
http://www.ee.iitb.ac.in/uma/~wel/da...iles/sl100.pdf

I want to know what is "input matching" and i want to know alternatives for such a transistor because it is no more available in market.

Thanks for your trouble.