Transistor
These also can be properly called electronic hearing aids. They appeared in a few models in late 1952 and virtually replaced vacuum tube hearing aids by the end of 1953. Transistors need only one battery. Therefore, the reduced size permitted development of a number of types of hearing aids.
1.) CIC (Completely-in-canal)
2.) ITC (In-the-canal)
3.) ITE (In-the-ear)
4.) BTE (Behind-the-ear)