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. This made them portable for the hard of hearing.

Unlike the carbon instruments, these had adequate power for severe hearing losses but were also usable by persons with a lesser loss. The first one appeared in 1921, but this type did not become practical until the early 1930's, and did not appear in a wearable version until 1936. Vacuum tube aids required two batteries, so costs were rather high.
Also, it should be noted that batteries were large and cumbersome.

These are based on the telephone principle; however, Alexander Graham Bell had nothing to do directly with their development. These appeared first in limited quantities in a table model about 1899, but in wearable and practical instruments beginning only in 1902.Carbon aids were popular through the 1940's. Most of these used a rather large 3-volt or 6-volt battery but did not have enough power to assist those with more than a moderate hearing loss.

Sources:
Henry Smith, founder of America Hears http://www.americahears.com/
a leading manufacturer and distributor  of hearing aids.

Hearing Centre Online http://www.hearingcenteronline.com/museum.shtml

Philippe Pango, Vitasound Inc. http://www.vitasound.com/