A book I am reading about the invention of transistors -- Crystal Fire -- includes the following about hearing aids. As a preface, Alexander Graham Bell was the founder of the Bell Telephone Company, and AT&T was initially a subsidiary of Bell Telephone (link).
In keeping with Alexander Graham Bell's devotion to helping the deaf and hearing-impaired, AT&T and its Bell Labs, where the transistor was invented, extended royalty-free licenses to hearing aid manufacturers. In 1952 Sonotone began selling hearing aids in which one of the three vacuum tubes was replaced by a transistor. A few days later the Maico Company came out with a model with three transistors and no vacuum tubes. A few months later Acousticon came out with a model with only one transistor and no vacuum tubes.
The principal limitation on vacuum tube hearing aids was the expense and encumbrance of the batteries needed to power the amplifying unit, generally worn around the waist. With the progress of miniaturization, solid-state circuits eventually allowed making hearing aids that could be worn entirely within the ear (p. 205).
The military was by far the biggest customer for transistors in the early 1950's. Their military use was in radar and guided missiles. Hearing aid manufacturers were probably the second largest customers. They were soon surpassed by the makers of transistor radios, which were introduced in October, 1954 by Texas Instruments, partnered with another company (Regency TR-1 radio).
For a little more on the history of hearing aids, Wikipedia has an article about it. Of course, hearing aids have become smaller along with microprocessors becoming smaller in the last 60 or so years
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