TRANSISTOR MUSEUM™ Historic Transistor Photo Gallery
Transistor Size (5/16”L X 3/16”W X 1/2”H) Type 21-A on left, Type 18-B on right. Bottom view of two type 21-A transistors. Date code 320 (1953, week 20).	NATIONAL UNION TYPE T21-A TYPE Germanium PNP Alloy Junction Transistor USAGE Low Frequency Audio – Hearing Aids   DATE INTRODUCED 1953 CASE STYLES Green Epoxy AVAILABILITY Very Rare (Limited Production)
HISTORIC NOTES The manufacture of both point contact and alloy junction transistors began at the National Union Radio Corp, Semiconductor Division at the Hatboro Pa. facility in 1952.  This is very early in the timeframe of transistor development, and establishes National Union as a pioneer in the history of semiconductor technology.  Both point contact and alloy junction transistors were manufactured, with the alloy junction type modeled after the RCA PNP developmental alloy junction transistors, such as the TA-153.  See the Transistor Museum™ PhotoGallery description of the NU point contact type T18-B  - note in the photo above (top image) a comparison of the case styles of the two different NU types.  The T21-A appears to be slightly more uneven, with a hand-made appearance.  Clearly different molds were used for these two types.   The lower photo shows the bottom view of two T21-As, and the crude construction technology is quite apparent.  The T21-A transistors were sold primarily to hearing aid companies, such as Sonotone.  Production of NU transistors ceased in 1954, and not many units were sold commercially.  The 1953/54 edition of the Radio Master catalog lists the T21A for a staggering $61.25. The April 1954 NU price list omits the T21A, and instead lists the later T34A for $7.50.
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