The KD503 have higher power and higher current than 2N3055. It has a transition frequency of 2.0 MHz. KD503 are packaged in a TO-3 case style (called T41 by Tesla), it is a 20 amp, 80 volt, 150 watt power transistor. It was produced exclusively by the Czechoslovakian electronics company Tesla.

KD503 is a higher power equivalent used in Eastern Bloc countries, and is intended for general purpose applications. Although this was adequate for most of the low-frequency 'workhorse' applications, and par with other high power transistors around 1970, it did bring some difficulty to high fidelity power amplifier designs around 20 kHz, as the gain begins to fall and phase shift increases. One limitation was that its frequency response was rather low (typically the unity-gain frequency or transition frequency, f T, was 1 MHz). It was second sourced by other manufacturers Texas Instruments listed a single-diffused mesa version of the device in an August 1967 datasheet.

The 2N3055 remains very popular as a series pass transistor in linear power supplies and is still used in for medium-current and high-power circuits generally, including low frequency power converters although its use in audio power amplifiers and DC-to-AC inverters is now less common and its use in higher frequency switch-mode applications never was very practical.

The team of design, production, and applications engineers received RCA Electronic Components achievement awards in 1965. The 2N3054 and 2N3055 were derived from the 2N1486 and 2N1490 after package redesigns by Milt Grimes. The historically-significant 2N3055 was designed by Herb Meisel's engineering group with RCA it was the first multi-amp silicon power transistor to sell for less than one dollar, and became an industry workhorse standard.

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