Semiconductors Ltd logo

In the late 1950s and the 1960s, transistors could be bought in the UK that were branded 'Semiconductors Ltd', made in the UK, using the same technology and encapsulation (and sometimes the same part numbers) as those made by Philco in the USA. What was going on?

Information is quite scarce on this aspect of British semiconductor manufacturing history, but enough exists to sketch it out. The manufacturer of these devices was actually Plessey Semiconductors Ltd., a division of the Plessey company, another longstanding British manufacturer with a very diversified product range. However, the word 'Plessey' never appears on Semiconductors Ltd documentation. Plessey management was farsighted enough to kick-start their business by licensing advanced technology from Philco, but then to develop their own in-house technology that was leading-edge in Europe if not the world. They purpose-built a semiconductor factory in Swindon where Plessey had other manufacturing facilities, and went from being a relatively small-scale manufacturer of transistors to become a leading and successful British manufacturer of integrated circuits in the 1980s (using the Plessey brand by then). This attracted the attention of GEC, the UK's largest electrical group. After initial attempts that were thwarted by UK monopoly laws, GEC finally teamed up with Siemens in Germany and bought Plessey in 1989. Subsequently parts of Plessey lived on in various subsidiaries of GEC that survived after GEC itself became defunct. In February 2009, one of these businesses was forced into receivership but survived after a management buy-out that was later reinforced by a second management buyout of another ex-Plessey site, the two then combining to re-create the original name of Plessey Semiconductors, now specialising in the development and manufacture of innovative products exploiting thier own CMOS and bipolar analog mixed signal process technologies.

Plessey logo Transistors by Semiconductors Ltd did not bear a logo other than the name 'SEMICS' in an unusual font. If you know anything more about the devices below, or have other information about Plessey's semiconductor history, please

I do not possess all the transistors below, in fact I am seeking many of them. It is noted in the text where I am seeking examples of any particular type: if you have some for sale or exchange, please Conversely, I am happy to help anyone looking for information on these devices. I have an extensive collection of original data sheets and books, and can provide some information on most Semiconductors Ltd types.


SB240 transistor I'm not entirely sure which type was Semiconductors Ltd's first transistor, but I suspect that it was the SB128, which is equivalent to the historic Philco surface-barrier type 2N128 from 1956. Semiconductors Ltd also made the germanium surface-barrier types SB129, SB240 (shown), SB344 to SB346 (SB345 shown). The SB types are marked 'UK' as are all Semiconductors transistors in my possession. The advantage of the surface-barrier technology was that the transistor gain extended to high-frequency, and so all these types are low-power RF amplifiers or oscillators, extending up to approximately 5 MHz.

I am looking for the SB128 and other SB types by Semiconductors, if you have any, please


MA393 transistor In the late 1950's Philco swiftly developed several improved transistor technologies and Semiconductors followed. After the original surface-barrier came an improved version called micro-alloy transistors (MATs). Again this was a technology developed by Philco for extending high-frequency performance. The MA393 shown dates from about 1960 and is equivalent to the Philco 2N393, with the data sheet stating that it will oscillate up to 60 MHz.


MDS31 transistor Semiconductors also followed Philco in selling germanium micro-alloy diffused transistors, or MADTs. At first they kept the American 2N numbering but I have never found one example of such a type marked 'SEMICS'. Did they just distribute the Philco types in the UK? However, later on they issued several germanium micro-alloy diffused transistors in the MDS series. I have data on some of these but my image shows the MDS31 for which I do not have data; if you have information about this type, or if you have any 2N type by Semiconductors, please


SA495 transistor Semics went on in early 1961 to produce 'surface alloy' silicon transistors, such as this PNP type SA495, which can oscillate up to 21 MHz, and switching PNP type SA496 which can switch up to typically 11 MHz. These figures are clearly inferior to that of the company's germanium devices, but the improved power handling and stability of silicon were valuable and other characteristics would follow. The image shows rather unusual encapsulation: the SA496 is a normal metallic colour, and the SA495 is gold-plated on the lower part only. I do have other examples that are all golden.


SAC40 transistor Semics produced some specialised silicon surface-alloy devices: the SAC series which contains 'chopper' transistors, and the SSA series of symmetrical transistors. A chopper circuit is a very specialised type of DC amplifier that 'chops' the input DC with a square wave and then amplifies that, later to filter out the square wave leaving the amplified DC level. It is quite hard to believe, but Semics made several transistor types specifically for this purpose. They also made some symmetrical switching types in the SSA series, this technology was believed to be advantageous in some circuit applications. I do not possess any of the SSA types, so if you have any, please


Semics also produced a good number of diodes of all types from miniature switching to high-power rectifiers, using both germanium and silicon. However, I do not possess a single one, which I find rather odd, as I have obtained parcels of mixed devices from hundreds of sources. If you have any diodes made by Semiconductors Ltd, please


MAT120 transistor Sinclair Micro Amplifier In 1962, Sinclair Radionics of Cambridge, Clive Sinclair's company, started selling the 'Sinclair Micro-Amplifier'. I quote here from their advertisement in 'The Radio Constructor':

'This microscopic amplifier, the smallest of its type in the world, out-performs amplifiers 20 times as large'.

The text goes on to describe its 60dB power gain, frequency response of 30Hz - 20KHz +/- 1 dB, and power 'sufficient to drive ANY type of earpiece or even a loudspeaker'. It is claimed that the amplifier is only made possible by the 'remarkable micro-alloy transistors it uses'. These were also available separately, comprising four types: MAT100 cost 7/9d, MAT101 cost 8/6d, MAT120 cost 7/9d, and MAT121 cost 8/6d.

As well as the advertisement, I have booklet number 181 from the series "Bernard's Radio Manuals" entitled '22 Tested Circuits using Micro Alloy Transistors' by Clive Sinclair. This gives the characteristics of these four devices: the MAT100 is a low-gain 60 MHz type, the MAT101 a high-gain 60 MHz type, the MAT120 a low-gain 120 MHz type, and the MAT121 a high-gain 120 MHz type.

ADT140 transistor I have a second Bernard's booklet, this one number 186 without a specified author, dated August 1963 and entitled "Tested Superhet Circuits for Shortwave and Communication Receivers using Micro Alloy Transistors (MAT's)". This lists the Sinclair types MAT120 and MAT121, but no MAT100 and MAT101, and also the 'new Sinclair subminiature alloy-diffused transistor type number ADT140'. As shown, this used a standard TO-18 can, but again with a label stuck on it.

Elsewhere on the Web it is claimed that the Sinclair transistors were 'rejects' that he bought cheaply and selected himself. This only seems possible to me if they were unable to meet their frequency specification but still possessed the high gains quoted for the MAT101 and MAT121. Another possibility is that Sinclair simply bought in bulk, either from Semiconductors Ltd or directly from Philco. He states in his book that the MATs are low-price because of high-volume fully-automated production. Some are plain metal coloured as shown and some are gold-plated like Semiconductors Ltd devices, but all my examples have labels stuck around them. Judging from their characteristics, I can guess that they might be low-spec examples of types 2N499 (100 MHz) and 2N500 (200 MHz), more-or-less the last MADTs that Philco produced in this encapsulation.



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