This shop is perhaps the most oddly-named in Deptford. Apparently, the business specialises in hard-to-find transistors and other electrical components as well as 'military and industrial obsolete semiconductors'.
I have to admit that my knowledge of semiconductors could be written on the back of a microchip. In marker pen. However, a friend who knows lots about such things helpfully explained to me* that semiconductors are in every electronic device, usually made of silicon, and can be used to create devices including one-way electronic valves, high-speed switching, voltage regulation, LEDs... They can handle extreme voltages and currents and allow good heat dissipation, which is why they are used in producing CPU chips for PCs. Their most common use is in transistors - a microprocessor chip contains thousands of these, each a couple of microns in size. Pretty important then, so (1) I can understand how there would be a specialist market in them; and (2) I feel a bit as if I'd had to ask what electricity or water is used for!
The shopfront makes it difficult to tell whether the shop is still open at all. A clue may be on the door: a year-old notice application for planning permission to redevelop the premises. The planning documents suggest that the premises were already vacant; permission has been granted, so the shopfront may not be with us for much longer.
*Any errors in this summary are, sadly, my own.