![]() ![]() P diffusion is relatively slow and therefore easier to control. It's the way early germanium transistors were made. I'm sure it's me not understanding what you're saying David, but to me it looks like you're describing a P-N-P 'sandwich' there and how critical and difficult a process it is? Yet PNP trannies are the more common? Silicon largely took over as silicon dioxide is a much more stable (and better insulating) oxide than germanium oxide (which is water soluble), and silicon's higher bandgap (1,1eV vs 0.7eV) meant higher operating temperatures and less leakage. In the case of germanium PNP devices, antimony was used as the N type dopant and Indium as P type the P type regions for these devices like the OC71 were formed by alloying in a blob of indium onto the front and back of an N type germanium wafer.Īs to why PNP germanium devices were easier to fabricate than NPN ones it is probably a combination of factors including the solubility of the dopants, their diffusion rates, the ease of making ohmic contacts and defect density of the resulting junctions. N type dopants that can be used are Phosphorus, Arsenic, Antimony. The solubility of these decreases in that order. P type dopants are group III - Boron, Aluminium, Gallium, Indium. ![]() I can remember seeing a crystal grower still used in Philips (formerly Mullard) Southampton site, I think around 1981. An impurity was deliberately added to the melt to dope the material. This caused a long tubular crystal to grow. It was then put into a 'puller' where it was meted and a single crystal dipped in and very slowly lifted out while rotating. This was done several times and the end bit was discarded. In the early days it was reasonably straightforward to grow single crystal germanium by first removing impurities using a technique called zone refining, which swept a heat source along a bar of the material, melting it in a zone which concentrated the impurities. It's very difficult doping carbon (Diamond), Silicon needs temperatures of 1000-1200C (it melts at about 1420C), germanium much less so (melts at 940C), so germanium devices were easier to make at first. ![]() Semiconductors are typically group IV elements (Carbon, Silicon, Germanium or SiC) or more complex like III-V (GaAs, InSb, AlP, GaN) or even II-VI (CdTe), but in the early days germanium and silicon were the main candidates as they could be grown as single crystals in high purity fairly easily compared to the others. It's a long time since I was involved in semiconductor physics but here goes: ![]()
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