Construction is more complex than the alloy diffusion technique, requiring many separate steps, depositing oxide layers on the silicon wafer and using photo lithographic techniques to etch away unwanted silicon areas.
#Transistor schematic silicon series
The disadvantages of diffused alloy transistors was overcome in the 1950s by developing transistors that use silicon as the semiconductor material and the planar (layered) type of construction, illustrated in Figures 3.2.2 to 3.2.3 These transistors are built up as a series of very thin layers (or planes) of semiconductor material built up rather like a multi layer sandwich). They have relatively high leakage currents across the junctions. Their collector current increases as their temperature increases, making them susceptible to destruction due to thermal runaway. They have a poor frequency response due mainly to their large junction capacitances. This junction therefore needs to be larger to dissipate this extra heat.Īlloy diffused transistors suffer from several drawbacks: This is because most of the heat generated within the transistor is generated at the base/collector junction. Notice that the P type region used for the collector is larger than the emitter region. The result is an extremely thin layer of N type semiconductor (the BASE) sandwiched between two thicker P type layers, which form the other two terminals, the emitter and the collector. Fusing is stopped just before the two P type regions meet. This creates a "hole" charge carrier at each Indium atom. As the two metals fuse together indium atoms (with 3 valence electrons) mix with the pure germanium atoms (with 4 valence electrons) creating P type material where the Indium atoms will appear to be short of one electron, and so will bond with only three instead of four neighbouring atoms. The fusion process causes the indium to diffuse into the germanium. The emitter and collector were made by diffusing two pellets of Indium (a trivalent material, having three electrons in their valence shell) into either side of a wafer of N type base, as shown in Fig 3.2.1.
Typically a thin wafer of Germanium was used for the base of the transistor. Diffused Transistor Alloy Diffused TransistorsĮarly bipolar junction transistors used a variety of methods to produce a current amplifier.
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