Germanium Alloy Diffused Transistor. Early bipolar junction transistors used Germanium as the semiconductor material and were of the Alloy Diffused type where, for a typical PNP transistor, two pellets of Indium (a trivalent material, having three electrons in their valence shell) are fused to either side of a wafer of N type Germanium, as shown in fig 2.1. Germanium Transistor The fusion process causes the indium to diffuse into the germanium. 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. This creates a "hole" charge carrier at each Indium atom. Fusing is stopped just before the two P type regions meet. 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. Notice that the P type region used for the collector is larger than the emitter region. This is because most of the heat generated within the transistor is generated at the base/collector junction. This junction therefore needs to be larger to dissipate this extra heat. Alloy diffused transistors suffer from several drawbacks: 1. They have a poor frequency response due mainly to their large junction capacitances. 2. They are not reliable at high temperatures. 3. Because they are made from germanium, they have relatively high leakage currents across the junctions. 4. They cannot withstand high voltages. Germanium PNP high frequency transitor Transistors made from germanium do have an advantage in some circuits due to the lower forward voltage required between emitter and base (0.15V instead of 0.6V for silicon) to make them begin to conduct. A feature of any semiconductor used for transistors and diodes is that it conducts more current, the warmer it gets. In germanium this is much more noticeable than in silicon. Germanium transistors will tend to easily go into "thermal runaway" i.e. the warmer they get the more current they pass, so they get warmer and pass more current. This situation can build up until the transistor destroys itself if the circuit is not carefully designed to avoid it. One feature of such a design must be efficient heat sinks fitted to any transistor (often output transistors) that will have to pass more than small currents. This unstable tendency in germanium transistors and diodes can however, be put to good use. Either device can be used as a simple temperature sensor, the current being approximately proportional to temperature. Silicon can however, also be used as a temperature sensor. The advantages and disadvantages of germanium and silicon transistors are summarised in table 2.1 below: Table 2.1 Transistors Parameter Germanium Silicon Comments Current Gain 50 to 300 Up to 850 Higher gain from silicon due to greater stability. Max. Power dissipation PT Up to 6 Watts More than 50 watts Silicon much better in high power applications. Leakage current A few micro-amperes A few nano-amperes Germanium 1000 times more leaky than silicon. Max. collector-emitter voltageVCE Tens of Volts Hundreds of volts Silicon the only real choice for high voltage applications. Temperature stability Poor Good Germanium more sensitive to thermal runaway. Normally a problem but can be useful in temperature sensing. Highest operating frequency fT A few MHz -Not normally used above audio frequencies. Useful up to hundreds of MHz Silicon transistors much better at high frequencies. Note: When reading parameters such as these in technical information, they are sometimes also written as Ice/Ibe. The difference between using capitals or lower case for the subscript letters denotes that the parameter refers to large signal or DC operating conditions (Capitals) or small signal AC operation (lower case).