Point-contact transistor

While point-contact transistors usually worked fine when the metal contacts were simply placed close together on the germanium base crystal, it was desirable to obtain as high an α current gain as possible. To obtain a higher α current gain in a point-contact transistor, a brief high-current pulse was used to modify the properties of the collector point of contact, a technique called 'electrical forming'. Usually this was done by charging a capacitor of a specified value to a specified voltage then discharging it between the collector and the base electrodes. Forming had a significant failure rate, so many commercial encapsulated transistors had to be discarded. While the effects of forming were understood empirically, the exact physics of the process could never be adequately studied and thus no clear theory was ever developed to explain it or provide guidance on improving it. Unlike later semiconductor devices, it was possible for an amateur to make a point-contact transistor, starting with a germanium point-contact diode as a source of material (even a burnt-out diode could be used; and the transistor could be re-formed if damaged, several times if necessary). ==Characteristics==

Some characteristics of point-contact transistors differ from the slightly later junction transistors: • The common base current gain (or α) of a point-contact transistor is usually around 2 to 3, whereas α of bipolar junction transistor (BJT) cannot exceed 1. The common emitter current gain (or β) of a point-contact transistor does not usually exceed 1, whereas β of a BJT is typically between 20 and 200. • Negative differential resistance. Point-contact transistors connected in the common emitter amplifier configuration will display negative output resistance, which may be undesirable for voltage/current amplifier applications. Switching circuits based around point-contact transistors often rely on negative differential resistance. • Until the development of the surface barrier transistor in 1953 point-contact transistors were the fastest transistors available, some operating in the lower part of the VHF band when the fastest junction transistors could still only barely operate at a few MHz. • Moisture attack was less damaging to point-contact transistors than to junction transistors, because their collector reverse resistance is lower and cutoff collector current higher. • When used in the saturated mode in digital logic, in some circuit designs (but not all) they latched in the "on" state, making it necessary to remove power for a short time in each machine cycle to return them to the off-state. ==See also==