Germanium Diodes.

Fig. 3.1 Germanium Point Contact Diode.

Germanium diodes are a type that has been in use since the early days of radio and although they have been largely superceded by silicon signal diodes and integrated circuits, they may still be found as signal diodes in older radio circuits, where they are used in such applications as radio signal detectors (demodulators) where the voltages involved in demodulating AM radio signals are likely to be small, less than 1Vpp (0.5Vpk). Germanium diodes will conduct at a forward voltage of only about 0.15V, but a silicon diode will not start to conduct until a forward voltage (positive on the anode) of 0.6V is reached.

As shown in Fig 3.2 the demodulator that recovers the audio signal from the amplitude modulated (AM) radio waves can use a germanium diode as a rectifier to remove the unwanted half of the AM signal. It works in a similar way to a rectifier diode in a simple power supply; it passes one half of the incoming radio signal and blocks the other half. The main differences being that the frequency of operation is much higher, also the voltages used in a demodulator are tiny by comparison to a mains (line) power supply.

Fig. 3.2 A simple diode demodulator using germanium gives an output for smaller signals than is possible using silicon.

Junction Capacitance.

The point contact diode illustrated in Fig 3.1 has another useful advantage over the silicon rectifier; the area of junction is very small. This also means that the junction capacitance will be very small, much smaller than that of a Silicon junction diode. This allows the point contact diode to operate at higher frequencies than an ordinary silicon junction diode, although some modern silicon diodes are available with very small juction areas allowing them also to operate at high frequencies.

The junction of a diode is basically an insulating layer (the depletion layer) sandwiched between two conductors, the P type and N type areas, it is therefore effectively a capacitor. If this junction capacitance is too large, it will reduce the effectiveness of the circuit at high frequencies; the whole waveform will pass through the diode, now effectively a capacitor, instead of one half cycle being removed by the rectifying action of the diode. Therefore at high frequencies no rectification would take place. Because the radio AM demodulator must work at high (radio) frequencies, the germanium diode, with its very small junction capacitance is ideal for the purpose.

The Germanium diode will not pass large currents - only a few milli-amperes, but that is all that is needed in a radio application such as a demodulator. Germanium diodes are therefore useful for radio frequency circuits involving small amplitude signals.

The advantages and disadvantages of silicon and germanium diodes are summarised in Table 3.1 below:

Table 3.1 Diodes

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