From the Ground Up
Eric Edwards GW8LJJ turns to semiconductors and, in this first instalment, diodes.
FromWikipedia
A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other semiconducting materials such as gallium arsenide and germanium are also used.
History
The first use of a diode was a ‘crystal’ semiconductor used in the early days of radio as a rectifier (detector). A crystal (crystalline material) was fixed in place and a flexible wire (cat’s whisker) made light contact with the crystal, Fig. 1. The cat’s whisker was carefully positioned for the received signal to be heard in a pair of high impedance headphones.
This was a delicate operation and many experiments were carried out by enthusiasts using various materials to use as the ‘crystal’, and Galena (Lead Sulphide) was commonly used. A razor blade with some corrosion (rust) and a light spring wire positioned in various spots to get the optimum sensitivity were also experimented with. Almost any combination of conducting material with ‘poor’ contacts would act as a detector.
It was common in the days of (vinyl) record players employing a crystal pickup (with poor connections to the ‘pickup’ leads) to receive off-air transmissions such as from taxis and even amateur radio operators, because the pickup was acting as a detector.
There are many different types of diodes made for various functions but they all share the same fundamental purpose of one-way conduction, Fig. 2.
Let’s make a Cat’sWhisker ‘Crystal’Set
The circuit in Fig. 3a is as simple as it gets. For this example, I wound a coil on a 2in (50mm) long, 1in (25mm) diameter piece of white plastic conduit using 60 turns of 26SWG (not critical) enamelled copper wire (magnet wire) and soldered the ends across a 300pF dual-gang variable capacitor of which only one section of a gang was used. A ‘fuse holder’ contact housing the galena (Iron Pyrite, as supplied by Trevor M0WDO, see ref, is actually used and is an iron Sulphide normally called Fool’s Gold) is soldered to the fixed (isolated) plates of the variable capacitor. The moving plates are connected to ground (earth connection). A 1nF (1000pF) capacitor is connected to the detector cathode (cat’s whisker), which is the output of the receiver, and the other end is soldered to the earth connection to decouple the RF signal so that only audio is recovered from the diode. The antenna is connected via a 100pF to add to the selectivity of the receiver. A 100pF trimmer (variable capacitor) would be better for improved selectivity in place of the fixed capacitor. A pair of high impedance headphones must be used so as not to heavily load the output, which would be the case if a standard pair is used. The output can, of course, be connected to an audio amplifier. The performance of this is comparative to using an OA81 or similar Germanium diode. Fig. 3b shows the set receiving off-air displaying the audio on an oscilloscope.
Germanium
The first of this type was a point contact diode (similar to the cat’s whisker) used in the Second World War for radar and other high frequency applications replacing thermionic (valve) diodes. Germanium is a common substance in raw form and can even be obtained from chimney soot! It has to be purified and this creates a crystalline construction like salt and sugar. Some common Germanium diodes are OA47, OA81, OA71, 1N34 and others that are not easily obtainable like a lot of electronic
Fig. 1: Using a ‘cat’s whisker’.
Fig. 2: A selection of diode types.
Fig. 3a: Circuit of a basic crystal set.
Fig. 3b: Off-air received signal displayed on oscilloscope. Fig. 4: Diodes under forward and reverse bias. Fig. 5: Diode construction, showing the barrier. Fig. 6: Regardless of the current flowing, the voltage drop will be the drop across the junction. Fig. 7: Current flow through a ‘signal’ diode and a ‘rectifier’ diode.
parts these days. These diodes were also used as video detectors and DC restorers in the days of CRT (Cathode Ray Tube) televisions. They also found their uses as FM detectors and in AGC circuits, speech clipping and noise limiters. They have lower losses than the Silicon types but they cannot handle the same power as silicon so they were primarily used for low current signal and fast switching circuits.
Silicon
Silicon can be obtained from sand on the beach but, as with germanium, it has to be purified. The atoms that make up the materials are rigidly locked together in a lattice, such that the electrons in the atom cannot be moved. The diode can be considered a good insulator. Well, an ideal diode is but, as with all things in life, nothing is perfect. After purification precise amounts of impurities are added and this is referred to as being ‘DOPED’. The impurities fit into the lattice and have their own electrons, which are free to move around and produce an electric current flow. There is now a surplus of electrons so the material is called an ‘N’ type as it is negatively charged.
There are other types of impurities that can be added to pure silicon and germanium and these produce a shortage of electrons in the lattice. In this case there are ‘HOLES’ in the lattice. Electrons can jump into these holes, producing what can be thought of as a flow of holes. It’s like sitting in a row of chairs and when someone gets up from the end, others move along, each leaving an empty chair (a hole) behind. People (electrons) move along nearer to the end of the row and a hole appears to travel in the opposite direction. Because there is a shortage of negative electrons there is a positive charge and the material is called ‘P’-type semiconductor.
Conduction
Diodes conduct in one direction only. When there is a positive voltage on the anode with respect to its cathode, it will fully conduct when a barrier voltage has been reached, referred to as being forward biased. This is considered as 0.2V (average between 0.2 and 0.3V) for a germanium diode and 0.6V (average between 0.6 and 0.7V) for a silicon diode. However, if a positive voltage is applied to the cathode with respect to the anode, no current flows, referred to as being reversed biased. The diodes in Fig. 4 show the biasing. The top row is all forward biased, which means the voltage on the anode is more positive than the voltage at the cathode. It also indicates that the diodes are silicon and they show the 0.6V drop across the diode. You will notice that the top far right diode has a negative voltage (−9.4V) on its anode; however, it has −10V on the cathode so the diode is conducting because the anode is more positive than the cathode by 0.6V. The bottom row of diodes is reversed biased as the anode is more negative than the cathode, so no conduction can take place. The voltages on the cathodes are there as a reference to show reverse bias and are