Although very many specialised diodes are used in the electronics industry to perform specific functions, the standard silicon or germanium PN junction diodes are used in even greater quantities. They are able to perform in a whole variety of roles ranging from being a signal diode, a switching diode, a power diode or a high voltage diode. These and many more functions can be performed by many standard PN junction diodes made from germanium, or more usually from silicon.
Diode circuit symbol and polarity
Like any diode, the PN junction diode has two connections. One is termed the anode and the other is termed the cathode. For a current to flow across the PN diode junction it must be forward biased. Under these conditions conventional current flows from the anode to the cathode, but not the other way around.
Diode circuit symbol and common package outlines
Key diode specifications
When using any diode it is necessary to ensure that it will meet the requirements for the job intended for it. There are a number of important specifications, some of which are more important than others dependent upon the type of role intended for the diode.
- Semiconductor material One of the major elements that will determine the properties of a diode is the material from which it is made. Most general purpose and rectifier diodes are made from silicon as this affords the best all round performance at an affordable price. For some applications silicon may be used. Other materials are generally reserved for more specialist diodes.
- Forward voltage drop While any diode will have a certain voltage drop because it is a semiconductor diode, and generally this is taken as about 0.6 volts for silicon and 0.2 volts for germanium, this is dependent upon the current flowing. There is also a resistive drop in the diode that adds to this. Therefore for any current there will be an overall voltage drop. While this may not be important for some circuits, it may be for others, especially where current levels start to rise. For some applications it may mean that enough heat is dissipated for the diode to need a heatsink.
- Leakage current When the diode is reverse biased no current should flow. Unfortunately this is not the case, because a small amount does flow. (This current flows because there are what are termed minority carriers in the semiconductor). The level of the leakage current increases as the temperature rises. The diode leakage current is specified at a certain reverse voltage and temperature and is normally only measured in microamps or picoamps. Silicon is much better than germanium.
- Junction capacitance Although varactor or varicap diodes have been optimised for their junction capacitance properties, all diodes exhibit capacitance across the junction. The level of capacitance falls as the reverse bias is increased. Accordingly any junction capacitance is specified for a given level of reverse bias. While the amount of junction capacitance is not important for power rectifier diodes, it is important for those operating at higher frequencies. Accordingly there are low capacitance diodes that are available. For example the 1N4149 is a low capacitance equivalent of the 1N4148.
- Peak Inverse Voltage (PIV) This is the maximum voltage a diode can withstand in the reverse direction. It is one of the key specifications for high voltage diodes and power diodes used in power supplies.
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