When at the end of XIX century the British scientist Frederick Guthrie, who is best known for the discovery of mustard gas, discovered the basic principle of operation of thermionic diodes, he hardly imagined that an electronic semiconductor device, based on a similar principle, will become one of the most commonly used components in various electronic circuits, process automation, electronic devices with various functions and applications. Moreover, one of the diodes – the LED, has recently been gaining more and more popularity as a source of light.
Of course, Thomas Alva Edison became even more famous as in the frosty February 13th 1880, while studying incandescent lamps, which were later named after him, rediscovered that when voltage is fed to the positive pole of a lamp and a metal plate is put close to it, there is electric current between the positive pole and the plate. Using this principle he created a DC voltmeter, patented it and basically at this point he stopped dealing with diodes.
The strange thing here is that none of them used the word diode. Later, scientists like John Fleming, a former associate of Edison, Charles Brown, Greenleaf Pickard, Jagadish Chandra Bose and others created a radio-receiver based on crystal diodes and as late as the 1919 another British scientist – William Henry Eccles introduced the term diode (from the Greek roots di meaning “two” and odos – “path”).
Diodes can be semiconductor, LEDs and special diodes. Here, we will mainly consider semiconductor diodes. Semiconductor diodes perform a p-n junction, and the current flows from the p-doped region to the n-doped region, but not in the reverse direction. The diode is like an electric current valve. Usually it is made from silicon or other semiconductor materials. The positive electrode is called anode and the negative electrode – cathode. When switching in forward direction, the diode has small resistance and there is electric current – when switching in reverse direction, the resistance is high and there is no current.
There are many classifications of diodes, but the main one is based on their function. According to this classification we distinguish the following types of diodes:
• A rectifying diode – as evidenced by its name it serves for rectifying alternating current, as it allows current to flow only in one direction. This diode is a key element in rectifier devices and various electronic circuits. The rectifying diode operates at mains frequencies and „cuts“ one of the half-waves of the sinusoid. They can be made of silicon or germanium. Silicon diodes are characterized by large breakdown voltages (1000V – 1600V) and a larger temperature range of operation. The breakdown in silicon diodes has an avalanche nature and the breakdown voltage increases with the temperature. Germanium diodes have lower breakdown voltages (100V-400V), they operate at lower temperatures and have less voltage drop in forward direction and less emitted power at the same current. Their breakdown has thermal nature.
• LED – emits incoherent light, and consists of one or more crystals behind glass, which generates luminous flux. LEDs are used in displays, in general and decorative lighting, indications of processes, etc. Gallium arsenide is one of the compounds, from which such a diode can be made. For example, almost every home electronic device has a LED based indication showing the power supply to the unit. At first LEDs were only red, but now there are white, orange, yellow, green, blue, blue-green, ultraviolet LEDs. These diodes are adjustable, i.e. dimmable.
• Tunnel diode – This diode uses the tunnel effect, which means transition of electrons through a potential barrier, higher than the electrons’ energy. The width of the transition is commensurate with the inter-atomic distance of the electrons. Then they seem to sneak through this passage (tunnel). Tunnel diodes have a p- n junction which has a very small width and high intensity of the magnetic field. They are made of heavily alloyed semiconductors. They can work in a very large temperature range as well as in a radioactive environment. An important advantage of tunnel diodes is their high frequency and the fact that they have very low power consumption, equivalent to about 1 % of the power of standard diodes.
• Zener diode (Stabilitron) is a diode, which allows current to flow in the reverse direction. At a certain reverse voltage there is a breakdown that does not damage the diode. The diode is named after Clarence Zener. It is used to stabilize the voltage. In circuits it is connected parallel to the load resistance. Low-voltage Zener diodes (up to 6 V) are made of low resistance material and work with a tunnel breakdown. By increasing the voltage above 7V, an avalanche breakdown is generated.
• Schottky diode – it is made of aluminum and silicon. It is named after the German scientist Walter Schottky. This diode uses the junction between a metal and a conductor, overcoming the so called Schottky barrier. The Schottky barrier has a small junction capacity, therefore the operating frequency can be very high. Schottky diodes are very fast, the triggering is less than 0.1nanosecond, faster then the triggering of the base-collector junction of the transistor, thus shunting it in integrated circuits. These diodes are used for voltage limiters, in photovoltaic systems where they prevent the flow of reverse current and discharging during the night. Schottky diodes are used as rectifying elements in switching power supplies, etc.
• A varicap diode or a varactor diode is used for frequency change, frequency multiplication, phase modulation, in oscillator controlling circuits, in frequency synthesizers, etc. Varicap diodes are used as capacitors with variable-capacity, as they use the dependence of the PN Junction capacity on the reverse voltage.
• Laser diode – its active medium is a semiconductor similar to the one used in LEDs. Laser diodes are used for transmitting signals in optical fibers, for recording data on disks (CD, DVD), etc. In practice, this type of device is a small laser.
• Gunn diode is a type of semiconductor diode, which is used for generating and converting variations in the UHF spectrum from 0.1 to 100GHz. Unlike other types of diodes, Gunn diodes are not based on the p-n junction, i.e., its properties are not determined by the junction effect of two different semiconductors, but on the properties of the semiconductor itself. It was invented in 1966 by the English scientist John Gunn.
•A high-frequency diode – it is designed for handling high-frequency signals. It rectifies currents and voltages with a frequency of 150 MHz. These diodes are used for detecting signals (separating a low frequency signal from an amplitude modulated high-frequency signal). They are also used for changing the carrier frequency of modulated oscillations, etc. They use point p-n junctions to extend the frequency range.
• An impulse diode – the transient processes last less than 1μs. They operate in a pulse mode. There are impulse diodes in the sub-nanosecond range 0.1-1ns. These diodes are alloyed with gold in order to become faster, thereby the rate of recombination increases. The forward current of these diodes is quite small – 20-50mA. A type of impulse diodes are mesa diodes. They are made by diffusion and their junction area is significantly reduced. Their recovering time is10 -50ns, the forward current is 500mA and the reverse voltage – around 100V.
• PIN diode – these are switching diodes. They are a part of a transmission line between the generator and the load. The energy that is transmitted along this line is determined by the impedance of PIN diodes. PIN diodes are used for adjusting UHF power for switching UHF signals with a bandwidth of 1 to 12 GHz, for amplitude modulation with a different depth, etc.
• Stabistor – a semiconductor diode in which the pressure in the forward volt- ampere characteristics depends only slightly from the current. This type of diode also serves to stabilize the voltage. The difference between the stabistor and the Zener diode is the little stabilization voltage. In order to obtain higher voltage, stabistors are connected in series.
• An avalanche diode – it operates under an avalanche breakdown mode and is used to generate VHF oscillations. If the diode is fed with continuous reverse voltage, lower than the breakdown voltage, and a variable signal is superimposed with an amplitude exceeding in one of its half periods the breakdown voltage in a specific narrow area, it creates conditions for avalanche multiplication. By repeating this half-period, a new quantity of avalanche electrons is generated, and they start drifting to the N-areas. The current through the diode is modulated by the frequency of the signal, superimposed on the input voltage. Due to the inertia of the avalanche, there is a phase shift between the voltage and the current flowing through the diode, and the phase angle is approximately equal to n/4, where n is the high-resistant narrow section called electrons multiplication layer. A region with a negative differential resistance comes out, which allows the generation of persistent UHF oscillations.
Diodes find a lot of various applications and are often used to protect sensitive and expensive electronic devices from over voltage, as under normal conditions they are non-conductive, and under over voltage they become conductive. In combination with other electronic components diodes form logic elements. They can also be used for measuring temperature, because in most cases they are directly dependent on its characteristics. Their earliest use was for amplitude demodulation of radio-signals. Each diode has specific volt-ampere characteristics. High-power voltage diodes up to 1200 V and current 200 A also find application in practice. These diodes are mounted on a cooler.