Use of Diodes in Electronics Engineering - Essay Example

Electronics Engineering: Diode A diode is a two-terminal semiconductor device that exhibits nonlinear current-voltage characteristics. The purposeof a diode is to permit current to flow in one direction and to prevent current flow in the opposite direction. The two terminals of a diode are referred to as the anode and the cathode. There are two types of semiconductor diodes: a P-N junction diode, which forms an electrical boundary at the interface between N- and P-type semiconductor layers, and a Schottky diode, whose boundary is formed between metal and semiconductor regions. Semiconductors are crystals that, in their pure state, possess electrical properties that lie between those of conductors and insulators. When the proper impurities are added in trace amounts, semiconductors display interesting and useful properties. This process is usually referred to as doping. The oldest ancestor of semiconductor devices was the crystal detector that was used in early wireless radios. This device was composed of a single metal wire (often called a "cat's whisker") that would touch against a semiconductor crystal. The result was a rectifying diode that allows current to flow easily in one direction but hinders the flow in the other direction. The rectifying diode had two terminals, but by 1930 vacuum-tube diodes had pretty much replaced the smaller crystal detector. The crystal and "cat's whisker" were abandoned, and would eventually emerge as popular children's toys known as crystal radios. The development of radar during World War II did a lot to revive the fate of crystal detectors. Though they were temperamental, crystals were better than vacuum-tube diodes at rectifying the high frequencies used by radar. During the war, a lot of effort was invested in improving semiconductors, particularly the silicon and geranium used in crystal detectors. Around the same time, Russell Ohl at Bell Laboratories discovered that these materials could be doped with tiny amounts of foreign impurity atoms in order to create interesting new properties. Depending on the selection of impurities added, semiconductor materials of two electrically different types can be created. One type is called N-type and is electron-rich while the other is called P-type and is electron poor. Most of the "magic" surrounding semiconductor devices occurs at the barrier between P-type and N-type semiconductor material. This type of barrier is called a P-N junction. Ohl and his peers found that a P-N junction created an effective diode. Like many other components, diodes possess a positive side and a negative side. The positive side is known as an anode, and the negative side is known as a cathode. When the voltage on the anode is higher than on the cathode, the current flows through the diode with very low resistance. When the voltage is lower on the anode than on the cathode, the current is prevented from flowing due to a very high resistance. An easy way to commit this to memory is by examining the symbol for a diode. The arrow in the diode symbol points in the direction in which it allows current to flow. The cathode of a diode is usually marked with a line next to it. A similar line can be observed in the schematic symbols above the arrow. Diodes are also sometimes marked with an identifying color code that is similar but not identical to those used for resistors. It is important to note that when current is flowing through a diode, the voltage on the positive leg is higher than that on the negative leg. This phenomenon is generally referred to as the diode's "forward voltage drop." The magnitude of the voltage drop is a function of the semiconductor material that the diode is made from. Silicon diodes are the most common and the cheapest. They have a forward voltage drop of about 0.65 volts. Geranium diodes have a forward voltage drop of approximately 0.1 volt. Geranium diodes, however, are typically much more expensive that silicon diodes. But, they are salvageable and can be collected from old circuit boards. The Zener diode is designed to have a specific reverse breakdown voltage or conduction voltage when reverse-biased. Due to this, Zener diodes can be used by themselves as voltage-sensitive switches, or in a series with a current-limiting resistor to provide voltage regulation. All P-N junctions are light sensitive; photodiodes are merely P-N junctions that are designed to optimize this effect. Photodiodes can be used in two ways-in a photovoltaic role or a photoconductive role. In a photovoltaic role, the photodiode becomes current source when illuminated. This is sometimes called a solar cell. In order to use a photodiode in its photoconductive mode, the photodiode is reverse-biased; the photodiode will then allow a current to flow when it is illuminated. All diodes emit some light when forward-biased. LEDs are made from a special semiconductor (materials like gallium arsenide phosphide), which optimizes the light output. Unlike light bulbs, LEDs rarely burn out unless their current limit is passed. When current is flowing through an LED, the voltage on the positive leg is nearly 1.4 volts higher than the voltage on the negative side. This sometimes varies with LED types. Infrared LEDs have a lower forward voltage requirement, and others may need up to 1.8 volts. There is very little resistance to limit the current, so a resistor has to be used in coordination with the LED to avoid destroying it. However, some panel-mount LEDs come from the factory with a current-limiting resistor soldered onto them. It is also important to note that LEDs can be used as photodiodes. But, their sensitivity is pretty low, so they are only useable in very bright conditions. A flashing LED is just an LED with a built-in microcircuit that causes it to flash periodically. Because the FLED draws current when it flashes, we can use FLEDs to drive a number of time-dependent circuits, because it periodically becomes conductive. Like other LEDs, FLEDs are light-sensitive, and they flash brighter in light. Some FLEDs need at least 3 volts minimum to work in, but most FLEDs do not generally require current-limiting resistors. There are several different applications for diodes. "For memory signals, you should use the clamping diode and the two-thirds rule as your first choice and the series-concept termination as your second. Clamping diodes typically provide a cleaner signal than series termination, especially when you use them at the first and last loads" (Johnson, Howard). There are other termination applications for diodes. "Schottky-diode termination works well in a multi-drop situation in which some of the receivers on the line can also drive the line" (Johson, Howard). Diodes are also useful in other applications. Diodes are used in converting AC power from the 60Hz line into DC power for radios, televisions, and telephone answering machines, computers, and several other electronic devices. Diodes are also used for converting radio frequency signals into audible signals in radios. Diodes have several identifying characteristics. They are two-terminal devices like resistors and capacitators. They do not have many terminals like transistors or integrated circuits. In diodes, the current is directly related to voltage just like it is in a resistor. Diodes are not like capacitators where current is related to the time derivative of voltage or inductors where the derivative of current is related to voltage. In diodes, the current is not linearly related to voltage, like it is in a resistor. Diodes only consume power. They do not produce power like a battery. Diodes are considered to be passive devices. In general, diodes tend to permit current flow in one direction, but tend to inhibit current flow in the opposite direction. When the voltage across the diode is positive, a lot of current can flow once the voltage becomes large enough. When the voltage across the diode is negative, hardly any current flows at all. It is important to connect a resistor in series with the diode, because it is the most common way to limit current. There are also circuits which function as nearly ideal current sources, and these may be used to drive a diode as well. Usually, however, if a diode is directly connected to a voltage source, the diode will burn up before the internal current limit of the voltage source is reached. Works Cited Johnson, Howard. "High-Speed Digital Design." Signal Consulting Inc. 1993-2005. http://www.sigcon.com/Pubs/news/2_19.htm Read More