Types of Diodes Help
A Gunn diode is made from a compound known as gallium arsenide (GaAs). When a voltage is applied to this device, it oscillates because of the Gunn effect , named after J. Gunn of International Business Machines (IBM), who first observed the phenomenon in the 1960s. Oscillation occurs as a result of a property called negative resistance . This is a misnomer because, as we have learned, there is no such thing as a device that conducts better than perfectly. In this sense, negative resistance refers to the fact that over a certain limited portion of the characteristic curve, the current through a Gunn diode decreases as the voltage increases, contrary to what normally takes place in electrical systems.
The acronym IMPATT (pronounced “IM-pat”) comes from the words impact avalanche transit time . We won’t concern ourselves in this book with the exact nature of this effect, except to note that it’s similar to negative resistance. An IMPATT diode is a microwave oscillating device like a Gunn diode, but it is manufactured with silicon rather than gallium arsenide.
An IMPATT diode can be used as a low-power amplifier for microwave radio signals. As an oscillator (a circuit that generates rf ac), an IMPATT diode produces about the same amount of output power, at comparable frequencies, as a Gunn diode.
Another type of diode that can oscillate at microwave frequencies is the tunnel diode, also known as the Esaki diode . It produces a very small amount of rf power.
Tunnel diodes work well as amplifiers in microwave receivers. This is especially true of GaAs devices, which act to increase the amplitudes of weak signals without introducing any unwanted rf noise , or signals of their own that cover a large range of frequencies. (An example of noise is the hiss that you hear in a stereo hi-fi amplifier with the gain turned up and no audio input. The less noise, the better.)
Leds And Ireds
Depending on the exact mixture of semiconductors used in manufacture, visible light of any color, as well as infrared (IR), can be produced when current is passed through a diode in the forward direction. The most common color for a light-emitting diode (LED) is bright red, although LEDs are available in many different colors. An infrared-emitting diode (IRED) produces energy at wavelengths slightly longer than those of visible red light. These are called near-infrared (NIR) rays.
The intensity of energy emission from an LED or IRED depends to some extent on the forward current. As the current rises, the brightness increases up to a certain point. If the current continues to rise, no further increase in brilliance takes place. The LED or IRED is then said to be in a state of in saturation .
An injection laser , also called a laser diode , is a special form of LED or IRED with a relatively large and flat p-n junction. The injection laser emits coherent electromagnetic (EM) waves, provided the applied current is sufficient. Coherent waves are all lined up and all have the same frequency, compared with the incoherent waves typical of most LEDs and light-producing devices in general.
Figure 16-4 is a simplified diagram of a laser diode. The substrate is the material on which the component is built; it is like the foundation of a building. It also serves to carry away excess heat so that the device can carry fairly high current without being destroyed. There are mirrors at opposite ends of the piece of n -type material. One of the mirrors (the one labeled in the drawing) is partially reflective. The opposite mirror (not shown) is totally reflective. The coherent rays emerge from the end with the partially reflective mirror.
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