Diodes - Surface Mounted
n
electronics, a
diode is a two-
terminal electronic component that conducts
electric current in only one direction. The term usually refers to a
semiconductor diode, the most common type today, which is a crystal of
semiconductor connected to two electrical terminals, a
P-N junction. A
vacuum tube diode, now little used, is a
vacuum tube with two
electrodes; a
plate and a
cathode.
The most common function of a diode is to allow an electric current in one direction (called the diode's
forward direction) while blocking current in the opposite direction (the
reverse direction). Thus, the diode can be thought of as an electronic version of a
check valve. This unidirectional behavior is called
rectification, and is used to convert
alternating current to
direct current, and extract
modulation from radio signals in radio receivers.
However, diodes can have more complicated behavior than this simple on-off action, due to their complex
non-linear electrical characteristics, which can be tailored by varying the construction of their
P-N junction. These are exploited in special purpose diodes that perform many different functions. Diodes are used to regulate voltage (
Zener diodes), electronically tune radio and TV receivers (
varactor diodes), generate
radio frequency oscillations (
tunnel diodes), and produce light (
light emitting diodes).
Diodes were the first
semiconductor electronic devices. The discovery of crystals'
rectifying abilities was made by German physicist
Ferdinand Braun in 1874. The first semiconductor diodes, called
cat's whisker diodes were made of crystals of minerals such as
galena. Today most diodes are made of
silicon, but other
semiconductors such as
germanium are sometimes used.
Types of semiconductor diode in Mobile Phones Circuit
Zener diodes
Diodes that can be made to conduct backwards. This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. In practical voltage reference circuits Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient to near zero. Some devices labeled as high-voltage Zener diodes are actually avalanche diodes (see above). Two (equivalent) Zeners in series and in reverse order, in the same package, constitute a transient absorber (or
Transorb, a registered trademark). The Zener diode is named for Dr.
Clarence Melvin Zener of Southern Illinois University, inventor of the device.
Light-emitting diodes (LEDs)
In a diode formed from a
direct band-gap semiconductor, such as
gallium arsenide, carriers that cross the junction emit
photons when they recombine with the majority carrier on the other side. Depending on the material,
wavelengths (or colors) from the
infrared to the near
ultraviolet may be produced. The forward potential of these diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 V to violet. The first LEDs were red and yellow, and higher-frequency diodes have been developed over time. All LEDs produce incoherent, narrow-spectrum light; “white” LEDs are actually combinations of three LEDs of a different color, or a blue LED with a yellow
scintillator coating. LEDs can also be used as low-efficiency photodiodes in signal applications. An LED may be paired with a photodiode or phototransistor in the same package, to form an
opto-isolator.
Photodiodes
All semiconductors are subject to optical
charge carrier generation. This is typically an undesired effect, so most semiconductors are packaged in light blocking material. Photodiodes are intended to sense light(
photodetector), so they are packaged in materials that allow light to pass, and are usually PIN (the kind of diode most sensitive to light). A photodiode can be used in
solar cells, in
photometry, or in
optical communications. Multiple photodiodes may be packaged in a single device, either as a linear array or as a two-dimensional array. These arrays should not be confused with
charge-coupled devices.