In electronics, an
avalanche diode is a
diodeIn electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
(made from
siliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
or other
semiconductorA semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
) that is designed to go through
avalanche breakdownAvalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche...
at a specified reverse bias
voltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
. The junction of an avalanche diode is designed to prevent current concentration at hot spots, so that the diode is undamaged by the breakdown. The avalanche breakdown is due to minority carriers accelerated enough to create ionization in the crystal lattice, producing more carriers which in turn create more ionization. Because the avalanche breakdown is uniform across the whole junction, the breakdown voltage is more nearly constant with changing current compared to a non-avalanche diode.
The
Zener diodeA Zener diode is a special kind of diode which allows current to flow in the forward direction in the same manner as an ideal diode, but will also permit it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage, "Zener knee voltage" or "Zener...
exhibits an apparently similar effect in addition to Zener breakdown. Both effects are actually present in any such diode, but one usually dominates the other. Avalanche diodes are optimized for avalanche effect so they exhibit small but significant voltage drop under breakdown conditions, unlike Zener diodes that keep voltage always higher than breakdown. This feature provides better surge protection than simple Zener diode and acts more like a gas discharge tube replacement. Avalanche diodes have a small positive temperature coefficient of voltage, where diodes relying on the Zener effect have a negative temperature coefficient.
Voltage reference
The voltage after breakdown varies only slightly with changing current. This makes the avalanche diode useful as a type of
voltage referenceA voltage reference is an electronic device that produces a fixed voltage irrespective of the loading on the device, power supply variations, temperature changes, and the passage of time. Voltage references are used in power supply voltage regulators, analog-to-digital converters and...
. Voltage reference diodes rated more than about 5.5 volts are avalanche diodes.
Protection
A common application is protecting
electronic circuitAn electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
s against
damaging high voltagesWhen the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous...
. The avalanche diode is connected to the circuit so that it is reverse-biased. In other words, its
cathodeA cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
is positive with respect to its
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
. In this configuration, the diode is non-conducting and does not interfere with the circuit. If the voltage increases beyond the design limit, the diode goes into
avalanche breakdownAvalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche...
, causing the harmful voltage to be conducted to ground. When used in this fashion, they are often referred to as
clamping diodesA clamper is an electronic circuit that prevents a signal from exceeding a certain defined magnitude by shifting its DC value. The clamper does not restrict the peak-to-peak excursion of the signal, but moves it up or down by a fixed value...
or transient voltage supressors because they "clamp" the maximum voltage to a predetermined level. Avalanche diodes are normally specified for this role by their clamping voltage
VBR and the maximum amount of transient energy they can absorb, specified by either energy (in
jouleThe joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
s) or
. Avalanche breakdown is not destructive as long as the diode is prevented from overheating.
RF noise generation
Avalanche diodes generate
radio frequencyRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
noise. They are commonly used as noise sources in radio equipment and
hardware random number generatorIn computing, a hardware random number generator is an apparatus that generates random numbers from a physical process. Such devices are often based on microscopic phenomena that generate a low-level, statistically random "noise" signal, such as thermal noise or the photoelectric effect or other...
s. For instance, they are often used as a source of RF for
antenna analyzerAn antenna analyzer is a device used for measuring the efficiency of antenna systems in radio electronics applications.-Theory of operation:...
bridges. Avalanche diodes can also be used as
white noiseWhite noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
generators.
Microwave frequency generation
If placed into a resonant circuit, avalanche diodes can act as
negative resistanceNegative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
devices. The
IMPATT diodeAn IMPATT diode is a form of high power diode used in high-frequency electronics and microwave devices. They are typically made with silicon carbide owing to their high breakdown fields....
is an avalanche diode optimized for frequency generation.