This entry is part 2 of 2 in the series Diode

Avalanche diode is a subcategory of diode designed to work in reverse bias and go through avalanche breakdown process at a certain voltage. Before going to the details of diode it will be quite interesting to know the origin of name itself.

Avalanche or Snow slide is a natural phenomenon when a small mass of snow slab triggers a massive amount of snow to barrel down the mountainside. The trigger of such an avalanche is a fracture in a cohesive snow slab lying Upon a week layer of snow. The gravitational pull or the potential energy of the slab at height creates a downward movement which sets the whole layer in action.

Avalanche in action

Extending the same analogy to the semiconductor when a single charge carrier particle knocks out many such particles due to an electrical potential and they all keep knocking out furthermore particles. This creates a massive current in that device. This phenomenon can cause catastrophic failure unless the device is designed to handle such a phenomenon also known as avalanche breakdown.

Avalanche breakdown is not only limited to semiconductors but can also occur in insulating materials. It can exponentially increase the charge carrier concentration causing the heavy electric current to flow even from an insulating material. The only required prerequisite is the high electric potential, which increases the momentum of travelling charged particle e.g. electron to such a great extent that they possess more energy than the valance bound.

Construction of avalanche diode is similar to Zener diode accept the fact that these are doped pretty lightly. Thus the depletion region is very wide. Low doping concentration helps the diode to withstand very high reverse voltage applied to it without breaking down. It also provides the required electric potential for Avalanche multiplication to take place. Very uniform doping is required during the manufacturing process as any current concentration can create a hotspot in the diode leading to burnout and permanent damage. Breakdown voltage is also controlled by controlling the amount of doping during the manufacturing process itself. Lighter the doping, higher is the breakdown voltage.

Working of Avalanche diode depends majorly on minority charged carriers while majority charge carriers creates a perfect environment for an avalanche to take place. When the reverse bias voltage is applied to avalanche diode, then the majority charge carriers are moved away from the junction increasing the size of the depletion region. This blocks any current flow and withstands a higher applied potential. Although minority charge careers keep flowing(reverse leakage current). While passing through the depletion region, these are exposed to high potential difference. The velocity of minority carriers increases and they acquire enough energy to knock out electron-hole pairs in this region. All newly created charge particles keep accelerating in the influence of electric potential leading to a chain reaction and massive current flow in a very short span of time. Going analogous to snow-slide, the minority charge carrier can be thought as snow slab, while majority charged carrier in depletion region will be comparable to layers of snow, and the electric potential as the gravitational pull.

The phenomenon of Avalanche is very quick hence this diode acts as the fastest surge suppression device. These are even faster than a MOV’s, Zeners and the gas tubes, hence use of these can result in greater system reliability. Application of avalanche diode is not only limited to protection circuits. These diodes generate radio frequency noises and often used as a source of RF for antenna analysers. In a resonant circuit, this can also act as a negative resistance. Another special application can be a very sensitive optical receiver. With very controlled doping these can be made so sensitive that it can detect the presence of single photon. also known as a single Photon Avalanche diode.

With avalanche in control let’s make the electron drive safer through semiconductor Valley even in turbulent potential conditions. Why not browse some applications of Avalanche diode as “The single-photon avalanche detector”, “The Surge protecting-Avalanche”, “White RF noise of avalanche diode” or find out other articles related to different diode types.

By Purnendu Kumar

Purnendu is currently working as Senior Project Engineer at QuNu Labs, Indias only Quantum security company. He has submitted his thesis for Masters (MS by research 2014-17) in electrical engineering at IIT Madras for doing his research on “constant fraction discriminator” and “amplitude and rise-time compensated discriminator” for precise time stamping of Resistive Plate Chamber detector signals. In collaboration with India Based Neutrino observatory project, he has participated in design and upgrade of FPGA-based data acquisition system, test-jig development, and discrete front-end design. After completion of his bachelors in Electrical Engineering (Power and Electronics), he was awarded Junior Research Fellowship at Department of Physics and Astro-physics, University of Delhi under same (INO) project. His current interest is in high-speed circuit design, embedded systems, IoT, FPGA implementation and optimization of complex algorithms, experimental high-energy physics, and quantum mechanics.