What is Avalanche Photodiode : Working & Its Applications

The avalanche photodiode or APD was designed by a Japanese engineer namely “Jun-ichi Nishizawa” in the year 1952. An APD is a very responsive semiconductor detector that used the photoelectric effect to change light into electricity. In 2020, a graphene layer is added to this diode to avoid degradation eventually to maintain these diodes.

In fiber-optic communication systems, the light is changed into electrical signals using a single component like avalanche photodiode or APD. In the avalanche process, charge carriers are produced through collisions. A light particle-like photon generates many electrons to produce an electric current. This article discusses an overview of what is avalanche photodiode and its works with applications.

What is Avalanche Photodiode?

The diode which uses the avalanche method to provide extra performance as compared to other diodes is known as avalanche photodiode.

These diodes are used to change the signals from optical to electrical. These diodes can be operated in high reverse bias. The avalanche photodiode symbol is similar to the Zener diode.

Avalanche Photodiode Construction

The construction of both the PIN photodiode and Avalanche photodiode is similar. This diode includes two heavily doped & two lightly doped regions. Here, heavily doped regions are P+ & N+ whereas lightly doped regions are I & P.

In the intrinsic region, the depletion layer width is fairly thinner in this diode as compared to the PIN photodiode. Here, the p+ region works like the anode whereas the n+ region acts as the cathode.

As compared to other photodiodes, this diode works in a high reverse bias condition. So this allows avalanche multiplication of the charge carriers formed through the light impact or photon. The avalanche action allows the gain of the photodiode to be enhanced several times to provide a high range of sensitivity.

Working Principle

Avalanche breakdown occurs mainly once the photodiode is subjected to maximum reverse voltage. This voltage enhances the electric field beyond the depletion layer. When incident light penetrates the p+ region then it gets absorbed within the extremely resistive p region then electron-hole pairs are generated.

Charge carriers drift including their saturation velocity to the pn+ region wherever a high electric field exists. When the velocity is highest, then charge carriers will collide through other atoms & produce new electron-hole pairs. A huge charge carrier’s pair will result in high photocurrent.

Avalanche Photodiode Operation

This diode operation can be done in a depleted mode completely. However, they can also work in the Geiger mode in addition to the linear avalanche mode. In this type of operation mode, the photodiode can be operated at the above breakdown voltage. At present, another mode is launched namely “Sub-Geiger mode”.

Avalanche Photodiode in Optical Fiber Communication

In optical fiber communication (OFC) systems, avalanche photodiodes are generally used for the recognition of weak signals but circuits need to optimize enough so that high Signal to noise ratio (S/N). Here, SNR is

S/N = Power from the photocurrent/photo detector’s power + Amplifier noise power

For obtaining the perfect signal-to-noise ratio, quantum efficiency should be high because this value is almost maximum, so most of the signals are noticed.

Avalanche Photodiode Characteristics

Avalanche photodiodes are highly sensitive, high-speed-based diodes which use an internal gain method that works through applying a reverse voltage. As compared to PIN type photodiode, these diodes measure low range light so used in different applications where high sensitivity is required like measurement of optical distance and optical communication for long-distance.

There are different avalanche photodiode families which are designed mainly for detecting short wavelengths otherwise near-infrared.

What is the Difference between Pin Photodiode and Avalanche Photodiode?

The difference between photodiode and avalanche diode includes the following.

What is the response time of avalanche photodiode?

Avalanche photodiode has a fast response time.

What is the dark resistance of photodiode?

A selenium cell otherwise other photoelectric device’s resistance within complete darkness is known as Dark Resistance.

Where are avalanche diodes used?

Avalanche diodes are mainly used as white noise generators and noise sources within radio gears. This diode protects the circuit against unwanted voltages.

Is Photodiode Reverse Biased?

Yes, it is reverse biased to operate within the photoconductive mode because when this diode is reverse biased then the depletion layer’s width will be increased. So this decreases the junction capacitance & the response time. The reverse bias can cause a quick response time for this diode.

What are Avalanche and Zener Breakdown Phenomena?

The Avalanche & Zener Breakdown are two dissimilar mechanisms where a PN junction breaks. This mechanism mainly occurs within the diode in reverse bias conditions. The avalanche breakdown mainly occurs due to the electrons ionization & hole pairs while the Zener breakdown takes place due to heavy doping.

Advantages & Disadvantages

The advantages of avalanche photodiode include the following.

• The sensitivity range is high.
• High performance.
• Quick response time.
• These diodes are applicable here the gain level is very important as the high voltage required, through lower reliability means that they are frequently less convenient to utilize.
• It detects low-intensity light.
• A single-photon generates a huge number of charge carrier pairs.

The disadvantages of avalanche photodiode include the following.

• The required operating voltage is high
• The output of this diode is not linear
• High range of noise
• It is not used regularly because of the low reliability
• It uses high reverse bias for its proper operation

Applications

The applications of avalanche photodiode include the following.

• LASER scanner
• Analyzer bridge of antenna
• PET scanner
• Barcode reader
• Laser microscopy
• Laser Rangefinders
• Speed gun
• APDs are used in receivers of OFC (optical fiber communications), imaging, finding the range, laser microscopy, laser scanners & OTDR (optical-time domain reflectometers).
• These are used in optical communications like receiving detectors. Their wide bandwidth & high sensitivity will make it very famous with designers. These diodes work through a reverse voltage beyond the junction that allows the formation of charge carrier pairs in reply to the radiation.

Thus, this is all about an overview of an avalanche photodiode and it’s working. This is a two-terminal PN junction diode that works in the reverse breakdown region. These photodiodes include high SNR (signal-to-noise ratio) as compared to PIN photodiodes, quick time response, high sensitivity & less dark current. Its spectral response range is normally from 200 to 1150 nm. Here is a question for you, what are the different types of a photodiode?