Avalanche Photodiode

An Avalanche photodiode (APD) is a highly sensitive semiconductor detector that uses the photoelectric effect to convert optical signals into electrical signals. In case of conventional photodiodes, PIN photodiodes operate in a linear mode and the output current is very small and also has limited gain. Whereas the APD operates in reverse bias and uses avalanche breakdown to amplify weak optical signals for higher sensitivity due to this process of detecting even the faint signals, the output currents are very large. It is also called a reach-through APD because this photodiode allows the electric field to extend or reach through the entire depletion region.

Symbol of APD (Avalanche Photodiode)

Its symbol is a graphical representation generally used in circuit diagrams. It contains an Anode and a Cathode

Construction of Avalanche Photodiode

The construction of avalanche photodiode is given below in the diagram

It has a p+ i-p-n+ Configuration as shown in the diagram. The construction of PIN photodiode and APD is similar is contains two heavily doped and two lightly doped regions. p+ and n+ are the heavily dopes regions whereas i and p are lightly doped regions. In the intrinsic region(i), the depletion layer width is fairly thinner in this photodiode as compared to PIN photodiode. Here p+ region acts like anode and n+ region acts like cathode. Here this p layer is having high resistivity, so all the reverse bias is mostly applied across p and n+ region. When the reverse bias is increased, the width of the depletion layer also gets increased.

Types of Avalanche Photodiode

There are 3 main types of APD

• Silicon APD
• InGaAs APD
• Ge APD

1) Silicon APD

They are sensitive to wavelengths in the visible to near-infrared range which is around 400-1100 nm.

It has p+ i-p-n+ construction, contains highly doped p+ and n+ regions and lightly doped I and p regions. The intrinsic (i) layer is where the main depletion region where photon absorption and electron-hole pair generation occur and the material used in intrinsic layer is silicon (Si). The p+ and n+ contacts are connected to provide efficient charge collection and to create electric field which is necessary for Avalanche multiplication process.

2) InGaAs APD

Indium Gallium Arsenide (InGaAs) APDs are suitable for use in near-infrared to short-wave infrared range, which is about 900-1700 nm.

Here in this type the basic structure is almost similar to silicon APD and the difference is there is a multiplication layer is added which is doped with Indium Phosphide (InP) material, through which device performance is enhanced by amplification of the electric field strength, having improved gain, lesser noise, and optimized for specific wavelength sensitivity. The intrinsic layer is doped with Indium Gallium Arsenide (InGaAs) material. The p layer which is lightly doped is also doped with a layer of InP material.

3) Ge APD

Ge APDs are sensitive to wavelengths in the mid-infrared range, typically from 800 nm up to around 1800 nm

The structure is similar to that of silicon APD whereas the key difference is in the intrinsic layer, material used is Germanium (Ge) which has smaller bandgap compared to silicon.

Working Principle of Avalanche Photodiode

The Reverse bias is applied to the photodiode which is near to the breakdown value. The incident light produces the electron hole pairs and these carriers travels with their saturation velocity. Now when they travel with the maximum velocity, they will colloid with the lattice. So new electron hole pairs are generated. These newly generated carriers travels along with initial carriers. Thus the multiplication of charge carriers takes place which increases the current. This process of generating more number of charged carriers is called as impact ionization.

Difference Between PIN Photodiode and Avalanche Photodiode

In case