Photodiode Types: PN, PIN, Avalanche, and Schottky Explained

Photodiodes are essential components in modern electronics, converting light into electrical current and enabling applications across telecommunications, medical imaging, and solar energy. There are various types of photodiodes such as PN, PIN, Avalanche, and Schottky, designed to meet specific requirements in sensitivity, speed, and amplification. This article explores the photodiode basics and types, explaining their working principles, key characteristics, and practical applications including data transmission systems.

What is a Photodiode?

Introduction:

• When light falls on the photodiode, it causes the generation of a very small current. This is known as the photovoltaic effect. It works similarly to a solar panel. Hence, a solar panel is also thought of as being an array of very large photodiodes.

• In order to deliver a large amount of current, the photodiode is connected with a DC power source in reverse biased mode. This mode is known as photoconductive mode.

Figure 1 above depicts the circuit symbol and a photodiode manufactured by OSRAM.

Photodiode Working Operation

Let us understand the photodiode working operation in photovoltaic mode and photoconductive mode and derive the difference between them.

• In photovoltaic mode, when light falls on the semiconductor material of the photodiode, it can excite electrons to a higher energy state. Due to this, electrons become mobile and leave behind holes. The electrons move toward the cathode terminal of the photodiode, and the holes move toward the anode terminal. This creates voltage between the two terminals. This happens even in the absence of visible light. The tiny amount of current produced without visible light is known as dark current. It is also called zero-bias mode.

Photodiode Photovoltaic mode vs Photoconductive mode

• In photoconductive mode, when light falls on the photodiode, it creates pairs of electrons and holes in the semiconductor material. These move toward opposite directions due to the applied bias voltage. As a result, a small current flows through the photodiode. Photoconductive mode delivers a fast response compared to photovoltaic mode. This is due to the wider depletion layer and reduction of capacitance, which is a result of the applied reverse bias voltage. It is also called reverse bias mode.

Photodiode Applications

Following are the applications of Photodiode:

• Optical disc drives
• Telecommunications (Optical wireless communication)
• Infrared data transfer
• Digital cameras
• Optical switches
• It is used in various sensors such as proximity sensors, optical encoders, light meters, etc.

Types of photodiodes

Following are the classification of Photodiodes based on their construction and principles of operation.

• PN Photodiode
• P-I-N Photodiode
• Avalanche Photodiode
• Schottky Photodiode

Photodiode types-PIN photodiode,Avalanche photodiode

Regular PN Photodiode

It was the first form of photodiode. Photo detection occurs in the depletion region of the diode. It is relatively small in size and hence sensitivity is not as good compared to other photodiode types. It is suitable for low light applications due to improved electrical noise performance. It does not require reverse bias unlike the PIN photodiode. Due to the latest photodiodes, this type has become less common now.

PIN Photodiode

P-I-N diode structure

PIN Photodiode: This photodiode type has an undoped semiconductor layer (viz. intrinsic) between the p-doped and n-doped layers. Hence it is known as a PIN photodiode. It is more sensitive than a regular PN photodiode. Moreover, it has a faster response than a PN photodiode. Many of the photodiodes available now-a-days are of PIN type.

Avalanche Photodiode

Avalanche Photodiode structure

• Avalanche Photodiode: When light falls on the undoped part of the avalanche photodiode, it triggers the generation of electron-hole pairs. The migration of electrons toward the avalanche region increases their velocity due to cumulative field strength. As a result, they collide with the crystal lattice and create further pairs of electrons and holes. Due to this behavior, the avalanche photodiode is more sensitive compared to the PIN photodiode.

However, higher sensitivity makes the avalanche photodiode vulnerable to electrical noise. Moreover, it is affected by heat. To overcome these drawbacks, a guard ring is enclosed around the p-n junction of the avalanche photodiode and a heat sink is used.

Schottky Photodiode

Schottky Photodiode structure

Schottky Photodiode: Sometimes it is impossible to realize P-I-N diodes for a given wavelength band. Moreover, the performance of such diodes is not up to par to be used as optical detectors. In these situations, a Schottky barrier photodiode is used. The figure depicts the Schottky Photodiode structure. As shown, a thin metal layer replaces either the P-region or N-region of the diode. Hence it is known as a “metal-semiconductor diode”. Depending upon the semiconductor and metal, a barrier is formed at the interface of these two materials. This barrier results in the bending of the bands. Due to the application of voltage, the bands can be bent more or less. In this region of band bending, electron hole pairs can easily be separated.

Conclusion

Each type of photodiode, from the basic PN to the advanced Avalanche and Schottky, serves a unique role in electronic applications. Their distinct features; whether it’s the high sensitivity of Avalanche photodiodes or the fast response of Schottky photodiodes; enable diverse applications in fields like communication, sensing, and light detection. Understanding these photodiodes and their functionalities helps in selecting the right component for optimized performance in various technological applications.