Photodiode basics | Photodiode types, applications, Working

This article covers Photodiode basics and Photodiode types including its applications and working operation. It describes different types of Photodiodes viz. PN photodiode, PIN photodiode, Avalanche photodiode, Schottky photodiode etc.

What is Photodiode?

Introduction:
• When light falls on the photodiode, it causes generation of very small current. This is known as photo-voltaic effect. It works similar to solar panel. Hence solar panel is also thought of being array of very large photodiodes.
• In order to deliver large amount of current, photodiode is connected with DC power source in reverse biased mode. This mode is known as photoconductive mode.

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

Photodiode Working Operation

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

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

Photodiode Types | Types of photodiodes

Following are the types of Photodiodes based on their construction and principles of operation.
• PN Photodiode
• P-I-N Photodiode
• Avalanche Photodiode
• Schottky Photodiode

Regular PN Photodiode

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

PIN photodiode

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

Avalanche Photodiode

• Avalanche Photodiode: When light falls on undoped part of the avalanche photodiode, it triggers generation of electron-hole pairs. The migration of electrons toward avelanche region increases their velocity due to cumulative field strength. As a result they collide with crystal lattice and create further pairs of electrons and holes. Due to this behaviour, avalanche photodiode is more sensitive compare to PIN photodiode. However higher sensitivity makes avalanche photodiode vulnerable to electrical noise. Moreover it is affected by heat. To overcome this drawbacks, guard ring is enclosed around p-n junction of avalanche photodiode and heat sink is used.

Schottky Photodiode

Schottky Photodiode: Sometimes it is impossible to realize P-I-N diodes for given wavelength band. Moreover performance of such diodes are not par to be used as optical detectors. In these situations, Schottky barrier photodiode is used.

The figure depicts Schottky Photodiode structure. As shown thin metal layer replaces either P-region or N-region of the diode. Hence it is known as "metal-semiconductor diode".

Depending upon semiconductor and metal, a barrier is formed at the interface of these two materials. This barrier results into bending of the bands. Due to application of voltage, the bands can be bended more or less. In this region of band bending, electron hole pairs can easily be separated.

Also refer difference between P-I-N photodiode vs Avalanche photodiode vs Schottky photodiode and advantages and disadvantages of Photodiode >> for more information.

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