Fundamentals of photodiode you need to learn

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At a glance: Photodiode

In today’s blog, we will learn about the Types of Photodiode, the Characteristics of Photodiode, modes of operation such as PHOTOVOLTAIC MODE and PHOTOCONDUCTIVE MODE also the Application of Photodiode.

Ever wonder why photodiode is not operated in forward bias condition ?

Introduction

The photodiode is an optoelectric device that converts an optical signal into an electric current.

The photodiode operated only in reverse bias conditions and not in forward bias conduction. The reason behind this is explained in the later part of the blog.

Conversion process

The incident light on semiconductor-generated carriers’ extraction causes the current flow in the external circuit. As a result, the carriers are transported through semiconductor electrodes.

Photoelectric effect

The photodiode works on the photoelectric effect.

 The emission of electrons when electromagnetic radiation (light) hits the material is known as the photoelectric effect. The emitted electrons are known as photoelectrons. 

This photoelectric effect was discovered by German physicist Henrich Rudolf Hertz in 1887

Construction

The depletion region is formed by diffusion of electrons from n- layer to player and diffusion of holes from Player to N- layer. Thus, the P-layer is made from a P-type semiconductor, and the N- layer is formed by N-type semiconducting material.

The P and N layers are thin, and most of the absorption is in the intrinsic region or lightly doped N region.

The depletion region is the region between two layers where there are no free charge carriers.

This region develops a built-in voltage to create an electric field across the depletion region allows the current to flow through anode-cathode in one direction.

The depletion region created between two layers acts as a capacitance. The P – layer and N- layer boundaries act as parallel plated capacitance, and this capacitance is inversely proportional to the width of the depletion region.

Types of Photodiode

Types of Photodiode, the Characteristics of Photodiode, modes of operation such as PHOTOVOLTAIC MODE and PHOTOCONDUCTIVE MODE also the Application of Photodiode.

PN Photodiode

PN photodiode has two layers P+ active area and N-type substrate, and between these layers, there is a depletion region. In the depletion region of the diode, photodetection occurs. However, it is relatively small in size and due to which sensitivity is low. 

The main disadvantage of PN photodiode is that the width of the depletion region is small and depends upon the doping concentration of the semiconductor.

PIN Photodiode

Types of Photodiode, the Characteristics of Photodiode, modes of operation such as PHOTOVOLTAIC MODE and PHOTOCONDUCTIVE MODE also the Application of Photodiode.

 To overcome the disadvantage of the small depletion region of PN photodiode, the PIN photodiode has a doped intrinsic semiconductor layer between p-doped and n-doped layers. Hence it is called a PIN photodiode. Sensitivity is more than regular PN photodiode. The response is faster than the PN photodiode. The PIN photodiode is similar to the P-N Junction photodiode, but the difference is that the intrinsic layer is placed between the two doped layers. Instead of putting the P and N layers together to create a depletion region.

 Avalanche Photodiode:

Types of Photodiode, the Characteristics of Photodiode, modes of operation such as PHOTOVOLTAIC MODE and PHOTOCONDUCTIVE MODE also the Application of Photodiode.Types of Photodiode, the Characteristics of Photodiode, modes of operation such as PHOTOVOLTAIC MODE and PHOTOCONDUCTIVE MODE also the Application of Photodiode.

 Avalanche photodiode uses the avalanche effect (impact ionization) to generate current.

 When light falls on the avalanche region of the avalanche photodiode, the generation of electron-hole pairs begins. The electrons migrating toward the avalanche region have increased velocity due to cumulative field strength. 

Due to which this, electrons collide with crystal lattice and create pairs of electrons and holes. That is why avalanche photodiode is more sensitive compare to PIN photodiode. On the other hand, it is affected by heat. Therefore, a guard ring is enclosed around the p-n junction of the avalanche photodiode, and a heat sink is used to overcome these drawbacks.

Working 

“PHOTOVOLTAIC” MODE – UNBIASED

Photodiodes can be operated without any voltage bias. This mode of operation is known as the PHOTOVOLTAIC mode. When the diode is unbiased, the dark current is near zero. (Dark Current – the current that flows through the device in the absence of radiation). In this mode, the current flow is due to the radiation falling on the junction, creating electron and hole pairs causing the current to flow.

Compared to the biased mode, the photovoltaic mode has less variation of photocurrent responsively with temperature. However, on the other hand, unbiased photodiodes have a slow response speed. 

The avalanche diode can be operated in the unbiased condition because it worked only in reverse bias, so this condition is only suitable for PIN photodiode and PN photodiode.

“PHOTOCONDUCTIVE” MODE- REVERSE BIASED

When the photodiode is reverse bias, the operating mode is known as PHOTO CONDUCTIVE mode. That P+ active area is connected to the battery’s negative terminal, and N-type substrate is connected to the battery’s positive terminal.

Part 1 -When a junction is not exposed to radiation

When the junction is not exposed to radiations (Light falls on junction) due to the minority carriers, dark current flows through the diode. (Dark Current – the current that flows through the device in the absence of radiation). This generates a depletion region of neutral atoms, causing a block in the flow of electrons and holes around the junction.

We already studied the reason behind this in the band theory of semiconductors; the conduction band and valance band are separated by bandgap. The conduction band contains free electrons, but the conduction band is empty when the junction is not exposed to radiation. On the other hand, the valance band is filled with electrons. This condition causes no charge carrier for conduction, and therefore without a charge carrier, there will be no conduction.

 Part 2 -When a junction is exposed to radiation

When the junction is exposed to radiation (Light falls on the junction), the temperature of the junction rises. This rise in temperature causes electrons and holes to separate from each other. During this separation, holes are attracted to the negative terminal, and electrons are attracted to the positive terminal.

On the semiconductor band level, when the junction is exposed to light if the photon has more energy than the bandgap generated during the absence of light.

 Then at that time, some of these electrons can be absorbed.

 And Jump into the conduction band from the valance band, resulting from which we notice the generation of free electrons and holes.

In the depletion region, electron-hole pairs are formed. This causes the current to flow through the diode. After reading this, you might understand that as the radiation increases, the flow of current increases. That is, we can say that current is directly proportional to the intensity of light.

The brighter the light, the more the photons are absorbed, which leads generation of more electron-hole pairs per second, leading to an increase in the current flow.

Photodiode in forward bias condition

Do you ever wonder why it is not operated in forward bias condition ?

There are three main reasons behind this.

  1. If we want that photon to generate electron-hole pair, which contributes to the current that has to be formed in the depletion region; otherwise, they will just recombine and not contribute to the current, which meant the photon effect will be useless.
  2. When the photodiode is forward, the biased depletion region becomes narrower. The electron-hole pair formed has fewer chances of being created in the depletion region, and result as most of the photon absorbed is not in use and will not contribute to current flow.
  3. In the forward bias condition, the depletion region becomes smaller. These holes and electrons will also start diffusing, which starts current flow, and we want current flow due to photons.

Characteristics of photodiode

As we operate photodiode in reverse bias condition only so the characteristics are obtained in the third quadrant.

When the junction is reverse biased but not exposed to the radiation at that time, only a small dark current will flow.

When the junction is exposed to radiation, the flow of current increases and becomes independent of applied voltage .the flow of the charge carrier will be only due to the intensity of light due to the temperature rise.

After the illumination, the curve shows equal spacing because the intensity of incident light is directly proportional to the current flowing through the diode.

Applications

  1. The photodiode automatically switches things, from the light attached to the circuit to the automatic street light. When the current is below a certain level, the light will be on, and when the current is above a certain level, the lights will automatically get off.
  2. Used in safety equipment like smoke and fire detector.
  3. Photodiodes are also used in solar cells.
  4. Optical communication and lightning regulation.

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