Short Notes: Photodiode | Electronic Devices - Electronics and Communication Engineering (ECE) PDF Download

Introduction

A photodiode is a PN-junction diode that consumes light energy to produce an electric current. Sometimes it is also called a photo-detector, a light detector, and photo-sensor. These diodes are particularly designed to work in reverse bias conditions, it means that the P-side of the photodiode is associated with the negative terminal of the battery, and the n-side is connected to the positive terminal of the battery. This diode is very sensitive to light so when light falls on the diode it easily changes light into an electric current. The solar cell is also branded as a large-area photodiode because it converts solar energy into electric energy. Though, solar cell works only in bright light.

Photodiode

A photodiode is one type of light detector, used to convert the light into current or voltage based on the mode of operation of the device. It comprises optical filters, built-in lenses, and also surface areas. These diodes have a slow response time when the surface area of the photodiode increases. Photodiodes are alike to regular semiconductor diodes, but that they may be either visible to let light reach the delicate part of the device. Several diodes intended for use exactly as a photodiode will also use a PIN junction somewhat than the usual PN junction.
Some photodiodes will look like a light-emitting diode. They have two terminals coming from the end. The smaller end of the diode is the cathode terminal, while the longer end of the diode is the anode terminal. See the following schematic diagram for the anode and cathode sides. Under the forward bias condition, the conventional current will flow from the anode to the cathode, following the arrow in the diode symbol. Photocurrent flows in the reverse direction.

Types of Photodiode

Although there are numerous types of photodiode available in the market and they all work on the same basic principles, though some are improved by other effects. The working of different types of photodiodes works in a slightly different way, but the basic operation of these diodes remains the same. The types of photodiodes can be classified based on their construction and functions as follows.

• PN Photodiode
• Schottky Photo Diode
• PIN Photodiode
• Avalanche Photodiode

PN Photodiode
The first developed type of photodiode is the PN type. As compared with other types, its performance is not advanced, but at present, it is used in several applications. The photodetection mainly happens in the depletion region of the diode. This diode is quite small but its sensitivity is not great as compared with others. Please refer to this link to know more about the PN diode.

PIN Photodiode
At present, the most commonly used photodiode is a PIN type. This diode gathers the light photons more powerfully as compared with standard PN photodiode because the wide intrinsic area between the P and N regions allows for more light to be collected, and in addition to this, it also offers a lower capacitance. Please refer to this link to know more about the PIN diode.

Avalanche Photodiode
This kind of diode is used in low light areas due to its high gain levels. It generates high levels of noise. So this technology is not appropriate for all applications. Please refer to this link to know more about the Avalanche diode.

Schottky Photodiode
The Schottky photodiode uses the Schottky diode, and it includes a small diode junction that means, there is small junction capacitance so, it operates at high speeds. Thus, this kind of photodiode is frequently utilized in high bandwidth (BW) optical communication systems like fiber-optic links. Please refer to this link to know more about the Schottky diode.
Each type of photodiode has its own benefits and drawbacks. The selection of this diode can be done based on the application. The different parameters to be considered while selecting photodiode include noise, wavelength, reverse bias constraints, gain, etc. The performance parameters of photodiode include responsivity, quantum efficiency, transit time or response time.
These diodes are widely used in applications where the detection of the presence of light, color, position, the intensity is required. The main features of these diodes include the following.

• The linearity of the diode is good with respect to incident light
• Noise is low.
• The response is wide spectral
• Rugged mechanically
• Lightweight and compact
• Long life

The required materials to make a photodiode and the range of electromagnetic spectrum wavelength range includes the following

• For silicon material, the electromagnetic spectrum wavelength range will be (190-1100) nm
• For Germanium material, the electromagnetic spectrum wavelength range will be (400-1700) nm
• For Indium gallium arsenide material, the electromagnetic spectrum wavelength range will be (800-2600) nm
• For Lead (II) sulfide material, the electromagnetic spectrum wavelength range will be <1000-3500) nm
• For Mercury, cadmium Telluride material, the electromagnetic spectrum wavelength range will be (400-14000) nm

Because of their better bandgap, Si-based photodiodes produce lower noise than Ge-based photodiodes.

Construction

The photodiode  is made using two semiconductors like P-type & N-type. In this design, the formation of P-type material can be done from the diffusion of the P-type substrate which is lightly doped. So, the P+ ions layer can be formed because of the diffusion method. On the substrate of N-type, the N-type epitaxial layer can be grown.

The development of a P+ diffusion layer can be done over the heavily doped N-type epitaxial layer. The contacts are designed with metals to make two terminals like anode and cathode. The front region of the diode can be separated into two types like active & non-active surfaces.
The designing of the non-active surface can be done with silicon dioxide (SiO2). On an active surface, the light rays can strike over it whereas, on a non-active surface, the light rays cannot strike. & the active surface can be covered through the material of anti-reflection so that the energy of light cannot lose and the highest of it can be changed into the current.

Working of Photodiode

The working principle of a photodiode is, when a photon of ample energy strikes the diode, it makes a couple of an electron-hole. This mechanism is also called the inner photoelectric effect. If the absorption arises in the depletion region junction, then the carriers are removed from the junction by the inbuilt electric field of the depletion region.

Therefore, holes in the region move toward the anode, and electrons move toward the cathode, and a photocurrent will be generated. The entire current through the diode is the sum of the absence of light and the photocurrent. So the absent current must be reduced to maximize the sensitivity of the device.

Modes of Operation

The operating modes of the photodiode include three modes, namely Photovoltaic mode, Photoconductive mode, an avalanche diode mode

Photovoltaic Mode: This mode is also known as zero-bias mode, in which a voltage is produced by the lightened photodiode. It gives a very small dynamic range & non-linear necessity of the voltage formed.

Photoconductive Mode: The photodiode used in this photoconductive mode is more usually reverse biased. The reverse voltage application will increase the depletion layer’s width, which in turn decreases the response time & the junction capacitance. This mode is too fast and displays electronic noise

Avalanche Diode Mode: Avalanche diodes operate in a high reverse bias condition, which permits the multiplication of an avalanche breakdown to each photo-produced electron-hole pair. This outcome is an internal gain in the photodiode, which slowly increases the device response.

Why is Photodiode Operated in Reverse Bias?
The photodiode operates in the mode of photoconductive. When the diode is connected in reverse bias, then the depletion layer width can be increased. So this will diminish the capacitance of the junction & the response time. In fact, this biasing will cause quicker response times for the diode. So the relation between photocurrent & illuminance is linearly proportional.

Which is better Photodiode or Phototransistor?
Both the photodiode and phototransistor are used for converting the energy of light to electrical. However, the phototransistor is more responsive as contrasted to the photodiode due to the utilization of the transistor.
The transistor changes the base current which causes due to light absorption & therefore the huge output current can be gained throughout the collector terminal of the transistor. The photodiodes time response is very fast as compared with the phototransistor. So it is applicable where fluctuation in the circuit occurs. For better understating, here we have listed out some points of photodiode vs photoresistor.

Photodiode Circuit

The circuit diagram of the photodiode is shown below. This circuit can be built with a 10k resistor and photodiode. Once the photodiode notices the light, then it allows some flow of current throughout it. The sum of current that supplies through this diode can be directly proportional to the sum of light noticed through the diode.

Connecting a Photodiode in an External Circuit
In any application, the photodiode works in reverse bias mode. The anode terminal of the circuit can be connected to the ground whereas the cathode terminal is connected to the power source. Once illuminated through light, then current flows from the cathode terminal to the anode terminal.
Once photodiodes are utilized with exterior circuits, then they are allied to a power source within the circuit. So, the amount of current generated through a photodiode will be extremely small, so this value is not sufficient to make an electronic device.
Once they are connected to an exterior power source, then it delivers more current toward the circuit. In this circuit, the battery is used as a power source to help in increasing the value of current so that external devices give a better performance.

Photodiode Efficiency
The photodiode’s quantum efficiency can be defined as the division of the absorbed photons which donate to the photocurrent. For these diodes, it is openly associated with the responsivity ‘S’ with no effect of an avalanche, then the photocurrent can be expressed as

Where,
‘η’ is the quantum efficiency
‘e’ is the charge of the electron
‘hν’ is the energy of the photon
Photodiodes’ quantum efficiency is extremely high. In some cases, it will be above 95% however changes extensively through wavelength. High quantum efficiency requires the control of reflections apart from a high inner efficiency like an anti-reflection coating.

Responsivity
The responsivity of a photodiode is the ratio of the photocurrent which is generated as well as absorbed optical power can be determined within the linear section of the response. In photodiodes, it is normally maximum in a wavelength area wherever the photon energy is fairly higher than the bandgap energy & declining within the bandgap region wherever the absorption reduces.
The photodiode calculation can be done based on the following equation

Here, in the above equation, ‘h ν’ is the energy of photon; ‘η’ is the efficiency of quantum & ‘e’ the charge of elementary. For instance, the quantum efficiency of a photodiode is 90% at an 800 nm wavelength, then the responsivity will be 0.58 A/W.
For photomultipliers & avalanche photodiodes, there is an extra factor for the multiplication of inner current, so that possible values will be above 1 A/W. Generally, the multiplication of current is not included within the quantum efficiency.

PIN Photodiode Vs PN Photodiode

Both the photodiodes like PN & PIN can be attained from a lot of suppliers. A photodiode selection is very important while designing a circuit based on the required performance as well as characteristics.
A PN photodiode doesn’t work in a reverse bias & consequently, it is more appropriate for the applications of low light to enhance the performance of noise.
The PIN photodiode that works in reverse bias can introduce a noise current to decrease S/N ratio
For the applications of high dynamic range, the reverse biasing will give good performance
For high BW applications, reverse biasing will provide good performance like the capacitance among the regions of P & N and the storage of charge capacity is small.

Advantages

The advantages of photodiode include the following.

• Less resistance
• Quick and high operation speed
• Long life span.
• Fastest photodetector.
• Spectral response is good.
• Doesn’t use high voltage.
• Frequency response is good.
• Solid and low-weight.
• It is extremely responsive to the light.
• Dark current is less.
• High quantum efficiency.
• Less noise.

Disadvantages

The disadvantages of photodiode include the following.

• Temperature stability is poor.
• Change within current is extremely little, therefore may not be enough to drive the circuit.
• The active area is small.
• Usual PN junction photodiode includes a high response time.
• It has less sensitivity.
• It mainly works by depending on the temperature.
• It uses offset voltage.

Applications of Photodiode

• The applications of photodiodes involve similar applications of photodetectors like charge-coupled devices, photoconductors, and photomultiplier tubes.
• These diodes are used in consumer electronics devices like smoke detectors, compact disc players, and televisions and remote controls in VCRs.
• In other consumer devices like clock radios, camera light meters, and street lights, photoconductors are more frequently used rather than photodiodes.
• Photodiodes are frequently used for exact measurement of the intensity of light in science & industry. Generally, they have an enhanced, more linear response than photoconductors.
• Photodiodes are also widely used in numerous medical applications like instruments to analyze samples, detectors for computed tomography, and also used in blood gas monitors.
• These diodes are much faster & more complex than normal PN junction diodes and hence are frequently used for lighting regulation and in optical communications.

V-I Characteristics of Photodiode
A photodiode continually operates in a reverse bias mode. The characteristics of the photodiode are shown clearly in the following figure, that the photocurrent is nearly independent of reverse bias voltage which is applied. For zero luminance, the photocurrent is almost zero excluding for small dark current. It is of the order of nano amperes. As optical power rises the photocurrent also rises linearly. The max photocurrent is incomplete by the power dissipation of the photodiode.