Before we dive into optoelectronic devices, let us first discuss what optoelectronics is.

Optoelectronics is the branch of electronics that combines optics and electronics. It is one of the fast-emerging technology fields. Optoelectronics makes use of the quantum mechanical effect of light. This property is used mainly in the materials that are used in the manufacture of semiconductors. It deals with applying electronic devices to the sourcing, detection, and control of light.

Now, we jump into optoelectronic devices. These are the devices that deal with optoelectronics technology. Optoelectronic devices can be categorized into two: light-sensitive devices and light-generating devices. Some examples of light-sensitive devices are photodiodes, phototransistors, light-sensitive resistors, photovoltaic cells, and more. Examples of light-generating devices include Light Emitting Diode (LED), cathode ray tube (CRT), and more. Such devices are used for numerous purposes, including telecommunications, monitoring and sensing, medical equipment, and general science.

Optoelectronics is widely used nowadays. It has many advantages: one of them is that it consumes less power. It helps even the military and aerospace industry immensely and gives a new dimension in designing satellites of the future. However, optoelectronics also has disadvantages. Optoelectronic devices are temperature-sensitive.

In this article, we will look more closely at some of the most common electronic devices: photoresistors, photodiodes, laser diodes, phototransistors, and optocouplers.


Photoresistors—the combination of photons (light particles) and resistors—are components made of semiconductors. Because it is dependent on light, it is definitely sensitive to light. When light hits this material, it absorbs the radiation, and electrons move from the semiconductor’s valance band to the conduction band. The more electrons in the resistor’s conduction band, the less the resistance of the resistor. Simply put, its resistance decreases when lighting increases.

Photoresistors are made of silicon or germanium, which are high resistance semiconductors. Aside from these two, photoresistors can also be made of other materials such as cadmium sulfide or cadmium selenide. In the absence of light, the photoresistors act as high resistance materials, whereas, in the presence of light, the photoresistors act as low resistance materials. Photoresistors have multiple uses, for example, for an automatic door opening or illumination control.

Types of Photoresistors

  • Intrinsic photoresistors – These photoresistors are made from pure semiconductor materials such as silicon or germanium. The resistance decreases slightly with the increase in light energy. Hence, intrinsic photoresistors are less sensitive to light. Therefore, they are not reliable for practical applications.
  • Extrinsic photoresistors – These photoresistors are made from extrinsic semiconductor materials. In extrinsic photoresistors, there is a large number of charge carriers. Providing a small amount of light energy generates a greater number of charge carriers. Thus, the electric current increases rapidly.
Photoresistor symbol


Photodiodes are semiconductor devices that convert light into an electrical current. Current is generated when photons are absorbed in the photodiode. Photodiodes may have large or small surface areas. They may also contain optical filters and built-in lenses. Photodiodes usually have a slower response time as their surface area increases. Photodiodes are similar to regular semiconductor diodes except that they may be either exposed or packaged with a window or optical fiber connection to allow light to reach the device’s sensitive part.

Photodiodes are a type of semiconducting devices with a PN junction. Between the p (positive) and n (negative) layers, there is an intrinsic layer. The photodiode accepts light energy as input to generate an electric current. This semiconductor light sensor generates electricity or voltage when light touches the junction (active p-n junction) and operates in reverse bias. When an excited photon strikes the photodiode, electron-hole pairs are created. Electrons then diffuse into the p-n junction to generate an electric field.

For simple day-to-day applications, photodiodes can be used. Photodiodes are applied in safety electronics like fire and smoke detectors. It is also used in TV units. When utilized in cameras, they act as photosensors. It is used as photoconductors and photomultiplier tubes. Photodiodes are also widely used in numerous medical applications like instruments to analyze samples, detectors for computed tomography, and blood gas monitors.

How to Read Schematics - Photodiode
Photodiode symbol

Laser Diodes

Laser Diodes

A laser diode is a semiconductor laser device similar to a light-emitting diode (LED) in both form and operation. Laser actually means Light Amplification by Stimulated Emission of Radiation. It is a source of directional, coherent, and highly monochromatic light. It functions under the conditions of stimulated emission. When a voltage is applied across the P-N junction, the electron population inversion and the laser beam are available from the semiconductor region. The P-N laser diode ends have a polished surface, and the emitted photons reflect on creating pairs. Thus, the photons generated will be in phase with the previous photons.

Laser diodes are the most common type of lasers produced, with a wide range of uses that include fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray disc reading/recording, laser printing, laser scanning, and light beam illumination. It can also be used in security systems and autonomous vehicles, and fiber optic communications. Laser Diodes are used in all major electronics, ranging from consumer electronics, medical machines, autonomous vehicles, scientific instrumentation, industrial applications, and many more.

How Does It Work

A laser diode works in three steps:

1. Energy Absorption – When a certain voltage is applied at the laser diode’s p-n junction, the electrons absorb energy, and they transition to a higher energy level. Holes are formed at the original position of the excited electron. The electrons stay in this exciting state without recombining with holes for a minimal duration of time.

2. Spontaneous Emission – After the upper-state lifetime of excited electrons, they recombine with holes. As the electrons fall from a higher energy level to a lower energy level, the difference in energy is converted into photons or electromagnetic radiation. The energy of the emitted photon is given by the difference between the two energy levels.

3. Stimulated Emission – A partially reflecting mirror is used on either side of the diode so that the photons released from spontaneous emission are trapped in the p-n junction until their concentration reaches a threshold value. These trapped photons stimulate the excited electrons to recombine with holes. Once the photon concentration goes above a threshold, they escape from the partially reflecting mirrors, resulting in a bright monochromatic coherent light.

Types of Laser Diodes

  • Double Heterostructure Laser Diode
  • Quantum Well Laser Diode
  • Quantum Cascade Laser Diode
  • Inter-band Cascade Laser Diode
  • Separate Confinement Heterostructure Laser Diode
  • Distributed Bragg Reflector Laser Diode
  • Distributed Feedback Laser Diode
  • Vertical External Cavity Surface Emitting Laser Diode (VCSEL)


Phototransistors are a form of bipolar transistor that is sensitive to light. These are more sensitive than photodiodes. Phototransistors are semiconductor devices that can sense light levels and alter the current flowing between emitter and collector according to the level of light it receives. Phototransistors are operated in their active regime, although the base connection is generally left open circuit or disconnected because it is often not required. The phototransistor base would only be used to bias the transistor so that additional collector current was flowing, and this would mask any current flowing. The collector of an NPN transistor is made positive concerning the emitter or negative for a PNP transistor. Applications of phototransistors include lighting control, alarm systems, level indicators, proximity detectors, punch-card readers, and encoders.

NPN Phototransistor

Phototransistor Operation

Phototransistors are made up of semiconductor material. When light strikes on the material, the free electrons/holes cause the base region’s current flow. The base of the phototransistor is for biasing the transistor. In the case of the NPN transistor, the collector is made positive concerning emitter, and in PNP, the collector is kept negative. The light enters into the base region of the phototransistor generates the electron-hole pairs. The generation of electron-hole pairs mainly occurs in reverse biasing. The movement of electrons under the influence of the electric field causes the current in the base region. The base current injected the electrons in the emitter region. The major drawback of the phototransistor is that they have a low-frequency response.

Bipolar vs Field Effect vs Darlington

Just like regular transistors, phototransistors can be both bipolar or field-effect transistors.

Field-effect phototransistors are light-sensitive field-effect transistors. Unlike bipolar transistors, field-effect transistors use light to generate a gate voltage used to control a drain-source current. Field-effect transistors are also susceptible to variations in light and are more fragile than bipolar phototransistors.

On the other hand, Darlington phototransistors are similar to a normal Darlington transistor with their internal structure. A photodarlington transistor’s sensitivity specifications may be approximately ten times higher than that of a normal phototransistor. However, these units’ working frequency is lower than the normal types and may be restricted to only some 10s of kilohertz.


Optocouplers can be described by a variety of different names: optoisolator and photocoupler. It is a semiconductor device that uses a short optical path or link to couple a signal from one electrical circuit to another whilst providing electrical isolation. These are typically contained in a single package, often about the size of an integrated circuit. Optocouplers can be used to link data across two circuits. And can also be used within optical encoders and in many other circuits where optical links and transitions are needed. They even form the essential element in solid-state relays. Optical coupling is used to isolate the input and output electrically whilst enabling the output to be switched according to the input state. As a result, they are found in a surprisingly high number of circuits.

An optocoupler is a component that contains two elements.

First is the light emitter. This is on the input side, taking the incoming signal and converting it into a light signal. Typically, the light emitter is a light-emitting diode. The second one is the light detector. It detects the light from the emitter and converts it back into an electrical signal within the optocouplers. The light detector can be any device, from a photodiode to a phototransistor, photodarlington, etc. The light emitter and detector are tailored to match one another, having matching wavelengths so that the maximum coupling is achieved.

Optocoupler symbol

Optocouplers are often used in AC-related operations. Some optocouplers are also used in DC circuits. The main application of the optocoupler is to isolate two circuits. But you can also use it in switching applications and in various microcontroller-related operations where digital pulses or analog information is needed from a high voltage circuitry.

Types of Optocouplers

  • Slotted Optocoupler – This type of optocoupler has a slot molded into the package between the LED light source and the phototransistor light sensor. The said slot houses transparent windows so that the LED light can normally and freely reach the transistor’s face but can be interrupted or blocked via an opaque object placed within the slot. The slotted optocoupler can thus be employed in various presence-detecting applications, including end-of-tape detection, limit switching, and liquid level detection.
  • Reflective Optocoupler – In this type of optocoupler, the LED and phototransistor are optically screened from each other within the package and both face outwards of the package. An optocoupled link can be set up by a reflective object placed in a short distance outside the package, in line with both the LED. The reflective coupler can thus be employed in applications such as tape-position detection, engine-shaft revolution counting or speed measurement, or smoke or fog detection, among others.