An Introduction to Optoelectronics

In this article, we talk about the basics of optoelectronics, including a brief lecture on photons. Also, various optoelectronic devices will be discussed including lamps, LEDs, photodiodes, photoresistors, laser diodes, and others.

What Are Optoelectronics?

The term optoelectronics is a specific discipline of electronics that focuses on light-emitting or light-detecting devices.

Light-emitting devices use voltage and current to produce electromagnetic radiation  (i.e., light). Such light-emitting devices are commonly used for purposes of illumination or as indicator lights.
In contrast, light-detecting devices, such as phototransistors, are designed to convert received electromagnetic energy into electric current or voltage. Light-detecting devices can be used for light sensing and communication. Examples of these include darkness-activated switches and remote controls. In general terms, light-detecting devices work by using photons to liberate bound electrons within semiconductor materials.
Figures 1 through 4 show a variety of optoelectronic devices.

Figure 1. Incandescent lamp.

Figure 2. LEDs. Image courtesy of Sinisa Maric.

Figure 3. Photoresistor. Image courtesy of Michigan State University (PDF).

Figure 4. Solar cell. Image courtesy of SparkFun

A Review of Photons

Photons are the fundamental units of electromagnetic radiation (EMR). Photons have a frequency of propagation, and we classify EMR based on this frequency—microwave EMR, infrared EMR, optical EMR, and so forth. The human eye is sensitive to optical EMR, which is further categorized into colors. Color is not an inherent property of photons; rather, photons have frequency, and human beings interpret these different frequencies as different colors.

Some Physics Behind Photons

The relationship between a photon's frequency and its wavelength (λ) is given by:

λ=ν/f (in units of meters)

where

• ν = velocity, or speed, of the photon (units of m/s)
• f = frequency (in units of Hz)

In free space, ν is the speed of light (c = 3.0 × 10⁸ m/s). But in other media, such as glass, ν becomes slower than the speed of light.
A photon with a longer wavelength (i.e., a lower frequency) has less energy than a photon with a shorter wavelength (i.e., a higher frequency).
See Figure 5 for more information on a photon's energy, frequency, and wavelength.

Figure 5. The Electromagnetic (EM) spectrum. Image courtesy of Inductiveload [CC-BY-SA-3.0]

Lamps

Lamps, such as incandescent light bulbs, are devices that convert electric current into visible light energy. Incandescent lamps have a filament made from tungsten wire. As current flows through this filament, the current collides with the filament's atoms causing the filament to generate heat resulting in photons being emitted. This particular process produces photons with a variety of wavelengths, resulting in emitted light that appears whitish in color.

Halogen lamps are similar to incandescent lamps. In fact, halogen bulbs are considered to be an advanced form of the incandescent bulb. Halogen lamps are commonly known for both their brilliant light and their very hot-to-the-touch bulbs. A halogen lamp uses a filament that resides inside a gas-pressurized bulb. The pressurized gas consists of an inert gas and a small amount of a halogen element such as bromine or iodine. Also, the glass of a halogen bulb is stronger than the glass in a regular incandescent bulb.

Fluorescent lamps are quite different. They consist of a mercury vapor–filled glass tube whose inner wall is coated with a material that fluoresces. When electrons, which are emitted from the fluorescent bulb’s cathode electrode, collide with the mercury atoms, UV (ultraviolet) radiation is emitted. This UV radiation is absorbed by the lamp's fluorescent coating, which in turn releases visible light.

LEDs

Light-emitting diodes (LEDs) are two-lead semiconductor devices that are similar to normal diodes except that they emit light that can be visible, infrared, or ultraviolet. When an LED's anode lead become more positive in voltage than its cathode lead (typically by a voltage ranging from of 0.6 to 2.2V), current flows through the LED device resulting in emitted light.
In historical terms, red was the first LED color; it was developed in 1962 and then mass produced in 1968. Next came yellow, green, and infrared LEDs. It wasn't until 1989 that the blue LED became commercially available. Nowadays, virtually any LED color, including white, is commercially available.

Laser Diodes

A laser diode is a semiconductor laser device that is very similar, in both form and operation, to a light-emitting diode (LED). More detailed information about laser diodes can be found here.

Photoresistors

Photoresistors are nothing more than light-controlled variable resistors, also known as light dependent resistors (LDRs). Typically, when a photoresistor is placed in the dark its resistance is very high (in the megaohms). In contrast, when a photoresistor is illuminated, its resistance drops dramatically—depending on the intensity of the light shining on it, the resistance may be as low as hundreds of ohms. Photoresistors are used in light-sensitive switching devices.

Photodiodes

Photodiodes are two-lead semiconductor devices that convert light energy (i.e., photons) directly into electric current. A photodiode is constructed using a very thin n-type semiconductor together with a thicker p-type semiconductor. The n side has an abundance of electrons and is considered the cathode while the p side has an abundance of holes and is considered the anode. When a photon (i.e., light) of sufficient energy strikes the diode, it creates an electron-hole pair. The holes move toward the anode while the electrons move toward the cathode, thus creating a light-induced current (i.e., a photocurrent).

Solar Cells

Solar cells are simply photodiodes with exceptionally large surface areas. These larger areas allow the solar cells to be more sensitive to incoming light as well as more powerful—in terms of both voltage and current—than photodiodes. Solar cells are commonly used in solar panels, but they are also often used as light-sensitive elements in detectors of visible light. Examples include light meters and light-sensitive relays.

Phototransistors

As you may have guessed, phototransistors are light-sensitive transistors. There are two common types: the first resembles the BJT (bipolar junction transistor) and the second is similar to a FET (field-effect transistor). The BJT-type phototransistor has its base replaced by a light-sensitive area; when this surface is kept dark the device remains off. The FET-type phototransistor, sometimes referred to as a photo-FET, uses light to generate a gate voltage that controls a drain-source current. FET-type phototransistors are more sensitive to variations in light compared to BJT-type phototransistors.

Optoisolators

Optoisolators (also known as optocouplers) are electrical devices that interconnect two circuits by means of an optical interface. For instance, a typical optoisolator is composed of an LED and a phototransistor, both of which are encased in a light-tight enclosure. The LED portion of an optoisolator is connected to a driver circuit, and the phototransistor is the output device. Accordingly, when the LED is energized it emits photons that are detected by the phototransistor. A typical application of an optoisolator is to provide electrical isolation between two separate circuits.

Fiber Optic

Optical fiber is used in conjunction with optoelectronic devices in order to transmit information via modulated light. Figure 6 below is a simple depiction of a fiber optic cable.

Figure 6. A simple depiction of a fiber optic cable.

Summary

Optoelectronics is the study and application of electronic devices that use light. Such devices include those that emit light (LEDs and light bulbs), channel light (fiber optic cables), detect light (photodiodes and photoresistors), or are controlled by light (optoisolators and phototransistors).