Technical Article

Introduction to Photodiodes: The Nature of Light and pn Junctions

December 16, 2020 by Robert Keim

Learn about how semiconductor devices interact with electromagnetic radiation to enable electronic detection of light, UV, and infrared.

Photodiodes are measurement devices that produce electrical signals in response to various types of high-frequency electromagnetic radiation—ambient light, light focused by a camera lens, laser signals used in communication systems, thermal emissions, and so forth.

This introduction to photodiodes will serve as preparation for further study of light-sensitive circuits, applications, and techniques. The introduction is organized as five separate articles:

  1. The Nature of Light and pn Junctions
  2. Physical Operation of Light-Sensitive pn Junctions
  3. Understanding Photovoltaic and Photoconductive Modes of Photodiode Operation
  4. Characteristics of Different Photodiode Technologies
  5. Understanding the Photodiode Equivalent Circuit


What Is Light?

If you’ve studied quantum mechanics, you know that this question is not as straightforward as it seems. Fortunately, we don’t need to unravel the mysteries of the universe in order to successfully incorporate photodiodes into our electronic systems. We do, however, need to have a basic scientific understanding of light.


Electromagnetic Radiation and Wavelengths

Electromagnetic radiation (EMR) propagates as a wave and also consists of massless particles called photons. We categorize electromagnetic waves according to their wavelength. Light is simply EMR that falls within a specific range of wavelengths.

If we adopt a strict interpretation of the word “light,” we would associate this word only with optical wavelengths, i.e., wavelengths of light to which the human eye is sensitive. The optical wavelengths extend from 400 nm to 700 nm, with different wavelengths corresponding to different colors.

As you can see in the diagram, the colors start at violet (which has the shortest wavelengths) and move through the rainbow toward red (which has the longest wavelengths).


Optical wavelengths


We can also apply the word “light” to electromagnetic radiation that is near but not actually within the optical range. Infrared light extends from 700 nm to 1 mm, and ultraviolet light extends from 400 nm to 10 nm. When “light” is interpreted in this broader sense, we can use the term “visible light” to distinguish optical EMR from infrared and ultraviolet.


Electromagnetic Radiation and Photons

Electrical engineers often emphasize the quantum nature of light, because photons play an important role in the interaction between light and electronic circuitry. Photons transfer energy, and the energy associated with an individual photon is determined by wavelength.

EMR with higher frequency (or shorter wavelength) has higher-energy photons, and EMR with lower frequency (or longer wavelength) has lower-energy photons.


The pn Junction and the Diode

Get some semiconductor-grade silicon (the really pure stuff). Dope a section of it with a pentavalent element to make n-type silicon, and dope an adjacent section of it with a trivalent element to make p-type silicon. You’ve got a pn junction—one of the pillars of postmodern civilization.

When a silicon pn junction is packaged and used in a circuit, we call it a diode (or a silicon junction diode if you want to be more precise). When we implement ordinary diodes, we’re usually thinking in terms of forward-bias operation: the diode blocks current when its forward-bias voltage is less than about 0.6 V, and it freely conducts current when its forward-bias voltage is greater than 0.6 V. (This is a major simplification, but a useful one. For a more in-depth discussion, consider reading my article on simplified circuit-analysis techniques for forward-conducting diode circuits.) 

With photodiodes, however, we’re interested in zero-bias operation or reverse-bias operation. This principle of photodiode implementation is crucial, so let’s discuss it a bit more before we finish up. 


The pn Junction as an Optical Detector

The purpose of a photodiode is to generate current that is proportional to the intensity of visible, infrared, or ultraviolet light. The technical term for light intensity as measured by a photodiode is illuminance.

A photodiode has transparent packaging that allows light to reach the pn junction, and in a properly designed photodiode circuit, incident light will create precise variations in the amount of current flowing through the photodiode.


The pn junction as an optical detector

If we forward bias a photodiode to the point of conduction, we no longer have an optical detector. Detection occurs when the energy transferred by incident photons significantly influences the total diode current. Current flows freely through a forward-conducting diode, regardless of the incident light. Thus, photodiode circuits are designed such that the photodiode has zero bias or reverse bias.

A photodiode implemented with zero bias operates in photovoltaic mode, and a photodiode implemented with reverse bias operates in photoconductive mode. These two modes are explored in detail later in this introduction.


Measuring Light, Infrared Radiation, and Ultraviolet Radiation

Photodiodes are semiconductor devices that can be used to measure visible light, infrared radiation, or ultraviolet radiation. A silicon photodiode is not fundamentally different from a typical silicon rectifier diode, but photodiodes take advantage of the zero-bias or reverse-bias characteristics of a pn junction.

In the next article, we'll discuss the physical operation of light-sensitive pn junctions.