What are light-emitting diodes (LEDs)?

A light-emitting diode (LED) is a small, energy-efficient device that produces light when electricity flows through it. Made from a semiconductor, it works by combining electrons and holes at a p-n junction, releasing energy as light in a process called electroluminescence (find out more about the p-n diode and light emission process in our videos 1 & 2 of in-depth course). LEDs require a certain voltage to turn on - typically around 1-2 volts for red light; 2-3 volts for green light and 3-3.6 volts for blue light. 

How is an LED made?

To make LEDs, manufacturers use a process called epitaxy, where thin layers of semiconductor material are carefully grown on a flat surface. These layers form a p-n junction, which produces light when electricity flows through. After the layers are grown, the material is cut into small chips, and tiny wires are attached. The chips are then coated with a protective lens and tested to ensure they work correctly. An example of fabricating the green LED device from an epitaxy wafer is shown in the figure below. The final product is packaged for use in devices like lights, TVs, and phones.  

Figure 1: (a) wafer after epitaxy, (b) close-up of a set of LEDs after metallisation; (c) single LED under test, (d) packaged LED

The LEDs you will test were fabricated from 4’’ epitaxy wafers in the semiconductor cleanroom at the University of Sheffield. They have a ‘mesa’ structure so that we can access the barrier layer, which is taken as a negative terminal. On the positive terminal is a very thin layer (100 nanometers) of ITO (Indium tin oxide), which is transparent and conductive to allow the light to pass through. Additional metal contact on the ITO provides better electrical contact. However, the metal contact is not transparent, so it will block some of the light. 

Figure 2: Cross-section of the under-test LED structure

What is the optical spectrum of LEDs? 

Light sources are characterised by the range of colours that they emit. Photon wavelength is a measure of the colour. A plot of wavelength versus intensity is known as the optical spectrum. The human eye can see just a small part of the optical spectrum - the colours of the rainbow, ranging from red to violet. Light with wavelengths below violet is known as ultraviolet and light with wavelengths above red is known as infrared.

In our daily lives, we encounter LEDs in many devices. TV screens, smartphones, traffic lights, and flashlights use LEDs to produce different light colours. The colour of the light from an LED depends on the wavelength of light it emits, which is part of the optical spectrum (refer to peak wavelength). LEDs are efficient because they can produce specific colours without wasting energy on other heat or colours (refer to spectral width), making them energy-saving and long-lasting. 

Here are two of the key performances you can extract from the optical spectrum:

Peak wavelength

The wavelength at which the LED emits the most light. For example, a green LED might have a peak wavelength around 530 nm, while a blue LED might peak around 450 nm.

Figure 3: Illustration of optical spectra of blue, green and red LED 

Spectral width

• This refers to the range of wavelengths emitted by the LED, usually measured as the Full Width at Half Maximum (FWHM), meaning the width of the spectrum at half of its peak intensity.

• LEDs typically have a narrow spectral width compared to other light sources like incandescent bulbs. However, semiconductor laser diodes have even narrower spectral widths than LEDs due to inherent physics mechanisms (find out in our videos 2 and 4 in-depth course).

What about white light LED? 

Unlike coloured LEDs, which emit light in a single part of the spectrum (like red,  green or blue), white LEDs produce light that covers a broader range of colours, combining them to create white light. There are two ways to make a white light LED.

Combination of Red, Green, and Blue LEDs

Some white LEDs mix light from red, green, and blue LEDs. These three colours blend together to form white light. 

The optical spectrum of a white LED shows a mix of wavelengths: 

1. a strong peak in the blue range (from the blue LED) and

2. a broader range of wavelengths in the green, yellow, and red parts (from the phosphor or RGB mixing). This broad spectrum is why white LEDs can mimic natural light and are suitable for various applications.

Blue LED with Phosphor Coating

Most white LEDs start with a blue LED. The blue light passes through a yellow phosphor coating, which converts part of the blue light into other colours. When combined, these colours appear white to us.

1. Obtain remote access

Please fill out the form if you want to get remote access. Your request will be processed during normal working hours (this is not an automatic process), so please be patient. If you have not received a notification for a long time, please email us at asisst@sheffield.ac.uk. 

 We appreciate your interest and patience. 

2. LED characterisation lab (Here is the lab sheet)

Aims

1. 1. • DC electrical behaviour of light emitting diodes (LEDs)

      • Optical emission spectrum of LEDs

Objectives

1. 1. • Use a probe station and Analog Discovery 3 to measure the DC current-voltage (I-V) characteristics of light-emitting diodes (LEDs) and work out the turn-on voltage (Von)

      • Use a fibre-coupled optical spectrometer to measure the LED emission spectrum

Results

1. Seeing the light

Figure 5: Blue LED under test and emitting blue light

2. Current-voltage characteristics

When a positive voltage is applied to the LEDs, the forward current rapidly increases after the turn-on voltage (Von). In contrast, the reverse current is small and does not dramatically increase with a negative voltage. Are you wondering why the LEDs have distinct current-voltage curves like a pn diode? Check out our video 1 on pn diodes (in-depth course)!

After lighting up the LEDs, can you tell what the turn-on voltage is?

Figure 6: Current-voltage characteristics of green and blue LED

3. Optical measurements

Before you light up the LEDs, please look at the optical spectrum. The optical spectrum belongs to a white light LED made by a blue LED and a yellow phosphor. This optical spectrum collects from a white light LED for the camera! 

After lighting up the LEDs, can you tell which spectra belong to blue, green and deep-red LEDs?

Figure 7: Optical spectra from different LEDs (deep-red, blue and green).

Do you know the turn-on voltage you measured is linked to the photon energy you measured from the spectra? 

Use the equation below to calculate the energy of different colour photons and compare it with turn-on voltage.