Image of the Month – March 2015
A Bright Idea!
Micro-CT of an LED
There has been a lot of talk about LED’s over the past few years, from LED flashlights to LED TV’s. So what are LED’s? Why are they so important to the world around us? And what happens when they burnout? Let’s take a closer look at a LED with Micro-CT imaging and answer some of these questions.
Figure 1. Actual LED (Left) and projection image of LED during scanning (Right)
What is an LED?
As you may know, LED stands for light-emitting diode and are used to emit visible, infrared, or ultraviolet light. The reason they are called a diode has to do with how they work. A diode is a device that allows current to flow in one direction and is made of two forms of silicon, N-type and P-type. N-type is silicon doped with phosphorus and P-type is silicon doped with boron. When these two types are placed next to each other they form a P-N junction (Figure 2)1.
The P-type silicon, with boron atoms, has only three valence electrons to pair with the four valence electrons of silicon. This creates “holes” or unpaired electrons on a silicon atom. In N-type silicon, phosphorus atoms with five valence electrons pair with silicon. This causes one unpaired electron on a phosphorus atom to be free to move. When the P-N junction is formed, the holes of the silicon (P-type) are filled with the electrons of the phosphorus (N-type). Once the regions around the P-N junction are balanced due to the exchange of electrons, this creates a depletion region, an area where no electrons flow1.
Figure 2. Components of a diode that consist of P-type and N-type silicon.
What happens now when we apply a voltage to the diode? That depends on whether a positive or negative voltage is applied. If the voltage of the N-type is more negative, then the free electrons of the phosphorus are repelled with enough force to cross the depleted region. Current then flows through the diode. Alternatively, when a negative voltage is applied to the P-type silicon, the “holes” move towards the incoming electrons. This expands the depletion region preventing current1.
Now that we understand how a diode works, the only difference with an LED is that light is emitted when current flows through the diode. There is also a reflector cup and an outer coating resin that are used to enhance the light given off.
Why are they so Important?
Their applications are broad from the blinking light on a cellphone to Christmas lights, and even backlight displays on computers and televisions. They are great because of their long lifetime, low energy consumption, and are very reliable. Despite their reliability, they do degrade over time and can be damaged if too much voltage is applied.
How they burnout
LED’s can burnout from over-voltage or over-heating. If too much voltage or the LED is overheated the leads going into the LED can melt causing an open circuit. This is what we were looking for in our LED as it no longer worked. By placing the LED in the scanner and taking a projection image we were able to see a possible open in the circuit. To ensure that was the case, the sample was scanned and reconstructed into coronal slice images to verify the open (Figure 3). The coronal image below solidifies the conclusion that there is indeed an open in the LED.
Figure 3. Open faintly visible in projection image (left) and clearly visible open in coronal image slice (Right).
We hope you enjoyed learning about LED’s and the benefits of MicroCT imaging in determining an open circuit. This example is one of many where Micro Photonics has provided open circuit analysis. If you have an electronic or another interesting industrial application that you would like showcased as the next image of the month, feel free to e-mail: email@example.com. We would love to include your work!
1. Scherz P. Practical Electronics for Inventors. New York (NY): McGraw-Hill. p. 414-419, 508-511.
SkyScan 1272 Micro-CT
Micro Photonics Imaging Laboratory, Allentown, PA