8 November 2012
There are many different fields where the detection of the colour or the wavelength of objects - in our case, LEDs - is interesting. These areas include the consumer market, the automotive sector and the industrial sector. For example, more and more LED backlights are being used for display equipment, and for such displays to maintain accurate colour rendition, it is mandatory to monitor the wavelength of these LEDs. One other possible application is the use in the LED lighting sector to provide a possibility to detect and compensate for colour changes due to temperature effects.
A multijunction photodetector is a sensor where multiple photodiodes are stacked vertically on top of each other. Because the penetration depth of the photons in the semiconductor is dependent of the wavelength each photodiode has a different responsivity. For instance the top photodiode is more sensitive to blue light than the bottom one. In our case we employed a standard BiCMOS process without any process modifications. In this process we were able to build three vertically stacked diodes. Due to the different responsivities for different colours we are able to use this photodetector as a colour sensor. In our work we show that this sensor is able to detect the wavelength of LEDs.
In our work, we have shown that multijunction photodetectors can detect the wavelength of LEDs over a wide spectral range. Today, in many applications a microcontroller with an integrated analogue to digital converter is already employed. This has to be kept in mind when the analogue part of the sensor is designed. We were able to minimise additional analogue circuitry for the wavelength detection, keeping the cost of the sensor low. We also concluded that if a calibration is necessary this can be accomplished by a simple two point calibration.
Previously, sensors were not calibrated because of the additional costs this incurs. If one wanted to use a custom, off-the-shelf colour sensor for wavelength detection, one would need to go through a time-consuming calibration process for each sensor. Typically no exact characterisation curves for the spectral responsivity of the sensor are available, so the calibration would be done, for instance, by a monochromator - a tuneable light source. This is very time consuming because the wavelength change takes time. The time of such a calibration can go easily into the minute time range.
Because of the working principle of our sensor we found that the calibration of the part to part variation can be done easily by measuring one or two known wavelengths. This can be for instance a red and blue laser source. Therefore the calibration can be finished in seconds.
Furthermore, because of the use of a multijunction photodiode, instead of a conventional photodiode with additional colour filters, only a standard BiCMOS process is necessary. Therefore, the costs are lower because no process modifications are required, and it is easier to transfer the design to different technologies. If a calibration is necessary for the application it can be done with our system easily at the wafer level for each sensor. With one or two fast measurements the whole sensor is calibrated.
We think that we will see many possible applications for this type of sensor. There is a great demand for colour sensors which are cheap, easy to produce and easy to integrate. There are many more potential applications, ranging from biological applications to chemical applications. The next logical step is that these sensors will be more and more integrated in complex system on chips because of the ease of integration.
For example, in the future, we hope to use this kind of sensor for biomedical sensor systems. For example the absorption characteristics of blood are strongly wavelength dependent, and we aim to perform non-invasive measurements of important human-parameters besides the typical oxygen saturation.
The Letter presenting the results on which this article is based can be found on the IET Digital Library.
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