How does a silicone photodiode detector work?
An alternative type of light detector is the solid-state diode detector. Silicone diode detectors have a greater wavelength range than a PMT, usually from 180 nm to 1100 nm. Unlike PMTs, diodes do not require a high voltage power supply (expensive). And finally the are more robust in being able to deal with high light intensities without saturating (overloading). The NIR region of UV/Vis/NIR instruments uses two different types of solid-state detectors.
A silicon photodiode utilizes the internal photoelectric effect, the phenomenon whereby the electrical properties of the detector itself change when light strikes it. As the name suggests, a silicon photodiode is a semiconductor. When light strikes this semiconductor, if the energy of the light is larger than the band gap, electrons in the valence band are excited into the conduction band, and holes are left in the original valence band. As shown at left, these electron-hole pairs are created throughout the semiconductor, but in the depletion region, the electric field causes electrons to be accelerated toward the N- region and holes to be accelerated toward the P-region. As a result, electrons accumulate in the N-region and holes accumulate in the P-region, and the two regions become, respectively, negatively and positively charged. If this is connected to a circuit, current flows. The band gap of silicon is approximately 1.12 eV, so current flows only for wavelengths that have an optical energy greater than this. This works out to a wavelength upper limit of around 1,100 nm.
The lead sulphide NIR detector has been the industry standard for over 40 years. It functions through the entire near infra-red range of 860 nm to 3300 nm and is fairly inexpensive. The newer wide band indium gallium arsenide (InGaAs) detector is more expensive but has over two orders of magnitude more sensitivity with less noise than the older PbS detector. When a wide band InGaAs detector is cooled to -50 degrees C it has a usable wavelength range of 800 nm to 2500 nm. Narrow band InGaAs detectors cover the range of 800 nm to 1600 nm.
The graph on the right shows the spectral sensitivity characteristic of a silicon photodiode.
Silicon photodiodes have some advantages over photomultiplier tubes: they are less expensive; there is little unevenness of sensitivity over their light-receiving surfaces; and they do not require a dedicated power supply. Even with respect to sensitivity, if the light intensity is relatively high, they can provide photometric data that is by no means inferior to that obtained with photomultipliers. If the light intensity is relatively low, however, because signals are amplified in the electronic circuit that gives a current, increasing the amplification factor decreases the response speed.