Radiation sensorsThere are a variety of radiation sensors for different types of radiation, including nuclear radiation as well as visible light, infrared and ultraviolet. Only a couple of the commonest will be considered here: the photodiode and phototransistor, charge coupled devices (CCDs), and pyroelectric sensors. The reader is referred to the further reading section for information on other types of sensor.
PhotodiodesThe simplest photodiode is a reverse biased p-n (diode) junction. When no light falls on the device only a small amount of current flows (the dark current). When light falls on the device, additional carriers are generated, and more current flows.Photodiodes typically work in the visible light - near infrared region of the spectrum. They are high impedance devices, and operate at relatively low currents (typically 10uA dark current, rising to 100uA when illuminated). They have fairly linear responses to increasing illumination, and generally have very fast response times.
PhototransistorsThe phototransistor has a much higher current output than a photodiode for comparable illumination levels. However, it does not operate as fast as photodiodes (about 100kHz being the top limit), and also has higher dark current.The phototransistor is essentially a transistor with the base current supplied by the current produced by illumination of the base-collector junction; it can be considered to be similar to a photodiode supplying the base current to a transistor (figure 3). Normal transistor action amplifies the small base current.
 Figure 3.
Charge coupled devices (CCDs)Charge coupled devices can be built as large linear and two dimensional arrays; the latter are often used for small video cameras. They consist of a large number of electrodes (gates) on a semiconductor substrate. A thin insulating layer is situated between the metal gates and the semiconducting substrate.The operation of a CCD is shown schematically in figure 4. The substrate has been doped so that the main current carriers are positive (i.e. "holes" - it's a semiconductors term; see further reading if you want to get into it). When a positive voltage is applied to every third gate (V1), the majority carriers are repelled from the region beneath (figure 4a), leaving "wells". When light falls on the device, additional carriers are generated (as with photodiodes). The positive carriers are repelled from the gate, but the negative charge carriers (electrons) are attracted to the gate and fill the wells (figure 4b). After time to allow carriers to accumulate, the entire array may be read out by shifting the carriers from one well to the next; the number of carriers being proportional to the amount of light that fell on each well. The electrical potential on gates to one side of those already biased (V2) is increased, so the charge is now shared between wells under two gates (figure 4c). Then the first potential (V1) is then switched off, so that the charge is transferred to the adjacent well (figure 4d). And so on.
 Figure 4.
Pyroelectric sensorsThese devices operate on the pyroelectric effect in polarised crystals (e.g. zinc oxide). These crystals have a built in electrical polarisation level which changes with the amount of incident thermal energy.These are generally high impedance devices, so are often buffered using field effect transistors. They can be made to automatically zero themselves to the ambient temperature, so they only respond to rapid fluctuations. One potential problem is that crystals that exhibit the pyroelectric effect may also exhibit piezoelectric effects (see mechanical sensors below), so pyroelectric sensors need to be designed to avoid strain on the crystal. One common application of these devices is in human motion detectors for intruder alarms. A lens cuts the sensor's field of view into discrete sections. As someone moves across the field of view, thermal radiation from their body falls on the sensor, resulting in discrete pulses as the person moves from one part of the field of view to the next. It is thus possible to build relatively cheap motion detectors, which can be tuned to respond to a particular rate of motion.
Integrated opticsAs mentioned in the "Microsystem Components" section earlier, the subject of integrated optics, being quite broad, is not discussed in this document. It is not possible, however, to discuss radiation sensors without making some mention of it.The use of integrated optics can potentially allow the analysis of optically acquired data (usually from fibre optic sensors), in the optical domain, on a chip. Optical fibres are aligned to waveguides on the surface of the chip by grooves machined into the substrate. Passive components for analysis are such things as:
Active component possibilities include:
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