Silicon photodiode transducers

Silicon Photodiode Transducers A silicon photodiode transducer consists of a reverse-biased pn junction formed on a silicon chip. As shown in Figure 7-32, the reverse bias creates a depletion layer that reduces the conductance of the junction to nearly zero. If radiation impinges on the chip, however, holes and electrons are formed in the depletion layer and swept through the device to produce a current that is proportional to radiant power. They require only low-voltage power supplies or can be operated under zero bias and so can be used in portable, battery-powered instruments. 

In a PDA, the individual photosensitive elements are small silicon photodiodes, each of which consists of a reverse-biased pn junction (see previous section). The individual photodiodes are part of a large-scale integraled circuit formed on a single silicon chip. Figure 7-3.1 shows the geometry of the surface region of a few of the transducer elements, F.ach element consists... 

Several types of photon transducers are available, including (I) photovoltaic cells, in which the radiant energy generates a current at the interface of a semiconductor layer and a metal (2) phototubes, in which radiation causes emission of electrons from a photosensitive solid surface (3) photomultiplier tubes, which contain a photoemissive surface as well as several additional surfaces that emit a cascade of electrons when struck by electrons from the photosensitive area (4) photoconductivity transducers in which absorption of radiation by a semiconductor produces electrons and holes, thus leading to enhanced conductivity (5) silicon photodiodes, in which photons cause the formation of... 

The linear photodiode array (LPDA) is a transducer developed to enable simultaneous measurement of light intensity at many wavelengths. The diode array consists of a number of semiconductors embedded in a single crystal in a one-dimensional linear array. A common procedure is to use a single crystal of doped silicon that is an n-type semiconductor. A small excess of a group 3A element, such as arsenic, is embedded into the surface at regular intervals. This creates local p-type semiconductors. The semiconductor device ideally has a cross-section such as that shown in Fig. 5.24. The surface contains a linear series or array of pn junctions, each of which is a photodiode. The individual diodes are called elements, channels, or pixels. 

SFM cantilevers and tips are often made of silicon or silicon nitride SFM tips possess radii of curvature at the apex between few and several tens to hundreds of nanometers (Fig. 1). A piezoelectric transducer is used in order to position the sample accurately. Depending on the scanner type (piezo tube length and design) the maximum scan sizes vary between ca 1 /rm and several himdred micrometers, with an accuracy of positioning in the best cases of O.Ol nm. The cantilever deflection is typically monitored by an optical beam deflection technique (Fig. la). Other possibilities to measure the deflection include STM, piezoresistive, capacitance, or interferrometric detection schemes (22). In the optical beam deflection setup, laser light is reflected off the end of the cantilever and is collected by a position-sensitive photodiode. For instance, a 4-quadrant photodiode can simultaneously measure deflections in vertical (surface normal) and horizontal (lateral) direction (Fig. la). 

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