The magnitude of the transverse base field depends on the width of the transistor base, the base resistance, and the spreading of the space-charge layer that results from the application of collector voltage. The carrier concentration in the pellet and, consequently, the severity of the hot spots are determined mainly by the magnitude of the transverse base field and by the applied collector voltage, which determines the intensity of the electric field across the space-charge layer formed at the base-collector junction. If unchecked, these hot spots initiate a regenerative cycle of high-density current that results in forward-biased second breakdown in the transistor. With the current focused into a small area, the heating effect is localized in this area, and hot spots may be formed within the silicon pellet. When the current flows through the space-charge layer, a significant amount of heat is generated. As current flows from emitter to collector, the transverse field focuses the current into a narrow region below the emitter edge. When the transistor is heavily forward-biased into the active region, a transverse electric field is produced in the base region, and a space-charge layer is formed at the base- collector junction. CLARK, in AC Power Conditioners, 1990 Forward-Biased Second Breakdown While this wavelength is below the SFH 310’s 880 nm peak sensitivity, the response is still about 60% of peak. If a visible LED is preferred, you could use the Kingbright WP7113SRC/DU red LED at 640 nm ( Figure 21.4). The SFH 310 can be paired with the IR LED QED123, mentioned above, with its 880 nm wavelength. Peak sensitivity of the SFH 310 is at 880 nm. The SFH 310 has a viewing angle of up to about 25 degrees off the central axis, and it is sensitive to light of wavelengths 450 nm to 1100 nm, which includes much of the visible spectrum (about 390 nm to 700 nm).
Like photodiodes, phototransistors may have filters to alter their sensitivity spectrum and lenses to control their viewing angle. For a sufficiently large resistance R, the sensor’s output voltage ranges from close to 0 V (no light on the phototransistor) to close to 3.15 V (transistor saturated, with 0.15 V drop from collector to emitter). The resistance R should be chosen to get the right voltage range at the input to the PIC32, which could be an analog or digital input, depending on the application. (Right) A circuit with a WP7113SRC/DU red LED illuminating an SFH 310 phototransistor. (Image courtesy of Digi-Key Electronics, .) (Middle) The circuit symbol for an NPN phototransistor. The shorter leg is the collector and the longer leg is the emitter.
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