Practical Electronics for Inventors, Fourth Edition by Paul Scherz & Simon Monk

Author:Paul Scherz & Simon Monk [Scherz, Paul]
Language: eng
Format: epub
ISBN: 9781259587542
Publisher: McGraw-Hill Education
Published: 2016-04-05T05:00:00+00:00

FIGURE 5.22

5.7.1 How a Phototransistor Works

Figure 5.23 shows a simple model of a two-lead bipolar phototransistor. The details of how this device works are given below.

FIGURE 5.23

The bipolar phototransistor resembles a bipolar transistor (with no base lead) that has an extra large p-type semiconductor region that is open for light exposure. When photons from a light source collide with electrons within the p-type semiconductor, they gain enough energy to jump across the pn-junction energy barrier—provided the photons are of the right frequency/energy. As electrons jump from the p region into the lower n region, holes are created in the p-type semiconductor. The extra electrons injected into the lower n-type slab are drawn toward the positive terminal of the battery, while electrons from the negative terminal of the battery are drawn into the upper n-type semiconductor and across the n-p junction, where they combine with the holes. The net result is an electron current that flows from the emitter to the collector. In terms of conventional currents, everything is backward. That is, you would say that when the base region is exposed to light, a positive current I flows from the collector to the emitter. Commercial phototransistors often place the pnp semiconductor in an epoxy case that also acts as a magnifying lens. Other phototransistors use a metal container and a plastic window to encase the chip.

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