phototransistor

phototransistor. A phototransistor is a semiconductor device that converts light signals into electrical signals, functioning as a light-controlled switch or amplifier. It operates on the principle of the Internal photoelectric effect, where incident photons generate electron-hole pairs within the semiconductor material. The device is essentially a Bipolar junction transistor whose base-collector junction is exposed to light, eliminating the need for a direct electrical base connection in many applications. First invented at Bell Labs by John Northrup Shive in the late 1940s, it has become a fundamental component in modern Optoelectronics.

Operating principle

The core operation relies on the Photogeneration of charge carriers within the base-collector P–n junction. When photons with sufficient energy strike the Depletion region, they create electron-hole pairs. In a typical NPN configuration, the generated electrons are swept into the collector, while the holes migrate toward the emitter, effectively acting as a base current. This photogenerated current is then amplified by the transistor's inherent Current gain, denoted as β or h_FE. This process is fundamentally different from the operation of a Phototube or Photomultiplier, which rely on the External photoelectric effect. The spectral response is determined by the Band gap of the semiconductor material, with Silicon being most common for detecting visible and near-Infrared light.

Structure and materials

Structurally, it is very similar to a standard Bipolar junction transistor, but with a lens or window to allow light to reach the base region. The most common construction uses a Planar process on a Silicon wafer, creating an NPN structure where the lightly doped P-type semiconductor base is exposed. Alternative materials include Germanium for longer infrared wavelengths and Gallium arsenide for high-speed applications. Specialized types, such as the Darlington phototransistor, incorporate two transistors for extremely high gain. The packaging is critical, often using Epoxy or other transparent materials like Glass in a TO-18 or TO-92 metal can, or a Surface-mount technology package such as an SOT-23.

Characteristics and performance

Key performance parameters include Responsivity, which defines the output current per unit of incident Optical power, and Dark current, the small leakage current that flows with no illumination. The Rise time and Fall time specify the switching speed, which is generally slower than a Photodiode due to the larger Capacitance of the base-collector junction. The Spectral sensitivity curve peaks at a specific wavelength, around 850 nm for silicon. Performance is significantly influenced by the Operating temperature, with higher temperatures increasing dark current. Manufacturers like Vishay Intertechnology and Everlight Electronics often specify these characteristics in datasheets under standard conditions defined by the International Electrotechnical Commission.

Applications

These devices are ubiquitous in applications requiring detection, switching, or amplification of light. Common uses include the object detection in opto-interrupters and Photoelectric sensors for Industrial automation on assembly lines at companies like Siemens and Omron. They form the receiver element in Infrared remote control systems for televisions from Samsung or Sony, and in Fiber-optic communication links for short-distance data transmission. Other applications include smoke detectors, Punch card readers, Laser rangefinders, and light meters in Digital cameras. They are also found in consumer electronics for screen brightness control in devices from Apple Inc. and for reading Compact discs.

Comparison with other photodetectors

Compared to a Photodiode, it offers much higher gain and output current but suffers from slower response times and higher noise. A Light-dependent resistor has no amplification and is slower, but can handle higher voltages and is used in simple light-sensing circuits like streetlight controls. The Avalanche photodiode provides gain through Impact ionization and is far faster and more sensitive, making it essential in long-haul Fiber-optic communication systems like those deployed by AT&T Corporation. The Photomultiplier tube offers extremely high sensitivity and speed for low-light scientific applications at facilities like CERN or the Hubble Space Telescope, but requires high voltage and is bulky. The choice depends on the specific requirements for gain, speed, cost, and size dictated by the application. Category:Transistor types Category:Optical devices Category:American inventions