Phototransistor

Phototransistor

What is a Phototransistor? Phototransistor is a three-layer semiconductor device. Phototransistor is

an electronic switching and current amplification component which relies on exposure to light to operate. When light falls on the junction, reverse current flows which is proportional to the luminance.

Phototransistors are used extensively to detect light pulses and convert them into digital electrical signals. A common type of phototransistor, called a photobipolar transistor, is in essence a bipolar transistor encased in a transparent case so that light can reach the base–collector junction. It was invented by Dr. John N. Shive

Characteristics : Low-cost visible and near-IR photo detection. Available with gains from 100 to over 1500.

Moderately fast response times. Available in a wide range of packages including epoxy-coated, transfermolded and surface mounting technology.

Symbol of Phototransistor

Phototransistor Construction

The construction of the phototransistor is quite similar to the ordinary transistor. Earlier, the germanium and silicon are used for fabricating the phototransistor. The small hole is made on the surface of the collector-base junction for placing the lens. The lens focuses the light on the surface.

Phototransistor Operation

The phototransistor is made up of semiconductormaterial. When the light was striking on the material, the free electrons/holes of the semiconductor material causes the current which flows in the base region. The base of the phototransistor would only be used for biasing the transistor. In case of NPN transistor, the collector is made positive concerning emitter, and in PNP, the collector is kept negative.

The light enters into the base region of phototransistor generates the electron-hole pairs. The generation of electron-hole pairs mainly occurs into the reverse biasing. The movement of electrons under the influence of electric field causes the current in the base region. The base current injected the electrons in the emitter region. The major drawback of the phototransistor is that they have low-frequency response.

Common emitter phototransistor circuit The common emitter phototransistor circuit configuration is possibly the most widely used, like its more conventional straight transistor circuit. The collector is taken to the supply voltage via a collector load resistor, and the output is taken from the collector connection on the phototransistor. The circuit generates an output that moves from a high voltage state to a low voltage state when light is detected.

Common collector phototransistor circuit The common collector, or emitter follower phototransistor circuit configuration has effectively the same topology as the normal common emitter transistor circuit - the emitter is taken to ground via a load resistor, and the output for the circuit being taken from the emitter connection of the device.

Photodiode Vs Phototransistor The photodiode and phototransistor both convert the light energy into the electrical energy. But the phototransistor is mostly preferred over the photodiode because of their following advantages. • The current gain in the phototransistor is more than the phototransistor even if the same amount of light strike on it. • The sensitivity of the phototransistor is higher than the photodiode. • The photodiode can be converted into the phototransistor by removing their emitter terminals.

Advantages of Phototransistors: Phototransistors have several important advantages that separate them from other optical sensor some of them are mentioned below • Phototransistors produce a higher current than photo diodes. • Phototransistors are relatively inexpensive, simple, and small enough to fit several of them onto a single integrated computer chip. • Phototransistors are very fast and are capable of providing nearly instantaneous output.

• Phototransistors produce a voltage, that photo-resistors cannot do

Disadvantages of Phototransistors: • Phototransistors that are made of silicon are not capable of handling voltages over 1,000 Volts. • Phototransistors are also more vulnerable to surges and spikes of electricity as well as electromagnetic energy. • Phototransistors also do not allow electrons to move as freely as other devices do, such as electron tubes.

Applications of Phototransistors The Areas of application for the Phototransistor include: • Punch-card readers. • Security systems • Encoders – measure speed and direction • IR detectors photo • electric controls • Computer logic circuitry. • Relays • Lighting control (highways etc) • Level indication • Counting systems