BASIC KNOWLEDGE - TRANSISTOR TECHNOLOGY Phototransistor: Definition, applications, and more

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The idea of the phototransistor existed even before the invention of the first bipolar transistor! The article explains the phototransistors of BJT and FET types in detail. It lists symbols, working principles, applications, advantages, and disadvantages.

1. What is a phototransistor?

Phototransistor definition

A phototransistor is a semiconducting detector that works on the principle of photoelectric effect to convert incident light into electric current. Type lll-V semiconductors are used to manufacture phototransistors in present times. A phototransistor is either a BJT (Bipolar Junction Transistor) or a FET (Field Effect Transistor). BJT phototransistor is a two-terminal three-layer semiconductor device manufactured with a light-sensitive base region. When compared to photodiodes, BJT phototransistors have high photocurrent gain in the order of 1000.

Phototransistor history

American Physicist John N. Shive invented the phototransistor in 1948. It was the same time when the first BJT was introduced. Phototransistors were invented to overcome the limitations of photodiode operation. Photodiodes produce less amount of output current. As a result, a phototransistor is used to amplify the current. A phototransistor can be called a combination of a photodiode and transistor (BJT or FET) to provide a high gain and amplify the signal. The phototransistor equivalent circuit represents a photodiode with a current amplifier.

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2. Bipolar Junction Phototransistor (Photo BJT)

Photo BJT symbol

The photo BJT symbol indicates that photons are incident on the base region to enable current flow.

In contrast, the base region in the phototransistor is unbiased. Photo BJT can be of two types: NPN Photo BJT and PNP Photo BJT. However, NPN Photo BJT is common.

Photo BJT working

The base terminal is either made light-sensitive or a lens is placed at the base-collector junction. The area of the base region in the phototransistor is slightly increased to allow the projection of a large number of photons.

The base is unbiased (open-circuit) while the collector is connected to the positive terminal of the external supply. The reason for unbiasing the base is to make the phototransistor sensitive to incident light and speed up its response time. The emitter-base junction is forward-biased and the collector-base junction is reverse-biased. When there is no external bias on the base terminal, a small leakage current termed “dark current” flows through the transistor due to minority charge carriers.

An increased number of incident photons knocks off charge carriers in the base region. The majority of the photoelectric effect is observed in the base-collector junction of the phototransistor. In simple words, the photoelectric effect absorbs incoming photons to generate electron-hole pairs, which are injected into the collector region. The initial base current controls the injection of charge carriers into the collector region. Hence, incident photons initiate a heavy current flow in the phototransistor. The current produced by the action of incident photons is termed “photocurrent”.

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NPN and PNP phototransistors undergo an amplification known as secondary photoconduction. After the transistor action of incident light, electron-hole pairs undergo multiplication due to impact ionization. The avalanche effect starts multiplication when charge carriers collide to generate extra electron-hole pairs. There is a significant increase in photocurrent. As a result, photodiodes have high photoconductive gains in the order of thousands.

The characteristics of a phototransistor are the graphical relationship between collector-to-emitter voltage on the X-axis and collector-to-emitter current on the Y-axis. The intensity of light is eventually responsible for the magnitude of the output current. Base current is directly proportional to collector current. Simply put, the base current is equal to beta times the collector current.

Phototransistor BJT is often used in Darlington pair configuration to enhance current gain. The resulting device is called a photodarlington.

3. Field Effect Phototransistor (Photo FET)

Photo FET symbol

Phototransistor FET is available in JFET or MOSFET transistor types. It can be constructed with a conductive n-channel or p-channel. Furthermore, a phototransistor MOSFET can be an enhancement type or a depletion type.

Photo FET working

Photo FET Phototransistor FET or Photovoltaic FET is a light detector that operates on the principle of the photoelectric effect. Similar to photo BJT, photo FET has a light-absorbing material to increase sensitivity, and speed up the response time. As it is a field effect transistor, a phototransistor FET has a conducting channel that functions as a current path between the drain and the source. However, the gate terminal is not made from the light-sensitive material. The light-sensitive material lies in the channel between the drain and source terminals.

When incident light falls on the sensitive channel region between the drain and the source, absorption takes place. The photoelectric effect generates new charge carriers. Unlike phototransistor BJT, phototransistor FET has a unipolar current flow depending upon the n-channel or p-channel configuration. Electrons modulate the conductivity in n-channel phototransistor FET and holes modulate the conductivity in p-channel phototransistor FET. Phototransistor FET is a voltage-controlled device because incident light modulates the conductivity of the channel. In simple words, the gate voltage can control the current flow between the drain and source terminals.

4. Phototransistor applications

• Historical use in direct distance dialing systems.

• Proximity detectors.

• Applications that require high current gain.

• High-sensitivity photodetectors that cover UV to IR Spectrum.

• Opto-isolators.

• Optical reader.

• Photo interrupters.

• Counting.

• Encoders.

• Radar tracking system.

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5. Phototransistor advantages

• Contains an optical window to cover sensitive areas.

• Amplifies the output current.

• Offers a current gain between 100-1500.

• Low to moderate noise

• Available in a variety of packages like SMT.

• Compact and cheap.

• High efficiency compared to photodiodes.

• Phototransistors made from gallium arsenide can handle high voltages.

6. Phototransistor disadvantages

• Slower response time compared to photodiodes. It is because the base region is increased for light incident light transistor action.

• Low transconductance.

• Failure to handle high voltages in high-power applications.

• Prone to voltage spikes.

• Limited bandwidth.

• Photodarlington has a large switching time.

7. Phototransistor vs Photodiode

References

https://www.sciencedirect.com/

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