Types Of Fet

Phototransistor basics | Phototransistor types, Phototransistor applications

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Different Types Of Transistors

This article covers Phototransistor basics and Phototransistor types including its applications.It describes different types of Phototransistors Viz. BJT phototransistor,FET phototransistor etc.

The Field-Effect Transistor (FET) is a type of transistor which uses an electric field to control the flow of current. They are commonly referred to as unipolar transistors because they involve single-carrier-type operation that is, they use electrons or holes as charge carriers in their operation, but not both. Types of FET Based on its construction FET’s are classified as (1) Junction Field Effect Transistor (JFET) These JFET’s working is based on the channels formed between the terminals. Junction FETs or JFETs were the first type of FET and these have a distinctive symbol showing the diode junction. Insulated gate FETs, including MOSFETs have circuit symbols that indicate the insulation on the gate. Obviously dual gate FETs have two gates and both need to be incorporated into the circuit symbol. A field-effect transistor (FET) is a three terminal (namely drain, source and gate) semiconductor device in which current conduction is by only one type of majority carriers (electrons in case of an N-channel FET or holes in a P-channel FET). It is also sometimes called the uni-polar transistor. MOSFET or Metal Oxide Semiconductor Field Effect Transistor is the transistor that operates under the applied electric field. The presence of an insulating oxide layer in MOSFET makes it different from other types of transistors. MOSFET is a type of FET (Field Effect Transistor). The other types of FET are MESFET (Metal Semiconductor Field Effect Transistor), MISFET (Metal Insulator.

What is Phototransistor?

• It is a transistor controlled by exposure to light. It is similar to photodiode controlling anordinary bipolar transistor.
• It can be either bipolar transistor or field effect transistor (FET).
• Its body is encased in resin or plastic or encased in a metal shell with window in it.

• The figure-1 depicts most common variant of phototransistor i.e. Bipolar NPN.
• Examples: Optek/TT Electronics OP506A, Vishay TEKT5400S, Vishay BPW17N etc.

Phototransistor applications

Following are the applications of Phototransistor:
• It may be used for light measurement or as light sensitive switch.
• Its output is interfaced with microcontroller containing ADC.
• To achieve clear ON/OFF signal, it is interfaced to input of the logic gatecontaining a schmitt trigger. It can also be processed by comparator for the same.
• Optocoupler contains phototransistor which is activated by internal LED device.Its purpose is to switch current while isolating one part of the circuit from the other part.

Types of Phototransistors

Following are the types of Phototransistors viz. BJT phototransistor and FET phototransistor based on theirconstruction and principles of operation.

• BJT Phototransistor: In the absence of light, bipolar phototransistor permits leakage between collector and emitter of 100 nA or less.When exposed to light, it conducts upto 50mA. This differentiates it from photodiode which can not pass much current.
• FET Phototransistor: Phototransistor has two leads which connect internally with its collector and emitter (or source and drain in FET).The base of transistor (or gate in FET) responds to light and controls the flow of current between the leads.

Other Phototransistor types or variants

Due to advancement in surface mount technologies, these variants are widely available. Although through-hole packages are alsoavailable very commonly. Phototransistors are encapsulated like LED and are also available with flat surface.Flat surface variants are sensitive to light from any direction in front of it.

➨Some phototransistor variants provide base connection or gate connection. This allows application of bias current in orderto prevent low amplitude light levels from triggering the phototransistor.
➨Photodarlington: It is similar to darlington transistor. It is made of pair of bipolar transistors.The first transistor is sensitive to light where as the second transistor acts as amplifier.
• Advantages: The photodarlington is more sensitive to light compare to regular phototransistor.
• Disadvantages: Its response is slower. It is less linear.
➨Photo-FET: It is not used as standalone component. It is used in optocoupler. It is symmetrical for either of the polarity andhence used for AC signals. It works as controllable resistance when applied voltage is low in contrast ro bipolar transistors.

Also refer advantages and disadvantages of Phototransistor >>for more information.

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