Photo Transistor:Working,Uses,Characteristics,Pros & Cons

  • What is photo transistor ?
  • Working principle of photo transistor
  • Photo transistor uses
  • Photo transistor symbol
  • Characteristics of photo transistor
  • Advantages and disadvantages of photo transistor

The photo transistor is an transducer which is capable of converting the light energy into the electrical energy. The parameters like wavelengths, alignments, interfaces, etc. should be considered with higher importance while designing the circuit.

Definition of Photo Transistor:

“The phototransistor is a semiconductor device that is able to sense light levels and alter the current flowing between emitter and collector according to the level of light it receives.”

As the name suggests, the phototransistor is a transistor which can sense the light and vary the flow of currents in-between the terminals of the transistor.

In general transistors are sensible to lights. This property of transistors are used in phototransistors. NPN type photo transistor is one of the types.

DG 1 3
photo transistor

Here, in a phototransistor light striking the base supplants the voltage, actually applied to the base, so a phototransistor amplifies disparities as per the light signal. The phototransistors may or may not be have a base terminal in it. If it is present, the base region permits it to biasing the phototransistor’s light impacts.

  • This type of transistor is controlled by exposure of light. It is like a photodiode controlling a BJT.
  • Photo Transistor can be any one type such as BJT or FET.
  • These types of transistors are typically covered with the plastic materials and one of the part is kept open or transparent for light.

Symbol of Photo Transistor:

symbol of a phototransistor
symbol of a phototransistor
 photo transistor
a photo transistor

Examples Photo Transistor:

  • KDT00030TR
  • PS5042
  • OP506A, OP550A, OP506B
  • TEKT5400S, TEMT1030
  • SFH314-2/3, SFH 325 FA-Z
  • QSE113E3R0
  • BPW17N, BPV11F, BPW85C etc.

Working Principle of Photo Transistor

Output of a photo transistor is taken from its emitter terminal; hence the light rays are allowed to the base region.

A photo-transistor can be three or a two terminal device as per our requirement. The base of the photo-transistor is used for only biasing purpose. For NPN transistor, the base is made +ve in respect to the emitter terminal, and in a PNP transistor the collector terminal is made –ve in respect to the emitter terminal.

At first, the light ray enters the base region of a photo transistor and generates an electron hole pairs. This process mainly occurs under reverse biasing. The active region of this type of transistor is used for generating current. The cut-off and saturation region are used to operate the particular transistor as a switch.

A photo-transistor and its working depends on so many internal and external factors, such as:

  • The intensity of the photocurrent will be more with higher DC current gain.
  • Luminous sensitivity is given by the ratio of the photo electronic currents to the incoming luminous fluxes.
  • If the wavelength gets increased, the frequency will be decreased.
  • If the area of the collector-base junction gets wider, the Amplitude of the photo current engendered by the photo-transistor will be higher.

Characteristics of Photo Transistor:

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Here the X axis is VCE– denotes the voltage applied to the collector-emitter lead and the Y axis is IC – denotes the collector current that carrying through the circuit in mA.

As we can see, the curve is clearly indicating that current is increasing with the intensity of the radiation which is at the base region.                  

DG 5 1
Here, the X axis denotes the illumination level and in Y axis base current has been plotted in it.

Advantages of Photo Transistor:

  • The efficiency of this type of transistor is greater than a photodiode. The transistor’s current gain is also more compare to photodiode; even if incident light is same, the photo transistor will produce more photo current.
  • In compared to a photo diode, the response time of a photo transistor is more. So, it means this type of transistor has a faster response time.
  • The photo-transistors are immune to any noise interference.
  • Photo-transistors are less costly.
  • The circuitry of a this type of transistor is less complicated.

Disadvantages of Photo Transistor:

  • The efficiency of the phototransistor decreases with the electromagnetic field interferes.
  • At higher frequencies, photo transistors do not function properly. Due to this problem it fails to convert the photo current effectively at high frequency.
  • Electric Spikes occur frequently.

Applications of Photo Transistor:

  • Photo-transistors are used in counting systems.
  • This type of transistors are utilized in the computing system.
  • This type of transistor can be used to generate variable voltage.
  • These types of transistors are used in.
  • Due to high light to current conversion efficiency these are widely used in remote, printing machine.
  • The most important application of this type of transistor is to use it as a light detector. It can also detect very less light also.
  • They also play important role in making punch cards.
  • This type of transistors are crucial optoelectronics device which are also used in optical fibers

Why is phototransistor reverse biased?

Photodiodes are connected in reverse bias to decrease the charges area and narrow the capacitance at the junctions. This permits higher bandwidth. The light acts as IB, so in an NPN phototransistor the collector have +ve voltage by a resistive load, whereas the emitter will be grounded one.

Difference between photo resistor and phototransistor

Responsive to lightLess sensitiveMore sensitive
Maximum Resistance in darknessLowHigh
Minimum resistance in bright lightHighLow
Current carrying capacityHigh (nearly Double)Comparatively Lower than photoresistor
DirectionPhotoresistor is sensitive to incident light from all direction. So directionlessPhototransistor is sensitive to incident light in certain direction and obtuse from other ways.
Temperature dependentResistance fluctuates with temperature variationsEffective resistance has less fluctuates with temperature variations.
Resistance change No variation in resistance observed for light intensity irrespective of voltage applied i.e. it remains equal.Effective resistance differs with the voltage applied.
CostComparatively costlyComparatively cheap

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