Luminous Sensitivity: The luminous sensitivity can be determined by the ratio between the photoelectric current and incident luminous flux.Time constant: Response time of the transistor also effects the efficiency of phototransistor to generate photocurrent.DC current gain of the transistor: The higher the DC current gain of the transistor, the higher will be the intensity of photocurrent generated.The current generated from phototransistor depends on several factors apart from luminous intensity such as The active region is used for generating current. The cut-off region and saturation region can be used to operate the transistor as the switch. The phototransistor can be operated in three regions that are the cut-off region, active region, and the saturation region. It can be of three terminals or two terminals we can omit base as per our requirement. The magnitude of the photocurrent generated by the phototransistor depends on the light intensity of the light falling on the transistor. The output of the phototransistor is taken from the emitter terminal and the light rays are allowed to enter the base region. This type of structure is used widely because the conversion efficiency increases several times as compared to the conversion efficiency of the homogenous transistor. The resulting structure becomes heterogeneous in nature. On the contrary, contemporarily, phototransistors are made up of Group-III and Group-V materials such as GaAs (Gallium Arsenide) in such a way that gallium and arsenide, each of these are used on either side of the transistor. The transistor which were used earlier was made of semiconductor material such as Germanium and Silicon and the resulting structure becomes a homogeneous material consist of either Silicon or Germanium. The collector and base region are formed by the techniques of ion-implantation and diffusion. This is because the more the light falls on the phototransistor the more current it will generate. The Phototransistors are manufactured in the similar way by which normal transistor is manufactured, the only difference is the area of the base and collector region in case of phototransistors is quite large as compared to the normal transistor.

The circuit symbol of the phototransistor is described in the diagram below.

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