PPC increases the response time of metal oxide semiconductor-based conventional two-terminal photodetectors. According to the literature, phototransistors have the ability to solve persistent photoconductivity (PPC). In many applications, the phototransistor is used in the two-lead version.The present study illustrates the fabrication of ZnO ultra-thin film (25 nm)-based bottom gate phototransistors using RF sputtering and thermal evaporation on SiO 2/Si substrate for UV detection. In the two-lead configuration, the base is not electrically available, and the device can be used only with light as the input.
In the three-lead configuration, the base lead is brought out so that the device can be used as a conventional BJT with or without the additional light-sensitivity feature. Thus, a typical phototransistor is designed to offer a large area to the incident light, as the simplified structure diagram in Below Figure illustrates.Ī phototransistor can be either a two-lead or a three-lead device. Since the actual photogeneration of base current occurs in the collector-base region, the larger the physical area of this region, the more base current is generated. The schematic symbol and some typical phototransistors are shown in Below Figure. The relationship between the collector current and the light-generated base current in a phototransistor is In many cases, there is no electrical connection to the base. Except for the way base current is generated, the phototransistor behaves as a conventional BJT. This action produces a collector current that increases with Iλ. When light strikes the collector-base pn junction, a base current, Iλ, is produced that is directly proportional to the light intensity. When there is no incident light, there is only a small thermally generated collector-to-emitter leakage current, I CEO this dark cur- rent is typically in the nA range.
The collector-base pn junction is exposed to incident light through a lens opening in the transistor package. In a phototransistor the base current is produced when light strikes the photosensitive semiconductor base region.
The phototransistor effectively converts light energy to an electrical signal. A phototransistor is similar to a regular BJT except that the base current is produced and controlled by light instead of a voltage source.
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