Image Credit: EG&G Optoelectronics
• Base current (Iλ) is produced when light strikes the collector base PN
• junction. The base current is directly proportional to the light intensity. If the size of the base region is doubled, the amount of generated base photocurrent is also doubled.
• Peak wavelength is the wavelength value at which a phototransistor is most sensitive. It is measured in nanometers (nm). Phototransistors respond to light over a broad range of wavelengths from fluorescent or incandescent light sources. They perform the best when matched with infrared (IR) LED light sources. This is because phototransistors have a peak spectral response in the near IR at about 840 nm.
• Collector-to-emitter breakdown voltage (VCE) is the maximum voltage allowed between the collector and emitter. Exceeding the maximum voltage can cause permanent damage to the phototransistor.
• Collect-emitter breakdown voltage (VBRCEO) typically ranges from 20 V to 50 V.
• Emitter-collector breakdown voltage (VBRECO) typically ranges from 4 V to 6 V.
• Dark current (ID) is the small amount of current can flow through a phototransistor even when it is not exposed to light. Dark current is thermally generated collector-emitter leakage current. It prevents a device from ever being considered completely "off." Dark current increases with the temperature and is measured in milliamps (mA).
• Power dissipation (PD or Ptot) describes power dissipation of the phototransistor measured in watts or milliwatts (mW). Actual dissipation isdeterminedby multiplying the voltage across the transistor and the current through the collector.Itis normally quoted for an ambient external temperature of 25°C.
• Rise time and fall time are measures of the phototransistor's speed of response. Both are expressed in nanoseconds (ns).
• Rise time (tR) is the amount of time it takes for a pulse waveform to increase from 10% to 90% of its maximum value.
• Fall time (tF) is the amount of time it takes for the output to fall from 90% to 10%.