An optically coupled amplifying device

This device consists of an electroluminescent p-n junction diode coupled to a phototransistor. The electrical properties of such amplifying devices are analyzed in the general case, and the properties of a GaAs diode--Si phototransistor pair are compared with this analysis. The spectra of electroluminescent GaAs diodes are shown to consist of a peak of 1.41 eV and another peak at 1.37 eV of approximately equal magnitude. The 1.41 eV peak is strongly absorbed in GaAs and special techniques must be employed to utilize this emission. The 1.37 eV peak, which is due to recombination via Zn or Cd atoms, is absorbed only slightly. The external quantum efficiency of these diodes is about 1% at room temperature. A description is given of the factors which determine this efficiency and of what efficiencies might reasonably be expected from GaAs in the near future. The speed of response of the GaAs diode is a few nanoseconds and does not limit the speed of the pair. The design considerations of the Si phototransistor are described. A very high value of hFEis required in the phototransistor, units used in this device having hFE∼ 500. The speed of the pair is limited by the transistor collector charging time, the emitter charging time, and by the diffusion of minority carriers to the collector junction. One of the major problems in constructing efficient optically coupled amplifiers is the maximization of optical coupling between the source and sensor. Various solutions to these problems and the design compromises involved are described. These devices typically have a current gain of 2 and risetime of about 500 nanoseconds. The outstanding characteristic of this device is the essentially complete electrical isolation of the input and output, the coupling capacitance being less than 1 pf and the coupling resistance being greater than 10¹³ohms. This provides greater flexibility to the circuit designer, because the input may be operated at a potential hundreds of volts different from the output, and because the feedback present in a normal transistor is absent.