Radiant energy – Photocells; circuits and apparatus – Housings
Patent
1977-02-04
1978-07-25
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Housings
H01J 502
Patent
active
041031571
ABSTRACT:
A photodetector capable of achieving an extremely high speed of response includes a Shottky barrier semiconductor diode in unpackaged chip form and a mount in which the diode is installed such that a photosensitive region on the diode is exposed forwardly toward the source of light which is to be analyzed. The mount serves to connect the diode with a source of bias voltage and also with a sampling oscilloscope, and it includes a parallel plate capacitor across which the bias voltage is applied. The center plane of this capacitor is located within 0.30 inch and preferably within 0.10 inch of the exposed photosensitive surface on the diode chip. The diode chip, constitutes a very high impedance in the circuit, and when illuminated by a light pulse, generates a signal in the form of a current which flows through the circuitry to the oscilloscope where the signal is displayed as a waveform. This signal is partially reflected at discontinuities in the circuitry, and the major discontinuity is the capacitor across which the bias voltage is applied. The reflected portion of the signal is re-reflected at the diode chip and follows the original signal with a net time delay equal to the round trip time between the photodiode and the capacitor. Further reflections of rapidly decaying intensity likewise occur, and with a conventional mounting all of the reflections, and particularly the first, significantly distort the trailing portion of the waveform. However, since the distance between the diode chip and the capacitor of this mount is so short, the reflected signal is close enough in time to the original signal to be completely embodied within the waveform of the original signal when it is displayed on the oscilloscope. Therefore, the reflection does not distort the waveform portraying the original signal. Rise times as short as 50 psec may be resolved with the photodetector.
REFERENCES:
patent: 3668612 (1974-10-01), Nepovim
patent: 3842262 (1974-10-01), Heitman et al.
patent: 3870878 (1975-03-01), Walter et al.
McDonnell Douglas Corporation
Nelms David C.
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