Proposed optoelectronic method of achieving very fast digital logic

In fast logic systems many difficulties are involved in electrical logical interconnections, which in the fastest systems have to be by correctly terminated matched lines. Because the line impedance is necessarily low, it is difficult to meet the requirements of (i) a useful fan-out ratio, and (ii) digit voltages large enough to overcome interference and tolerances at the inputs of succeeding logic elements, without demanding considerable energy dissipation at each logic step. Some of these difficulties may be avoided by transmitting the digital information in the form of light, but the losses inherent in this process, and the large gain required to offset these losses, have hitherto precluded the realisation of fast optoelectronic digital systems. A method is proposed in which a large current gain is obtained by accelerating photoelectrons in vacuo and using their kinetic energy to generate electron-hole pairs in a semiconductor electron detector. The time delay associated with this gain is far smaller than would be introduced by other means. It is shown that an optoelectronic logic system using refinements of techniques already available should yield logic speeds comparable with the fastest so far reported for other methods, larger fan-in and fan-out ratios, and a power dissipation several times smaller. Foreseeable developments should permit substantial further increase in speed and reduction of power dissipation.