Theory of the p + n Diode SAW Storage Correlator in the Flash Mode

Abstmct-The theory of the gap-coupled surface acoustic wave p'n diode storage correlator operated in the flash mode is presented. The circuit model used in the analysis of both the storage and the read-out processes is reviewed. The effect of the minority carrier lifetime and the voltage dependence of the diode capacitance are included for the first time in the analysis of the flash-mode writing process. A straightforward transmission line theory approach is developed to find the surface wave amplitude excited by the diode potential during the readout process. The calculated predictions of this theory are in good agreement with experimental data and tend to confirm the possibility of ohtaining efficient storage with a single write-in short pulse. HE FLASH technique was applied to the p'n diode SAW storage correlator by Gautier et al. [l], who were unable to store any signal in the diodes with one write-in short pulse when using a planar vidicon array with a long minority carrier lifetime (60 p). This negative result is explained as follows. The pulse used to charge the array has a duration of 5-10 ns. With a long minority carrier lifetime the holes injected from the p-island into the n-side do not recombine and a large number of these carriers will tend to diffuse back across the junction after the pulse is turned off. The general consensus then was that the use of diffused diode devices was limited to narrow bandwidth signal applications because of the long charging times required; i.e., that the parametric technique should be used in conjunction with diffused pn diodes. Using a V-groove mesa diode structure, Borden and Kino [2] demonstrated that p+n diodes can be charged with a single short pulse and that the charge is stored for milliseconds. They attributed their success to the fact that with such structures the minority carrier lifetime is several orders of magnitude shorter than the planar vidicon array used by other groups. However, the large magnitude of the storage time which they reported seems to contradict the obvious fact that a reduction in the minority carrier lifetime will undoubtedly reduce the storage time. Loh et al. [3], commenting on Borden and Kino's results, concluded that the deliberate introduction of asymmetric, rather than symmetric, trapping levels was responsible for their success in efficiently charging a pfn diode with a single short pulse since the presence of these