Pulse-response characteristics of position-sensitive photodetectors

A key factor in designing position-sensitive detectors operating through lateral photoeffects in semiconductors concerns the nature of the pulse response over the entire active area of the device. It is demonstrated that the limiting behavior can be approximated by means of a fundamental RC-filter time constant τ
c
together with position-dependent initial delay times of up to 0.6τ
c
. A comparison with recorded pulse-response traces suggests that the time constant τ
c
of two-dimensional position-sensitive photodetectors takes on values of R
s
C
A
d
²
/π
²
, where R
s
is the backlayer sheet resistance, C
A
the junction capacitance per unit area, and d the electrode separation. Illustrations of the dependence of pulse shape on spot position for representative ion-implanted silicon units are shown to substantiate the analysis in the context of evaluating position-sensitive photodetectors for applications involving very short laser pulses.