  In this paper we examine the validity of the concept of impulse response
employed to characterize the time response and the signal-to-noise ratio of
p-i-n and similar photodetecting devices. We analyze critically the way in
which the formalism of analog linear systems has been extrapolated, by
employing results from macroscopic electromagnetic theory such as the
Shockley--Ramo theorem or any equivalent approach, to the extreme case of a
single-photon detection. We argue that the concept of "response to an optical
impulse" is ill-defined in the customary terms it is envisioned in the
literature, this is, as an output current pulse having a certain predictable,
calculated temporal shape, in response to the detection of an optical "Dirac
delta" impulse, conceived in turn as the absorption of a single photon.
