Photodetector efficiency decreases as bandwidth increases, Bandwidth-e
fficiency limitations of traveling-ware photodetectors (TWPD's) are su
bstantially greater than those of lumped-element photodetectors becaus
e the velocity-mismatch bandwidth limitation is independent of device
length, TWPD's can be long for high efficiency without significantly c
ompromising bandwidth, The TWPD is modeled by a terminated section of
transmission line with a position-dependent photocurrent source propag
ating on it at the optical group velocity, A wave model fur the transm
ission line confirms the accuracy of an equivalent-circuit model for e
lectrical wave propagation, The velocity-mismatch impulse and frequenc
y response are determined by absorption coefficient and wave velocitie
s rather than junction capacitance and load resistance, The velocity-m
ismatch bandwidth limitations can be written in a simple form which el
ucidates the factors affecting device response, A discretized periodic
TWPD is described by the same equations as the fully distributed vers
ion, This more complicated device offers additional degrees of freedom
in design and potentially improved performance.