Trends in the efficiency and small signal modulation behavior of porous sil
icon light emitting diodes (LEDs) are reported for devices formed by the an
odization of bulk silicon p-n junctions. As the average size of the silicon
skeleton is decreased, the external electroluminescence (EL) efficiency in
creases from 0.001% to 0.18% and there is a corresponding blue shift in the
EL peak from 776 to 633 nm. An associated tenfold increase is observed in
the photoluminescence efficiency while the diode resistance, at 2 V, increa
ses from 3x10(3) to 1x10(6) Omega. Under small signal pulsed operation, the
voltage dependence of the rising edge of the EL is well described by a car
rier mobility of 3x10(-4) cm(2) s(-1) V-1 which is independent of the avera
ge size of the luminescent regions of the silicon nanostructure. The fallin
g edge of the EL transient is dominated by radiative recombination of quant
um confined excitons. The modulation speed is found to be limited by a comb
ination of carrier mobility in the silicon wires and radiative recombinatio
n processes. Evidence of charge trapping and discharge is found in an EL ov
ershoot phenomenon. The major application of this type of porous silicon LE
D, with modulation speeds below 1 MHz, appears to be for displays integrate
d with circuitry rather than for optical interconnection. [S0021-8979(99)07
717-8].