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In n-i-p-i structures with selective ohmic contacts to the n- and the p-layers both, carrier densities and electric fields, can be tuned over a wide range by applying moderate external voltages. The resulting absorption changes due to phase space filling and Franz-Keldysh effect, respectively, can be superimposed constructively by a suitable sample design. In contrast to optically excited n-i-p-i structures whose dynamic behavior is governed by the internal electron-hole recombination lifetimes (well up to milliseconds), the time constants for structures provided with selective n- and p-contacts are RC times given by the resistance of the doped layers including contact resistances, and by the capacitance of the interdigitated n- and p-layers. Although the areal capacitance of such n-i-p-i structures is relatively large compared to p-i-n structures, very short RC times can be achieved for sufficiently small devices as RC time constants scale basically quadratically with the device width. We have investigated the dynamical response on a series of n-i-p-i modulators grown by epitaxial shadow mask MBE with a width of the n-i-p-i region ranging from 100 micrometer down to 5 micrometer. For the smallest devices time constants as low as 1.5 ns have been measured. Modulation at up to 250 MHz has been demonstrated with a decrease in switching contrast from 2.2 at dc operation to 1.75 at 250 MHz. The voltage swing used in these experiments as only 3.7 V. We stress that these devices were not optimized. The switching time for the 5 micrometer sample was increased by about a factor 10 due to high contact resistances. The switching contrast can be enhanced easily and with no penalty on the high frequency response by using n-i-p-i structures with a larger number of periods, as the RC time constant is independent of the number of periods. Thus, with a suitable design operation in the multi-GHz range as well as an improved switching contrast should be possible.
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