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Carrier-injection-type high-speed semiconductor optical switches have been of interest in recent years due to
their nanosecond switching times, their immunity to variations in temperature, wavelength, polarization, etc, and the
ease with which they can be monolithically integrated with other optoelectronic components and electronic circuitry.
Their drawbacks, however, have been high insertion loss and excessive power dissipation. To overcome these limits,
a novel, large cross-section, single-mode AlGaAs/GaAs optical switch has been designed and fabricated. The
switch's strip-loaded waveguide uses a five-layer W-shaped heterostructure and a 1.7&mgr;m-thick core layer, which
provides high fiber-coupling efficiency. Since the constituent heterojunction band discontinuities can impede the
current across the junction, the addition of 20nm-40nm thick, compositionally graded interfaces significantly
reduces the switching voltage. In addition, using a lightly doped core layer can reduce the series resistance of the
switch, which is important in heat reduction. The core doping needs to be low otherwise it will cause increased free-carrier
absorption, which contributes to high insertion loss. We have fabricated switches with different core doping
levels using both abrupt and graded heterojunctions. The measured on-chip optical propagation losses are 0.3dB/cm
for unintentionally doped core, 1.5dB/cm for n = 1x1016cm-3 doped core, and 2.7dB/cm for n = 5x1016cm-3 doped
core. The measured I-V curves show that the switching voltage can be reduced by changing abrupt heterojunctions
to graded ones. The calculated theoretical band structure for switches with abrupt/graded heterojunctions based on
thermionic emission clearly demonstrated the advantages of applying grading in semiconductor optical switches.
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