A polymer/silica hybrid 2 \times 2 multimode-interference Mach–Zehnder interferometer thermo-optic (TO) switch is designed and fabricated. Instead of polymer, silica is used as under-cladding to accelerate heat release because of its large thermal conductivity. The developed switch exhibits low power consumption of 6.2 mW, low crosstalk of about –28 dB, and short response time. The rise and fall times of 103 and 91 \mu s for this hybrid switch are shortened by 40.8% and 52.4%, respectively, compared with those of the fabricated TO switch (174 and 191 \mu s) using polymer as both upper- and under-claddings.