A vertical cavity multiple quantum well (MQW) modulator, which offers high contrast ratio, low insertion loss, low power consumption, low jitter and chirp, and easy two-dimensional integration with silicon electronics, is one of the essential components for dense optical interconnects. The devices we have investigated consist of quantum wells in a Fabry-Perot cavity configured as p-i-n diode. The absorptive characteristics of the MQW region can be modified through field-induced absorption change known as quantum-confined Stark effect (QCSE), which translate to the modulation in the device reflectivity. Modulators can also perform the dual function of a high-efficiency photodetector at the receiver end by using a different electronic control circuit. This has the advantage of doubling the optical I/O of a photonic layer and reducing power losses. In this paper, the high-speed performance of modulators and photodetectors will be presented. We use 26 pairs of 75-Å GaAs/AlGaAs quantum wells. The modulators have a high contrast ratio of 11 dB, small driving voltage of 3.5 V, and f3dB bandwidth greater than 18 GHz. If the device is used as a high-speed photodetector, it has a quantum efficiency of 95% and f3dB bandwidth greater than 10 GHz. Results of devices of different sizes, with different bias voltages and termination resistors, are presented.