High-speed and high-performance optical phase and amplitude modulators are critical components of many photonic systems. Semiconductor-based modulators are very attractive, since they can be monolithically integrated with other semiconductor devices. Unfortunately, the commonly used modulators based on square quantum wells have inherent properties that limit their modulation performance. We present a new class of quantum wells called "stepped quantum wells" (SQW) with extra degrees of freedom that can be used to design high performance optical modulators. We demonstrated SQW phase modulators with nearly one order of magnitude higher efficiency than their counterparts. Also, linearized modulators based on SQW with more than two orders of magnitude higher linearity than the existing semiconductor modulators are presented. Finally, high-performance surface-normal modulators based on SQWs with nearly two times better efficiency and 7 dB higher extinction ratio compared with the conventional devices with rectangular and coupled-quantum well active layers are demonstrated.