Efficient active control of optical nonlinearity has long been a critical goal for advancing nonlinear optoelectronic devices. However, this remains a significant challenge, as nonlinear susceptibilities are inherently dictated by the microscopic properties of materials. In this work, we propose a microscopic mechanism to electrically reconfigure the Kerr nonlinearity by modulating the concentration of free electrons in heavily doped semiconductors under a static bias. Our theory incorporates electrostatic and hydrodynamic frameworks to describe the electronic dynamics, demonstrating electrically tunable linear and nonlinear modulations. The power threshold of achieving optical bistability shows unprecedented tunability over 2 orders of magnitude, reaching values as low as $10\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{W}$ through surface-charge control. These findings offer insights into understanding and actively controlling Kerr nonlinearities, paving the way for efficient refractive-index engineering as well as the development of advanced linear and nonlinear electro-optical modulators