Abstract: A closed loop control system for controlling the temperature of a heated substrate is presented. In the preferred embodiment, the closed loop control system is implemented substantially or completely on a single substrate. The closed loop control system comprises means to reduce or eliminate parasitic feedback in the control system. In one aspect of the present invention, the mechanism for reducing the effects of parasitic feedback comprises a mechanism to prevent minority carriers from collecting around the base of the temperature sensing device. The mechanism for preventing minority carriers from collecting around the base of the temperature sensing device can be one or more guard rings which attract the minority carriers and prevent them from collecting around the base of the temperature sensor.