Transistor with integrated photodetector for conductivity modulation

Abstract

A semiconductor device and a method of modulating the conductivity of a DMOS transistor included in the device utilize photocurrent generated by a photodetector for minority-carrier injection. The injection of minority carriers into the DMOS transistor of the device reduces the on-resistance of the transistor. The semiconductor device may be used in an optocoupling application. In a first embodiment, the semiconductor device includes a lateral DMOS transistor, a minority-carrier injector, and a photodetector. In a preferred embodiment, the semiconductor device is an integrated device, such that the transistor, the injector and the photodetector are collectively formed on a single semiconductive substrate. The photodetector of the device includes at least one electrically isolated photodiode. As an example, the photodetector may include two dielectrically isolated photodiodes. The photodiodes are serially connected between the drain terminal of the transistor and the minority-carrier injector. The connection to the minority-carrier injector allows the photocurrent generated by the photodiodes to be transmitted to the injector to provide the current/voltage needed by the injector to introduce minority carriers into the transistor. The connection to the drain terminal ensures that the voltage applied to the injector, when the photodetector is generating photocurrent, is maintained at a higher voltage than the drain voltage applied to the drain terminal, even if the drain voltage is fluctuating. In a second embodiment, the semiconductor device includes a vertical DMOS transistor, instead of a lateral DMOS transistor.