ESD protection for open drain I/O pad in integrated circuit with parasitic field FET devices

摘要

An open drain FET driver circuit at an input-output pad of a semiconductor chip and a frame of the same conductivity type as the drain and source diffusions of the driver is formed around the driver (or partly around the driver). The frame is connected to Vdd and forms the diffusion for the Vdd end of a field FET. The drain of the driver forms the diffusion for the pad end of this field FET and the pad to Vdd FET breaks down in response to an ESD voltage between the pad and Vdd and provides a path for ESD current that the open drain driver itself does not provide. Optionally, a second field FET is formed between the source of the driver FET and the frame and this FET conducts an ESD current between the pad and Vdd in series with the driver. With this cell array structure, the junction capacitance which the ESD protection devices contribute to the pad can be significantly reduced for high speed I/O applications.