Patent ID: 7027316

Claim:
An access circuit structure for selectively providing an externally accessible terminal with access to each of a plurality of circuit wells fabricated in a semiconductor substrate and isolated from each other, the access circuit structure comprising for each of the circuit wells: a first transistor fabricated in a well formed in the semiconductor substrate that is isolated from the circuit wells, the first transistor having a first source-drain region fabricated in the well and being coupled to the externally accessible terminal and to the well, a second source-drain region fabricated in the well, and a gate electrode fabricated between the first and second source-drain regions of the first transistor and coupled to receive a first select signal for the respective circuit well, the first transistor being fabricated in the well in a manner that causes a first diode to be formed between the source and drain regions of the first transistor; and a second transistor fabricated in a well formed in the semiconductor substrate that is isolated from the circuit wells, the second transistor having a first source-drain region fabricated in the well and being coupled to the second source-drain region of the first transistor, a second source-drain region fabricated in the well and being coupled to the well and to a respective one of the circuit wells, and a gate electrode fabricated between the first and second source-drain regions of the second transistor and coupled to receive a second select signal for the respective circuit well, the second transistor being fabricated in the well in a manner that causes a second diode to be formed between the source and drain regions of the second transistor, the second diode being coupled to the first diode in a back-to-back configuration.