Method for detecting mobile ions in a semiconductor device

A P-channel floating-gate MOS transistor is used to detect and measure positive mobile ions in the oxide layers of a semiconductor device. The transistor is first "programmed" by applying a voltage close to the breakdown voltage of the transistor, which causes electrons to tunnel through the oxide underlying the floating gate and to become trapped on the floating gate. This results in a negative voltage on the floating gate, which turns the transistor on and causes a first current, IDS0 to flow through the transistor. The semiconductor device is then baked, or heated, to accelerate the movement of positive mobile ions attracted to the negative charge previously trapped on the floating gate. Any positive mobile ions collected by the floating gate will neutralize a portion of the negative charge on the floating gate and will create a less negative voltage on the floating gate, resulting in a lesser current through the device after the bake. A second transistor, identical to the floating-gate transistor but with a gate connected to a test pad, is used to measure the charge gained by the floating gate during the bake and the corresponding concentration of collected mobile ions.