Abstract An ungated FET having an annular drain surrounding a central source is formed on a test site on the same semiconductor chip with product FET devices. All FETs are covered with a passivating film whose charge level is to be determined. An off-chip operational amplifier circuit is connected to the ungated FET to provide automatically whatever source-to-substrate bias is required to cause a negligibly small source-to-drain current, e.g., 0.5 microamps to flow. The potential difference between the source and substrate is measured while the aforesaid negligibly small source-to-drain current is flowing. The charge level in the passivating material covering the product FET devices as well as the ungated FET device is calculated using the measured sourceto-substrate bias.
A method for determining the charge density within an insulating layer on a semiconductor substrate comprising providing an ungated field effect transistor formed on said substrate and having said insulating layer as its gate insulating layer, applying a fixed voltage between the source and drain electrodes of said transistor, applying a variable voltage between said substrate and said source, monitoring the magnitude of source-to-drain current flowing in said transistor, adjusting said variable voltage to a value at which said sourceto-drain current is reduced to a negligible value, and determining said charge density using said value of said variable voltage.
Claim 2
Independent Claim
99 words
A method for determining the charge density within an insulating layer on a semiconductor substrate comprising providing an ungated field effect transistor formed on said substrate and having said insulating layer as its gate insulating layer, applying a fixed voltage between the source and drain electrodes of said transistor, applying a variable voltage between said substrate and said source, monitoring the magnitude of source-to-drain current flowing in said transistor, adjusting said variable voltage to a value at which said sourceto-drain current is reduced to a negligible value, and determining said charge density using said value of said variable voltage.
Claim 3
Dependent Claim
35 words
The method defined in claim 1 and further including forming the drain and source junctions of said ungated field effect transistor by placing an annular drain junction in said substrate surrounding a central source junction.
Claim 4
Dependent Claim
18 words
The method defined in claim 1 wherein said substrate is silicon and said insulating layer is silicon oxide.
Claim 5
Dependent Claim
23 words
The method defined in claim 3 wherein said insulating layer is selected from the group consisting of quartz and thermally grown silicon oxide.