Cascode MOS Measurement for Low Conductance Accuracy
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Solution Overview
Problem
Conventional wafer acceptance testing (WAT) systems face accuracy limitations at low conductance values, particularly below 1e-6 Mho, due to high impedance in semiconductor devices, which makes direct measurement of impedance sensitive to noise and limited in accuracy.
Innovation Solution
A cascode structure is formed by connecting a high impedance MOS device under test with a low impedance dummy MOS device in series, allowing for the derivation of conductance and impedance through the gain of the cascode structure, transconductance, and output impedance, enabling accurate measurement of low conductance values down to approximately 1e-8 Mho.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If direct measurement method is used for high impedance MOS devices, then measurement simplicity is maintained, but measurement precision deteriorates due to noise sensitivity and accuracy limitations at low conductance values
Solution Approach 1:
A cascode structure is introduced as an intermediary measurement system between the high impedance MOS device and the measurement instrument. The cascode structure includes a first MOS device (the device under test) and a second MOS device (a dummy device with known characteristics). This intermediary structure transforms the difficult-to-measure high impedance node into a measurable form by utilizing the known characteristics of the dummy device and the relationship between gain, transconductance, and output impedance in the cascode configuration.
Solution Approach 2:
A dummy MOS device is created as a copy or replica of the device under test, but with known and controllable characteristics. This dummy device serves as a reference element that allows the measurement system to indirectly determine the properties of the high impedance MOS device through the cascode structure's gain relationship, avoiding direct measurement of the difficult-to-access high impedance node.
2Measurement precision
If cascode structure is formed to measure low conductance values, then measurement precision improves, but device complexity increases due to additional components and measurement steps
Solution Approach 1:
The cascode structure acts as an intermediary that bridges the gap between the high impedance device under test and the measurement system. By introducing the dummy device and the cascode configuration, the system transforms the measurement problem into a solvable form using relationships between gain (A), transconductance (gm), and output impedance (Zout), where gds = gm/A. This allows accurate measurement of conductance values down to approximately 1e-8 Mho.
3Measurement precision
If high impedance nodes are directly measured, then measurement simplicity is maintained, but noise sensitivity increases causing measurement accuracy to deteriorate
Solution Approach 1:
The cascode structure serves as an intermediary that isolates the high impedance node from direct measurement. The dummy device in the cascode configuration provides a known reference point that allows the measurement system to calculate the output impedance without directly measuring the high impedance node, thereby reducing noise sensitivity and improving measurement accuracy.
Solution Approach 2:
The dummy MOS device creates a controlled copy of the measurement path that allows indirect determination of the high impedance node's properties. By measuring the gain of the cascode structure and using the known transconductance of the dummy device, the system can calculate the output impedance without directly probing the noisy high impedance node.
Data Source
AI summary
A dummy MOSFET is connected in series with a device under test to form cascode structure. The conductance of the low conductance MOSFET is derived from the measurements done on the cascode structure. An open loop gain stage is connected to the cascode structure in case the signal at the internal node of the cascode structure is extremely small to be measured directly and accurately. Impedance measurements can also be done on high impedance MOS devices without noise distortion with the help of the cascode arrangement.