RDS(on) Test

A challenging test for FETs is drain-to-source on-state resistance, or RDS (On). Typically, a MOSFET is designed to have a low RDS (On). Let's say we have a spec of RDS (On) < 100 mΩ. To generate a measurable voltage drop, say 50mV across a 100 mΩ resistance, 500mA current is required. So, this measurement requires a high-resolution voltmeter and a high-current source.

We can use a VI to pull current through the FET, but because of IR drop through the DIB traces, we cannot use the same VI to measure voltage. To ensure a accurate measurement at the FET drain/source, we need to use a full kelvin 4-wire connection and a high -resolution differential voltmeter as shown in the figure below.


Test Procedure:

  1. Force, say 1V using VI1, set current capability lower than VI2 to make VI2 dominate VI1
  2. Operate VI2 in current mode and use it to pull say, I1=500mA. Since VI2 dominates over VI1, 500mA will flow through FET.
  3. Use the differential voltmeter to measure the drop across the FET, voltage meas. = V1
  4. Repeat 1-3 pulling say, I2=300mA, voltage meas. = V2
  5. Calculate the resistance as $$\ R_{DS} (On)= {(V2-V1) \over (I2-I1)} $$
The dual measurement method cancels any offset in the measurement instrumentation

DIB design considerations:

  1. kelvin force/sense planes/traces
  2. differential voltmeter traces
  3. don't damage the FET

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