MOSFET switching devices operate in the on and off states. In the “on” state, the impedance of the switch is theoretically zero and no power is dissipated in the switch no matter how much current is flowing through it. In the “off” state, the impedance of the switch is theoretically infinite, therefore no current is flowing and no power is dissipated.

Thedrain-source on-resistance (RDS(on)) is the effective resistance between the drain and the source of a MOSFET when it’s in the on state. This occurs when a specific gate-to-source voltage (VGS) is applied. In general, as the VGS increases, the on-resistance decreases. The lower the MOSFET on-resistance, the better because a low resistance reduces undesired power dissipation, improving the power efficiency of the device.

Answer: Drain-Source On-Resistance - RDS(on)

What is drain-source on-resistance?

Drain-source on-resistance (RDS(on)) is the resistance between the drain and the source of a MOSFET when a specific gate-to-source voltage (VGS) is applied to bias the device to the on state. As the VGS increases, the on-resistance generally decreases. The measurement is made in the ohmic (i.e. linear) region of the device. Generally speaking, the lower the MOSFET on-resistance, the better.

One of the ways to trace this resistance is to use a curve tracer. On a curve tracer, the so called “Collector Supply” drives the drain while the “Step Generator” drives the gate. For step-by-step instructions on how to test a MOSFET for drain-source on-resistance using a curve tracer, see below. For instructions on how to use an oscilloscope or SMU to measure MOSFET on-resistance, see our “What is the drain-source on-resistance of a MOSFET?” FAQ.

What the display shows:

The display shows VDS on the horizontal axis, and the resulting ID on the vertical axis. The specification is met when at the specified VDS, VDS/ID is less than or equal to the specified maximum.

How to test a MOSFET for drain-source on-resistance on a curve tracer:

1. Under Controls, set:

            A:Max Peak Volts to the lowest setting above the specified VDS

            B: Max Peak Power Watts to the lowest setting that satisfies (ID x VDS)

            C: Collector Supply Polarity to (+DC) for N-channel or (-DC) for P-channel  

            D: Horizontal Volts/Div to display VDS between the 5th and 10th horizontal divisions

            E: Vertical Current/Div to display ID between the 5th and 10th vertical divisions

            F: Number of steps to minimum (zero)

            G: Step Generator to Voltage

            H: Step Generator Polarity to apply forward bias (+ for N-channel), (- for P-channel)

            I: Step/Offset Ampl to approx 50% of the specified VGS

            J: Pulse to Long       

            K: Configuration to (Base/Step Gen, Emitter/Common)

            L: Variable Collector Supply to minimum % (full ccw)

            M: DotCursor ON

2. Apply power to the MOSFET:

            A: Position the Left/Right switch as appropriate

            B: Slowly increase the Variable Collector Supply until the specified VDS is reached

3. Compare to data sheet specifications:

            A: Check that VDS/ID is less than or equal to the specified minimum

Tektronix Curve Tracers are discontinued products. More efficient and accurate methodologies and solutions have been designed to support curve tracing functionality on a much more compact form factor. One such solution is based on using a dual channel SMU or two single channel SMUs and software to control the bias voltage step generation and the relative drain to source voltage drop. To learn more, see our “What is the drain-source on-resistance of a MOSFET?” FAQ.

Transconductance is a key test for validating the MOSFET performance in power electronics designs. It ensures that a MOSFET is functioning properly and helps engineers choose the best one when voltage gain is a key spec for their circuit designs. This, in turn allows companies to take power semiconductor devices to market faster while minimizing failures in the field.

Transconductance is the ratio of drain current (ID) to gate-source voltage (VGS) when a constant drain-source voltage is applied. The current to voltage ratio is commonly referred to as gain. Transconductance is a critical parameter strictly connected with the threshold voltage (VTH) of MOSETs and both are related to the size of the gate channel. The formula for deriving the transconductance of a MOSFET from I-V measurements is:

gm = ΔID / ΔVGS

The approach shown in the first configuration calls for three source measure units (SMUs), allowing every node to be held at a feedback-controlled voltage and every current to be measured simultaneously. If you don’t have enough SMU channels to cover each device channel connection, it is possible to proceed as shown in the second configuration. It should be noted that this configuration is more susceptible to a noisy ground connection and can produce ground loops if long cables are used. Also, the current and voltage at the source terminal cannot be measured, which can lead to errors in calculations.

Measuring transconductance

1. Sweep the gate voltage (VGS) over the desired range, while maintaining a constant drain/source voltage (VDS)
2. Measure the drain current (ID) at each increment step of VGS.
3. Calculate transconductance (gm) by dividing the small changes in the current ID by the small changes in VGS.

The red plot line shown here illustrates the transconductance (gm) and the maximum transconductance value (Vth).

Learn more about safe, precise and fast MOSFET device characterization tests.

Answer: Zero Gate Voltage Drain Current - IDSS

What is Zero Gate Voltage Drain Current? 

Zero gate voltage drain current is the ID that flows when VGS=0.  It’s the on-state current in a depletion mode MOSFET and the off-state current in an enhancement mode MOSFET.

On the IV curve tracer, the Collector Supply drives the drain and the gate is shorted to the source so that VGS=0.

What The Display Shows:

The display shows VDS on the horizontal axis, and the resulting ID on the vertical axis.  The specification is met when with VGS=0 and the specified VDS applied, ID is less than or equal to the specified maximum.

How To Do It:

1. Set controls:

            A:Max Peak Volts to the lowest setting above the specified VDS

            B: Max Peak Power Watts to the lowest setting that satisfies (ID x VDS)

            C: Horizontal Volts/Div to display VDS between the 5th and 10th horizontal divisions

            D: Vertical Current/Div to display the ID between the 5th and 10th vertical divisions

            E: Collector Supply Polarity to (+DC) for N-channel or (-DC) for P-channel

            F: Configuration to (Base/Short, Emitter/Common)

            G: Variable Collector Supply to minimum % (full ccw)

            H: DotCursor ON

2. Apply power to the MOSFET:

            A: Position the Left/Right switch as appropriate

            B: Slowly increase the Variable Collector Supply % until the specified VDS is reached

3. Compare to data sheet specifications:

            Check that at the specified VDS, ID is less than or equal to the specified maximum

Answer: Gate Threshold Voltage - VGS(th)

What is Gate Threshold Voltage? 

Gate threshold voltage is the lowest VGS at which a specified small amount of ID flows.  The test is run with VGS = VDS.

On the curve tracer, the Collector Supply provides VDS.  Patch cords are used to short the gate to the drain so that VGS=VDS.

What The Display Shows:

VGS is displayed on the horizontal axis, and the resulting ID is displayed on the vertical axis.  The specification is met when, at the specified ID, VGS is within the min/max limits.

How To Do It:

1. Set controls:

            A: Max Peak Volts to the lowest setting above the specified VGS

            B: Max Peak Power Watts to the lowest setting that satisfies (ID x VDS)

            C: Horizontal Volts/Div to display VGS between the 5th and 10th horizontal divisions

            D: Vertical Current/Div to display the specified  ID between the 5th and 10th vertical divisions

            E: Collector Supply Polarity to (+DC) for N-channel or (-DC) for P-channel

            F: Configuration to (Base/Open, Emitter/Common)

            G: Variable Collector Supply to minimum % (full ccw)

            H: DotCursor ON

2: Attach patch cords:

            A: Connect a patch cord between the base and collector terminals on the unused side of the interface area

            B: Connect a second patch cord between the base sense and collector sense terminals on the unused side of the fixture area

3. Apply power to the MOSFET:

            A: Position the Left/Right switch to Both

            B: Slowly increase the Variable Collector Supply % until either the specified ID or the maximum threshold voltage is attained – whichever comes first

4. Compare to data sheet specifications:

            Check that the gate threshold voltage is within the specified min/max limits

Answer: Transconductance (gFS) and Forward Admittance

What is Transconductance and Forward Admittance? 

Transconductance is the ratio of ID to VGS.  The I/V ratio is commonly referred to as gain.

On the curve tracer, the Collector Supply drives the drain and the Step Generator drives the gate.

What The Display Shows:

The display shows VDS on the horizontal axis, and the resulting ID on the vertical axis.  With the Step Generator providing gate drive, the curve will be displaced upward from the horizontal axis as the gate drive causes a proportional ID.  The specification is met when, at either the specified VGS or the specified ID, the ratio of ID to VGS is equal to or greater than the specified minimum.

How To Do It:

1. Set controls:

            A:Max Peak Volts to the lowest setting above the specified VDS

            B: Peak Power Watts at the lowest setting to satisfy (ID x VDS)

            C: Collector Supply Polarity to (+DC) for N-channel or (-DC) for P-channel

            D: Horizontal Volts/Div to display the specified VDS between the 5th and 10th horizontal divisions

            E: Vertical Current/Div to display the specified ID between the 5th and 10th vertical divisions

            F: Number of steps to minimum (zero)

            G: Step Generator to Voltage

            H: Step Generator Polarity to apply forward bias  (+ for N-channel),  (- for P-channel)

            I: Step/Offset Ampl to approx 1% of the specified VDS

            J: Pulse to Long       

            K: Configuration to (Base/Step Gen, Emitter/Common)

            L: Variable Collector Supply to minimum % (full ccw)

            M: DotCursor ON

2. Apply power to the MOSFET:

            A: Position the Left/Right switch as appropriate

            B: Slowly increase the Variable Collector Supply % until the specified  VDS is reached

3. Adjust to parameters:

            Press and hold Offset Aid until an appreciable vertical displacement of the   curve occurs. It will be necessary to readjust Variable Collector % to maintain VDS.  Continue adjusting Step Offset and VDS alternately until the specified operating point is reached.

4. Calculate transconductance (gFS):

             Read gFS directly from the cursor readout

5. Compare to data sheet specifications:

              Check that the value is equal to or greater than the specified minimum

Forward admittance is an alternative way of expressing transconductance and is measured by setting the curve tracer up to measure transconductance (as above), switching Horizontal Volts/Div to STEP GEN, using SWEEP to complete the curve, then changing the cursor to F line and positioning the slope of the F line until it’s tangent to the curve.

Answer: On-State Drain Current - ID(on)

What is On-State Drain Current?

On-state drain current is ID with a specified VGS to bias the device to the on-state.  The measurement is made in the ohmic (i.e. linear) region of the device.

On the curve tracer the Collector Supply drives the drain and the Step Generator drives the gate.

What The Display Shows:

The display shows VDS on the horizontal axis, and the resulting ID on the vertical axis.  The specification is met when at the specified VDS, ID is greater than or equal to the specified minimum.

How To Do It:

1. Set controls:

            A:Max Peak Volts to the lowest setting above the specified VDS

            B: Max Peak Power Watts to the lowest setting that satisfies (ID x VDS)

            C: I: Collector Supply Polarity to (+DC) for N-channel or (-DC) for P-channel  

            D: Horizontal Volts/Div to display VDS between the 5th and 10th horizontal divisions

            E: Vertical Current/Div to display ID between the 5th and 10th vertical divisions

            F: Number of steps to minimum (zero)

            G: Step Generator to Voltage

            H: Step Generator Polarity to apply forward bias (+ for N-channel),  (- for P-channel)

            I: Step/Offset Ampl to approx 50% of the specified VGS

            J: Pulse to Long       

            K: Configuration to (Base/Step Gen, Emitter/Common)

            L: Variable Collector Supply to minimum % (full ccw)

            M: DotCursor ON

2. Apply power to the device:

            A: Position the Left/Right switch as appropriate

            B: Slowly increase the Variable Collector Supply until the specified VDS is reached

3. Compare to data sheet specifications:

            A: Check that ID is equal to or greater than the specified minimum

Answer: Drain-Source Breakdown Voltage - V(br)DSS

What is Drain-Source Breakdown Voltage?

Drain-source breakdown voltage is the VDS at which a specified value of ID flows, with VGS=0.  Since it's the reverse current through a pinched-off channel, ID exhibits a knee shaped rise, increasing rapidly once breakdown occurs.

On the curve tracer, the Collector Supply drives the drain and the gate is shorted to the source so VGS=0.

What The Display Shows:

The display shows VDS on the horizontal axis, and the resulting ID on the vertical axis.  The specification is met when, at the specified ID, VDS is greater than or equal to the specified minimum.

How To Do It:

1. Set controls:

A: Max Peak Volts to the lowest setting above the specified minimum

     VDS

            B: Max Peak Power Watts to the lowest setting that satisfies (ID x VDS)

            C: Horizontal Volts/Div to display VDS between the 5th and 10th horizontal divisions

            D: Vertical Current/Div to display ID between the 5th and 10th vertical divisions                  

            E: Collector Supply Polarity to +Leakage (for N-channel) or -Leakage (for P-channel)

            F: Configuration to (Base/Short, Emitter/Common)

            G: Variable Collector Supply to minimum % (full ccw)

            H: DotCursor ON

2. Apply power to the MOSFET:

            A: Position the Left/Right switch as appropriate

            B: Slowly increase the Variable Collector Supply % until the specified ID is attained

3. Compare to data sheet specifications:

            Check that at the specified ID, VDS is greater than or equal to the specified minimum

Answer: Zero Gate Voltage Drain Current - IDSS

What is Zero Gate Voltage Drain Current?

Zero gate voltage drain current is the ID that flows when VGS=0.  It’s the on-state current in a depletion mode MOSFET and the off-state current in an enhancement mode MOSFET.

On the curve tracer, the Collector Supply drives the drain and the gate is shorted to the source so that VGS=0.

What The Display Shows:

The display shows VDS on the horizontal axis, and the resulting ID on the vertical axis.  The specification is met when with VGS=0 and the specified VDS applied, ID is less than or equal to the specified maximum.

How To Do It:

1. Set controls:

            A:Max Peak Volts to the lowest setting above the specified VDS

            B: Max Peak Power Watts to the lowest setting that satisfies (ID x VDS)

            C: Horizontal Volts/Div to display VDS between the 5th and 10th horizontal divisions

            D: Vertical Current/Div to display the ID between the 5th and 10thvertical divisions

            E: Collector Supply Polarity to (+DC) for N-channel or (-DC) for P-channel

            F: Configuration to (Base/Short, Emitter/Common)

            G: Variable Collector Supply to minimum % (full ccw)

            H: DotCursor ON

2. Apply power to the MOSFET:

            A: Position the Left/Right switch as appropriate

            B: Slowly increase the Variable Collector Supply % until the specified VDS is reached

3. Compare to data sheet specifications:

            Check that at the specified VDS, ID is less than or equal to the specified maximum