What Is a Curve Tracer? How I-V Curves Show Semiconductor Health

What Is a Curve Tracer? How I-V Curves Show Semiconductor Health

ByKen

What Is a Curve Tracer? How I-V Curves Show Semiconductor Health

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What is a curve tracer? A curve tracer is an instrument that applies a changing voltage or current to an electronic component and plots how the component responds. The result is a graph of current against voltage, called an I-V curve. Keysight describes a curve tracer as a tool that applies a varying voltage or current to a device under test and plots the response on a screen. It is used mainly on semiconductors, including diodes, transistors, MOSFETs, IGBTs, and power devices, because their behavior is easiest to understand as a curve rather than a single number.

Quick answer

Short answer to what is a curve tracer: A curve tracer sweeps a voltage or current across a diode, transistor, or power device and plots the response as an I-V curve, which shows conduction, breakdown, and leakage behavior.

The basic idea: sweep, measure, plot

If you have used a multimeter to check a diode with its diode mode, you have measured one point on the diode’s curve: a forward voltage drop at one test current. A curve tracer draws the whole curve. It shows where the device starts to conduct, how it behaves at higher voltage, and where it breaks down or leaks. This guide explains what the instrument does, how to read its display, and where it fits compared with a multimeter.

A curve tracer works in a simple loop. It steps or sweeps a voltage or current across the device, measures the matching current or voltage at each step, and plots the pairs. Keysight’s blog describes the process in the same terms: the instrument sweeps through a range of values, records the matching responses, and produces the I-V curve. Pulsed modes use short pulses to limit device heating, which matters for power parts that warm up during a long measurement.

The display is usually an oscilloscope-style screen. The horizontal axis carries voltage, and the vertical axis carries current. Tektronix’s datasheet for its 370B and 371B programmable curve tracers describes the display axes this way: the vertical axis shows current, either collector or base current, or emitter current, and the horizontal axis shows voltage, either collector voltage or base or emitter voltage. A family of curves can be drawn at several base currents or voltages, which is how a transistor’s output characteristics are shown.

What the curves reveal

Each region of a curve tells you something. For a diode, the curve is nearly flat until the forward voltage is reached, then rises sharply. The reverse side is flat until breakdown, where current rises again. For a transistor, the curves show how much current flows for a given control signal, and where the transistor saturates. Keysight’s power device analyzer page says the curves show behavior across the conduction, saturation, breakdown, and leakage regions, and that the method is used to check the safe operating area of power devices.

Keysight’s blog lists the characteristics a curve tracer typically checks: breakdown voltage, threshold voltage, on-resistance, leakage currents, gate charge, and capacitance, in parts such as MOSFETs, bipolar transistors, IGBTs, diodes, and wide bandgap devices. Not every instrument measures all of these. The basic I-V sweep is the common core, and extra functions depend on the model.

Two-terminal and three-terminal use

A curve tracer can test a two-terminal part, such as a diode, by connecting it across two pins. The instrument then draws one I-V curve for that device. Three-terminal parts, such as transistors, need a step generator to set the control current or voltage. Tektronix’s datasheet describes a step generator that provides stepped stimulus in current or voltage mode, with a set number of steps, selectable polarity, and an offset control. Each step produces its own curve, so the result is a family of traces.

The three-terminal case is more involved. The control terminal is stepped to several levels, and the output terminal is swept at each level. The shape of the family shows the device’s gain and saturation. Reading it correctly needs an understanding of which terminal is which, which is why a datasheet or the device’s own pinout is essential before you connect anything.

Curve tracer compared with a multimeter

A multimeter measures one value at a time. Its diode test sends a fixed small current and displays the voltage drop, which is useful for a quick go or no-go check. A curve tracer sweeps the full range, so it can show a leaky junction, a soft breakdown, or a device whose gain has drifted, which a single reading might miss. The trade-off is time and setup. A curve tracer takes longer to set up and needs the device to be isolated from the circuit it normally sits in.

For a quick check of a diode or transistor on the bench, a multimeter is often enough. For failure analysis, incoming inspection, or matching parts, the curve is more informative. Tektronix’s datasheet lists typical uses as device characterization, failure analysis, incoming inspection, component matching, and manufacturing test.

Curve tracers and source measure units

Keysight’s blog notes that a curve tracer can be instrument hardware or software running on source measure units (SMUs) and power device analyzers. An SMU is a flexible module that sources and measures voltage or current in multiple quadrants, and it can be programmed for a precise measurement. A curve tracer is optimized for fast graphical visualization of I-V characteristics across wide ranges. In practice, many labs use both: an SMU for precise single points, and a curve tracer or analyzer for the overall shape.

Power devices and high voltage

Power devices are the main reason curve tracers have grown more capable. Keysight’s power device analyzer page lists the measurements its models cover, including DC and pulsed I-V, capacitance, gate charge, switching behavior, breakdown voltage, on-resistance, and threshold voltage. It lists output voltage ratings that run from 1,200 V to 3 kV, and output current from 200 A to 3,000 A, depending on the model. Those are high-power instruments, and the same principle applies at lower power: the curve shows the device’s limits.

Testing at high voltage carries real risk. A curve tracer applies voltage that can be lethal, and it can destroy a device if the sweep exceeds its rating. Always work from the device’s datasheet ratings, use the instrument’s current and voltage limits, and follow the manufacturer’s safety procedures. This guide does not cover setting up a live high-voltage test.

Older instruments and the Tektronix 370B

Tektronix’s 370B and 371B programmable curve tracers are older instruments. Tektronix’s datasheet states that both are no longer sold by Tektronix and points to its Encore program for reconditioned units. The datasheet lists the 370B with up to 20 A and 2,000 V of sourcing, a measurement resolution of 1 pA and 50 µV, and a GPIB interface. Those figures describe that specific model and are not a guide to what a current curve tracer can do. Look at the current manufacturer’s datasheet for any instrument you plan to buy.

What you can and cannot check with a curve tracer

A curve tracer shows how a device behaves at the test conditions you set. It cannot tell you the device’s full lifetime, its behavior in your circuit under real heat and load, or whether a fault is in the board around the part rather than the part itself. A healthy curve is evidence that the device works under those conditions, not a guarantee for every application. When you need a long-term reliability answer, you need a test plan designed for that goal.

Practical checklist

If you are considering a curve tracer for a bench or lab, start with the device types you will test. A diode-and-transistor bench needs a lower voltage and current range than a power-electronics lab. Next, check the sweep ranges, the step generator, and whether the instrument supports pulsed measurements. Then check the connections: a three-terminal part needs the right fixture or leads, and a poor connection can produce a distorted curve that looks like a device fault.

Frequently asked questions

Is a curve tracer the same as an oscilloscope? No, although the display looks similar. An oscilloscope shows voltage against time. A curve tracer shows current against voltage, which is a different view of the device.

Can a curve tracer check a diode? Yes. A two-terminal sweep produces the diode’s forward and reverse curve.

Is a curve tracer a multimeter? No. A multimeter reads single values. A curve tracer draws the characteristic across a range.

How we chose these sources

We opened each source listed below and used only facts it stated. We used manufacturer learning pages, product datasheets, and the Used Equipment knowledge guides, and we did not use retailer listings, forums, or encyclopedias. Where a source was a patent or a search summary, we did not treat it as a citation.

What we could not verify

We could not verify current Tektronix curve tracer models or their specifications from the sources used here, because the search returned the discontinued 370B and 371B datasheets and patent literature rather than a current Tektronix product page. We also could not verify a current Tektronix learning guide on curve tracing. The general principles in this guide come from Keysight’s technical material. Specifications for any particular instrument should be taken from its current manual or datasheet, not from this page.

Related reading

For how a multimeter reads a single value at a point, see how a multimeter works. For a related test that reads a single value on a battery, see how a battery tester works.

Sources

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