A semiconductor curve tracer is a specialised piece of electronic test equipment used to analyse the characteristics of discrete semiconductor devices such as diodes, transistors, and thyristors. Based on an oscilloscope, the device also contains voltage and current sources that can be used to stimulate the device under test (DUT).
The basic operating principle of the device is to apply a swept (automatically varying) voltage to the main terminals of the DUT while measuring the amount of current that the device permits to flow. This so-called V-I (voltage versus current) graph is displayed on an oscilloscope screen. The operator can control the maximum amount of voltage applied to the device, the polarity of the voltage applied (including the automatic application of both positive and negative polarities), and the load resistance inserted in series with the device.
For two terminal devices (such as diodes and DIACs, this is sufficient to fully characterize the device. The curve tracer can display all of the interesting parameters such as the diode's forward voltage, reverse leakage current, reverse breakdown voltage, and so on. For triggerable devices such as DIACs, the forward and reverse trigger voltages will be clearly displayed. The discontinuity caused by negative resistance devices (such as tunnel diodes) can also be seen.
The main terminal voltage can often be swept up to several thousand volts with load currents of tens of amps available at lower voltages.
Three-terminal devices require an additional connection; this is usually supplied from a stepped voltage or current source attached to the control terminal of the DUT. By sweeping through the full range of main terminal voltages with each step of the control signal, a family of V-I curves can be generated. This family of curves makes it very easy to determine the gain of a transistor or the trigger voltage of a thyristor or TRIAC. For most devices, a stepped current is used. For field effect transistors, a stepped voltage is used instead.
Curve tracers usually contain convenient connection arrangements for two- or three-terminal DUTs, often in the form of sockets arranged to allow the plugging-in of the various common packages used for transistors and diodes. Most curve tracers also allow the simultaneous connection of two DUTs; in this way, two DUTs can be "matched" for optimum performance in circuits (such as differential amplifiers) which depend upon the close matching of device parameters. This can be seen in the image to the right where a toggle switch allows the rapid switching between the DUT on the left and the DUT on the right as the operator compared the respective curve families of the two devices.
Ironically enough, the original semiconductor curve tracers were originally based on vacuum tube circuits. The Tektronix model 575 curve tracer shown in the gallery was a typical instrument of this generation. At this time, the available semiconductor devices simply didn't have enough performance to generate the test waveforms adequate to characterise themselves.
Nowadays, curve tracers are entirely solid state (with the possible exception of the CRT) and are substantially automated to ease the workload of the operator, automatically capture data, and assure the safety of the curve tracer as well as the DUT.
Reconditioned curve tracers may still be obtained from various suppliers of used test equipment.
Curve tracers are capable of generating lethal voltages and currents and so pose an electrocution hazard for the operator. Modern curve tracers often contain mechanical shields and interlocks that make it more difficult for the operator to come into contact with hazardous voltages or currents. The DUTs can also become hot during operation, posing a burn hazard to the operator.