Current-voltage (I-V) measurements are among the most fundamental methods for characterizing electronic devices. They can be used to evaluate the behavior of active and passive electronic devices under a required set of operating conditions, to find optimum power supply values, and explore performance limits. Most device measurements are either to determine the current flow for a given voltage or the capacitance of an electronic device for a given voltage (C-V measurements). The x-y plots and curves that result can tell a great deal about the electronic device, to better understand its operation and behavior in an electronic circuit or to develop a computer-aided model that can be used in circuit simulation software.
Electronic devices that can be evaluated by means of I-V measurements include transistors, diodes, resistors, and capacitors, whether in discrete form or as parts of integrated circuits (ICs). Measurements can be made statically, under constant test conditions, or dynamically, where the test conditions are changing, such as swept measurements, where current or voltage is fixed while the other parameter is steadily increased.
Ideally, I-V measurements can be performed quickly, so that the measurements are suitable for production line testing or, they can provide a generous amount of data about a device under test (DUT) even when measurements are performed in an R&D setting. As with many electronic measurements, however, tradeoffs will exist between measurement speed and accuracy, with longer test times allowing capture of more data and more averaging to minimize errors. Although shorter test times can introduce greater measurement uncertainties due to the lack of averaging time, but shorter test times can also minimize the impact of sustained applied I-V test signals on the self-heating effects of some device materials, and the variations in measured values that can result from substrate heating.
I-V measurements can be performed under continuous-wave (CW) or pulsed conditions, so that the device measurements more closely reflect the conditions under which an electronic device and its associated circuitry will operate. The CW measurements will reveal the steady-state behavior of a device, while pulsed I-V measurements can show its transient operation.
I-V measurements under CW conditions may apply a single current or voltage value to a DUT and perform analysis once the output of the DUT has fully stabilized, or they may simply perform analysis during some set time following the application of the test voltage or current. I-V measurements using CW values of voltage and current can also be performed as swept measurements, in which the current or voltage source remains connected to the DUT, but the value of the current or voltage is constantly increasing from some starting point, using a predetermined step size of current or voltage. Analysis on the DUT is performed continuously, to determine the effects of the steadily increasing voltage or current on the DUT, such as its power consumption under different power-supply levels. Pulsed I-V measurements employ precisely controlled pulsed inputs to a DUT, such as pulse width and pulse period, which is the timing between each pulse.
As electronic devices and I-V measurements evolve, towards smaller, lighter, and lower-power electronic products, test instruments for performing I-V or C-V measurements are being designed with higher levels of integration and greater functionality than before. They are also being tasked to measure DUT power at lower levels than ever before. Excessive device leakage current, for example, can compromise the power consumption of a circuit, and sensitive I-V measurements can detect and analyze leakage currents. To do so, the measurement instruments must themselves employ effective circuit designs and achieve extremely low noise levels.
Investigating I-V Instruments
The test tools required for performing I-V measurements are known as source/measurement units (SMUs): instruments that integrate a source of test signals as well as a means of measuring them: typically, a voltage source, a current source, an ammeter, and a voltmeter. They can generate precisely known values of current and voltage, which are applied as the input to a DUT and then measured at the output of the DUT. SMUs have advanced from simple test tools to powerful, programmable instruments that can automatically perform a wide range of I-V and often C-V measurements while storing measured data for analysis and modeling. In contrast to making I-V measurements with discrete instruments, such as an oscilloscope and programmable power supply, many SMUs and parametric device analyzers integrate the different functions needed for I-V (and often C-V) measurements within a single instrument for ease of synchronization and setup. In fact, many modern SMUs or device parametric analyzers provide the capabilities to perform all three types of basic electronic device measurements: DC I-V measurements, pulsed I-V measurements, and C-V measurements, often with high resolution over wide ranges of test source values.
For example, the Keithley 4200-SCS Semiconductor Characterization System from Keithley Instruments is a fully integrated device characterization system, including a self-contained Windows-based operating system for programming, control, and data collection and analysis. It is capable of highly accurate DC CW and pulsed I-V and C-V measurements and can analyze and plot graphs of test results that provide meaningful insight into a DUT’s behavior under any number of operating conditions. In addition to I-V and C-V measurements over standard values of current and voltage, the Keithley 4200-SCS system can perform extremely low-power, low-current measurements, with current-measurement resolution to 0.1 fA. Such measurements are effective for evaluating the leakage currents of a DUT under different conditions. The Keithley 4200-SCS also provides many useful additional measurement functions, such as stress measurements to evaluate the reliability of a DUT.
The Keysight / Agilent B1500A Semiconductor Device Analyzer is a modular test instrument that allows users to customize their measurement capabilities according to which modules are added. The mainframe provides 10 slots for different function modules to run under the control of an embedded PC with Windows operating system and Keysight’s EasyEXPERT test software. Depending upon the choice of instrument modules, the Keysight B1500A can perform low-voltage, low-current, or high-voltage CW and pulsed I-V or C-V measurements. It features current and voltage measurement resolution of 0.1 fA and 0.5 µV, respectively. Pulse widths can be set as narrow as 10 ns and as much as 80-V peak-to-peak (pulsed) voltage (±40 V DC) is available for high-voltage testing. Depending upon which modules are in place, the Keysight B1500A can perform single- and multiple-channel sweeps, list sweeps, arbitrary waveform generation, and time sampling for a wide range of measurement capabilities.
For characterizing higher-power devices, the Keysight /Agilent B1505A Semiconductor Device Analyzer also takes the modular instrument approach to keep pace with characterizing devices operating at high voltages and high currents, to as high as 3000 V and 20 A, respectively. The instrument can be configured for CW and pulsed source conditions for evaluating power MOSFETs and motor-control ICs used in automotive applications as well as the latest wide-bandgap power transistors, such as those based on silicon carbide (SiC) and gallium nitride (GaN). Modules are also available for filling its 10 instrument slots for performing multiple-frequency C-V measurements to 5 MHz.
Tektronix offers several test solutions for I-V testing—semiconductor parameter analyzers that were once known simply as curve tracers for their capabilities in providing plots of semiconductor behavior as simple-to-follow graphic displays. The Tektronix 370A semiconductor DC parameter analyzer provides extensive device testing and curve tracing within the same instrument. It can perform device measurements at source levels to 2000 V or 10 A and power levels to 220 W, with 2 mV voltage measurement resolution and 1 nA current measurement resolution. It can be operated manually from a front-panel display and is simple to program for automatic measurements, and offers a host of useful functions including waveform averaging, waveform comparisons, and envelope displays. The Tektronix 370B Curve Tracer provides built-in test source capabilities to 2000 V voltage or 20 A current, with 1 pA current resolution and 50 µV voltage resolution, for accurate control of semiconductor device testing from extremely low to high power levels.
While many of these instruments provide the capabilities for performing I-V measurements on photovoltaic (PV) devices, the Daystar DS-100C I-V Curve Tracer is a portable tester designed for testing PV systems. It has simple, switch-selected low- and high-voltage settings and uses a capacitive load to modify the impedance connected to a PV unit under test. By performing measurements as the capacitors are charged and the load changes, highly accurate measurements can be performed and I-V curves displayed for several device parameters, including short-circuit current, open-circuit voltage, peak power, current at peak power, voltage at peak power, and fill factor.
As a complementary test tool to these more traditional SMUs, device parametric analyzers, and curve tracers, the Huntron 2800S Tracker uses a power-off test method to eliminate the risk of damaging electronic devices under test when power is applied. It applies a current-limited AC signal across a DUT. For the I-V plot, the current flow causes vertical deflection through the device while the applied voltage causes a horizontal deflection for the I-V plot. The measurement results in a unique I-V signature for the DUT, which can quickly reveal the operating health of the DUT.
Of course, this is only a sampling of the many instruments available for I-V (and C-V) measurements on the Axiom Test Equipment website, even when those measurements are to be performed on the latest device types, such as high-power SiC or GaN transistors for emerging Fifth Generation (5G) wireless network applications.
More information, including data sheets, for each of these instruments can be found by visiting Axiom’s website at www.axiomtest.com contacting Axiom Test Equipment’s sales department at firstname.lastname@example.org or by calling an Axiom sales representative at 760-806-6600.