To characterize semiconductors requires a special class of test instruments capable of evaluating voltages and current levels under different conditions. RF/microwave engineers are all too familiar with the customary tools of the measurement trade, including power meters, spectrum, and even vector network analyzers (VNAs). But additional capabilities are needed to evaluate the direct-current (DC) characteristics of integrated circuits (ICs) and discrete semiconductors, such as diodes and field-effect transistors (FETs). Flexibility is a key to any successful semiconductor test system, since at some point it will likely be called upon to make measurements on any number of functions.
Assembling an effective semiconductor test system first requires defining test requirements, such as types of measurements; frequency range; and whether a device under test (DUT) is packaged, in chip form, or on an undiced wafer. The system really begins where signals are injected into and withdrawn from a DUT, making a probe station or test fixture a critical component in any semiconductor measurement system. Whether a circuit, wafer probe, or test fixture, it must not only provide adequate RF/microwave bandwidth but also suitable DC biasing range. The latter is necessary to support high-power requirements, as might be needed for testing RF/microwave discrete power transistors or even higher-power monolithic-microwave-integrated-circuit (MMIC) amplifiers.
In many cases, it is first necessary to understand the operating limits for a DUT (such as breakdown voltage and leakage voltage for FET) and the voltage drop across a diode to better test those devices. Parametric semiconductor test systems, such as the 4080 series of parametric test systems from Agilent Technologies, can evaluate the DC limits and recommended operating ranges for discrete devices and ICs. However, they can also be used in conjunction with the company's PNA series of microwave vector network analyzers to perform S-parameter measurements through 20 GHz on a DUT. The model 4083A system, for example, can work with a PNA system for microwave measurements, but also perform its own DC parameter measurements to determine device or IC breakdown voltage, high-speed capacitance, and other key device parameters under different operating conditions.
Additional suppliers of semiconductor DC parameter test systems include LTX-Credence, Aeroflex with its AX Series of test systems, Teradyne with its popular J750 test system, Keithley Instruments, and National Instruments. For example, Keithley's S530 series parametric test systems are designed to perform all of the DC and current-voltage measurements required for process control monitoring, process reliability monitoring, and device characterization. The systems can be equipped for both production and laboratory environments, with low-current as well as high-voltage (to 1000 V) versions, including those with probe-station integration.
National Instruments employs the flexibility of a PXI architecture to perform DC, digital, and RF measurements on semiconductors. Combined with the firm's popular LabVIEW test software, a wide range of measurements can be automated for semiconductor design and test purposes. This can help reduce the time and cost of device DC characterization, to list one potential benefit. National Instruments alone provides more than 300 instruments in the PXI format, with well over one thousand PXI instrument modules available from a wide range of industry suppliers in support of semiconductor test.
It is often necessary to know critical points for leakage voltage and breakdown voltage for transistors and diodes, and many of these companies offer systems that are fully programmable for a wide range of discrete-device and IC characterization tests. In addition, a system such as the model 7BT discrete semiconductor tester from Lorin Test Systems can evaluate FETs for essential operating parameters. These include drain-source resistance, drain-source voltage, and gate-source current operating under the control of a Windows-based personal computer.