What are the challenges when measuring power semiconductors in pulse conditions?
To get a picture of the transistor behavior in known conditions, it is essential to perform measurements in the pulsed mode to have quasi-isothermal measurement data. In addition, both self-heating and trapping phenomena have the same order of magnitude regarding their time constants, which must be determined separately.
To get quasi-isothermal IV curves, pulsed signal generators coupled with broadband and precise measurement units are needed to determine the voltage and current levels at the component reference planes. Indeed, engineers in charge of modeling work devote a significant amount of time to fit the model behavior with the reference data. It is then essential to trust the input information on which all modeling efforts are based. Therefore, the test accuracy in a pulsed regime must not be questioned.
Timing accuracy is just as critical as the measurement itself. Gate and drain pulses must be tightly synchronized: if their edges are not properly aligned, the transistor can momentarily pass through an intermediate operating region that belongs to neither the quiescent nor the measurement state. In some cases, this transient condition can push the device outside its safe operating area and damage it. The quality of pulse width, delay, and synchronization control therefore determines not only measurement repeatability, but also the safety of the device under test.
In such conditions, the use of standard equipment (Oscilloscopes, classic SMUs) can have the following disadvantages :
- Difficulty in controlling and synchronizing multiple instruments for different voltage and current ranges and time scales (pulse width, duty cycle, bias points).
- Instrument calibration drift can be observed over time. It may be necessary to recalibrate the current probes (degauss) during measurements or to recalibrate the system if a time parameter of the tests is modified.
- A family of pulse generators with precise and embedded broadband measurement units has been developed to address these challenges.
Why Pulsed IV Measurements Are Essential for RF Power Transistors
The continuous evolution of RF and microwave technologies has considerably increased the demand for accurate transistor characterization techniques. Modern devices such as GaN HEMTs, GaAs FETs, LDMOS transistors, HBTs, and advanced MMIC power amplifiers now operate at very high frequencies and power densities, under electrical conditions that conventional DC characterization methods cannot properly represent.
In real RF operation, transistors are subjected to rapid voltage and current variations occurring over nanosecond and microsecond time scales. Under these dynamic conditions, several physical phenomena alter the intrinsic behavior of the device. Thermal effects, charge trapping, and memory effects progressively modify the current-voltage characteristics, creating significant discrepancies between static DC measurements and the actual RF operating behavior. This phenomenon, commonly referred to as dispersion, is particularly critical in modern GaN and GaAs technologies.
As RF systems become more demanding in terms of efficiency, linearity, and power performance, the limitations of conventional DC measurements become increasingly evident. Pulsed IV characterization has therefore become an essential technique for accurately evaluating RF power transistors under realistic operating conditions.
The Limitations of Conventional DC IV Measurements
Traditional DC IV measurements consist of progressively sweeping the drain voltage and current while the transistor remains continuously biased. During the measurement sequence, the device dissipates significant power over relatively long periods of time.
As the junction temperature rises, the electrical characteristics of the transistor begin to change. The measured IV curves progressively exhibit a “droop” caused by self-heating and trapping effects. Consequently, the extracted characteristics no longer represent the intrinsic behavior of the device but rather a thermally degraded operating condition.
This issue is particularly severe for high-power RF technologies such as GaN, where trapping effects and thermal phenomena strongly influence device performance. Under RF excitation, these mechanisms can produce current collapse, dynamic ON resistance variations, gain compression, and nonlinear distortion effects that cannot be predicted accurately from static DC data alone.
As a consequence, transistor models extracted exclusively from DC measurements often lead to inaccurate circuit simulations. Designers may then face discrepancies between simulated and measured RF performance, resulting in additional design iterations, increased development costs, and longer time-to-market.
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Pulsed IV Measurements: Accessing the True RF Behavior
Pulsed IV characterization was developed to overcome these limitations by minimizing the thermal and trapping effects that occur during conventional DC measurements.
Instead of continuously stressing the transistor, the device is maintained most of the time at a low-power quiescent operating point, commonly referred to as the “Q-point” or quiescent bias point. Very short synchronized voltage pulses are then applied to the gate and drain in order to move the device temporarily to a non-quiescent measurement state. Current is sampled during this extremely short interval before the transistor immediately returns to its quiescent condition.
Because the duty cycle is very low, the device has sufficient time to cool between successive pulses. In many practical cases, the transistor may spend only 1% of the total time in the active pulsed state and 99% at the quiescent bias condition. This approach enables measurements to be performed under quasi-isothermal conditions, much closer to the actual operating environment encountered in RF applications.
The resulting IV characteristics therefore provide a much more realistic representation of the intrinsic large-signal behavior of the transistor. Pulsed measurements allow engineers to accurately observe phenomena such as current collapse, dynamic trapping effects, thermal dispersion, and RF operating trajectories. This information is essential for developing reliable nonlinear transistor models and accurately predicting RF circuit behavior.
Moreover, pulsed characterization provides valuable insight into the physical mechanisms responsible for dispersion. By varying pulse duration, duty cycle, and quiescent bias conditions, engineers can study the time constants associated with trapping and thermal effects, helping both technology developers and model engineers better understand device physics.
The Importance of Timing Accuracy
Although the principle of pulsed IV measurements appears straightforward, achieving accurate and reliable results requires extremely precise timing control.
The synchronization between gate and drain pulses is particularly critical. If the pulse edges are not properly aligned, the transistor may momentarily transition through intermediate operating regions that do not correspond to either the quiescent or measurement state. In some situations, these transient conditions may force the device outside its safe operating area and potentially damage the transistor under test.
Accurate pulsed characterization therefore requires careful control of pulse widths, delays, synchronization timing, and acquisition windows. The quality of the measurement system plays a decisive role not only in measurement accuracy and repeatability, but also in protecting expensive RF power devices during characterization.
The AMCAD Pulse IV System: A Solution Designed for RF and Microwave Devices
The AMCAD Pulse IV System was specifically designed to address the challenges associated with advanced RF transistor characterization. Unlike conventional laboratory setups assembled from multiple independent instruments (scopes, external DC power supplies), the AMCAD platform provides a fully compact and integrated solution optimized for pulsed IV and pulsed RF measurements.
The system combines synchronized pulsed IV measurements generation and measurement capabilities, and trigger signals to perform synchronized pulsed S-parameter using an RF vector network analyzers equipped with pulsed RF measurement options, allowing both DC and RF characteristics to be evaluated under identical operating conditions. This is particularly important for advanced nonlinear model extraction workflows, where consistency between static and RF measurements is essential.
One of the major strengths of the AMCAD architecture lies in its timing control capabilities. The system provides independently adjustable gate and drain pulse timing, including pulse widths, delays, rise time of the pulse, and acquisition apertures. This high level of synchronization precision ensures accurate dynamic measurements while maintaining safe operating conditions for the device under test.
Its integrated fast current and voltage measurement capabilities complete it’s unique performances on the market.
Consequently, the AMCAD platform enables very short pulse measurements, allowing characterization to be performed under near-isothermal conditions even for high-power GaN and LDMOS devices. By minimizing self-heating and trapping artifacts, the system captures transistor behavior much more representative of actual RF operation.
Another important advantage is the integrated protection capability implemented within the measurement architecture. High-power RF transistors are often expensive and sensitive to electrical overstress. The AMCAD system incorporates advanced protection mechanisms capable of detecting abnormal operating conditions and rapidly disabling the pulse generators when necessary. This provides a highly secure environment for characterizing valuable devices safely and repeatably.
The platform is equally suitable for packaged devices and on-wafer measurements, making it highly versatile for both semiconductor manufacturers and RF design laboratories. Because pulsed measurements can safely explore operating regions that would be difficult or impossible to access under continuous DC bias, the system is particularly valuable for characterizing high-voltage and high-current RF power technologies.
Improving RF Modeling and Circuit Design
One of the primary motivations for pulsed IV characterization is the extraction of accurate nonlinear transistor models.
Modern RF simulation tools rely heavily on compact device models to predict circuit behavior under large-signal operating conditions. The quality of these models directly determines the accuracy of simulations involving gain compression, harmonic distortion, efficiency, and power performance.
Pulsed IV and pulsed S-parameter measurements acquired under controlled thermal conditions provide the highly accurate data required for advanced model extraction. More accurate models lead directly to more reliable circuit simulations, reducing the number of design iterations and accelerating product development cycles.
For semiconductor technology developers, pulsed measurements also provide a powerful diagnostic tool for process optimization and reliability analysis. The ability to analyze trapping phenomena, thermal behavior, and dynamic dispersion contributes significantly to the development of more robust and higher-performance RF technologies.
Conclusion
As RF and microwave technologies continue to evolve toward higher frequencies and power levels, accurate dynamic transistor characterization has become indispensable.
Conventional DC IV measurements are no longer sufficient to describe the true operating behavior of modern RF power devices because they are strongly affected by self-heating and trapping phenomena. Pulsed IV characterization overcomes these limitations by enabling measurements under quasi-isothermal conditions that closely reproduce real RF operation.
By providing accurate dynamic transistor characteristics, pulsed measurements improve nonlinear model extraction, increase simulation reliability, reduce development cycles, and help engineers optimize RF circuit performance.
The AMCAD Pulse IV System was specifically designed to meet these advanced characterization requirements. Through its integrated architecture, precise timing control, synchronized pulsed RF capability, and advanced device protection features, the system provides a highly effective and reliable solution for RF and microwave transistor characterization.
For RF designers, model engineers, and semiconductor technology developers, pulsed IV characterization is no longer simply an advanced measurement option — it has become an essential tool for accurately understanding and predicting the behavior of modern RF power devices.
Why are our solutions unique?
Other SMUs can also generate and measure pulsed signals for transistor testing.
Our AM3100 and 3200 systems are compact and easy to implement. They are not made of a heterogeneous assembly of different measuring instruments, not initially designed to work together.
Our systems don’t need external precision multimeters nor require new calibration for every new measurement condition. Therefore, it offers an easy way to control the bench for various measurement configurations.
Product brochures offer essential information such as Recovery measurement time delay after power switching, bandwidth, absolute measurement accuracy versus signal ranges, settling time, and noise…, which allows the user to validate the results he gets.
Our probes are natively equipped with Electronic Fuses, which can be triggered for customized current thresholds. In case of a short circuit, electrical shutdown, or unexpected events, the system is immediately turned off with a specific sequence offering users a safe operating environment.
The pulse width can range from a few hundred nanoseconds (a must for measurements in a quasi-isothermal regime) to a few milliseconds to catch long-time constants (trapping effects, dynamic self-heating).
Because our system does not incorporate oscilloscope-based measurement hardware and has low output impedances, it is possible to have almost immediate feedback between measurement and generation, allowing rapid plotting of IV curves for a grid of defined voltages, whatever the current consumption, thanks to the ALCs integrated into the probes.
Finally, AMCAD is not just a manufacturer of measurement equipment; we use these instruments daily in our laboratories for testing and modeling service activities. We know exactly how to guide our customers in the use of these solutions and know their problems. This source of information also allows us to draft the road map toward future generations of our products.
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