Device Modeling and Characterization
Advances in mobile device design, as well as emerging technologies used in automobiles and microwave communication systems, have placed greater importance on semiconductor modeling and characterization. While both have historically been integral in creating time- and cost-efficient on-wafer processes, they are becoming more critical – and daunting. This is due to designs stretching into millimeter wave (mmWave) frequencies, as well as the implementation of advanced materials with low Dk/Df and processes.
For all these reasons, accurate semiconductor modeling and characterization will continue to stretch the limits of engineering and testing. And it’s only the beginning. Advanced R&D is underway in the D band (110 – 170 GHz) and higher frequencies for 6G and other next-generation technologies, including artificial intelligence (AI). Developing accurate models, as a result, often requires single sweep measurements that start at DC and continue to 220 GHz and extend up to 1.1 THz.
Effective Device Modeling
Semiconductor device modeling is vital because it helps engineers determine the behavior of discrete devices and sub-systems through circuit simulations rather than reliance on proto-type wafers, a very expensive and slow alterative. It can help uncover potential issues before actual semiconductor production begins in earnest. To create accurate models, devices need to be characterized across the widest possible frequency range to properly simulate real-world operating conditions.
Device characterization is important to identify out-of-band spurs and harmonics and out-of-band oscillations. Discovering these issues and their causes are particularly important at frequencies beyond the band of application.
Extensive On-wafer Testing
For device modeling, engineers need as much information as possible from the RF and DC measurements data. Accurate, stable measurements over extended time periods are a necessity for foundries and fabless semiconductor companies. Engineers must conduct extensive testing of on-wafer devices utilizing silicon CMOS, bipolar, compound gallium arsenide (GaAs), gallium nitride (GaN), and many other device technologies and substrates.
It has been demonstrated that characterizing devices well beyond 125 GHz dramatically improves device model accuracy. By providing single-sweep coverage over an extremely wide frequency range the potential for first-turn design success improves considerably. The end result is lower cost-of-test, as well as faster time-to-market, and a potential competitive advantage in the market.
Overcoming Measurement Challenges
As microwave communication systems migrate to the mmWave spectrum, on-wafer measurements must span into those higher frequencies to accurately characterize devices and accurately extract the necessary data. There are significant technical challenges associated with ensuring accurate measurements at higher frequencies. Among the factors are measurement repeatability, reliable probe contact, noise (internal or external), probe current leakage, thermal performance, temperature variables, and reliable connections between the device and test equipment.
To complete the entire physical model for accurate prediction of how the device will perform under real-world conditions, more than conventional S-parameters, gain compression, and distortion measurements are necessary. Harmonics of the second, third, and fifth order of magnitude must be measured to accurately verify devices. Additionally, spurs must be measured to ensure device characteristics do not impact subsequent circuitry.
Measurement of these parameters during device characterization must be accurate to ensure proper specifications are set for a device. In production, minimizing test time must be balanced with ensuring proper pass/fail binning.
On-wafer Test Solutions
Accurate device modeling and characterization requires a test solution that has a broadband vector network analyzer (VNA) that can conduct single-sweep measurements from DC to 220 GHz at its center. As the frequency range of communications systems continue to evolve, these VNA-based broadband systems should also integrate advanced mmWave waveguide modules to achieve stable power even at extremely low levels at frequencies up to 1.1 THz.
