Power density in active devices is increasing according to the demands of transistor users. Applications in commercial wireless, avionics, broadcast, industrial, and medical systems are pushing the envelope for solid-state power, with growing requirements for higher output power levels from fewer output-stage devices. At Freescale Semiconductor, supplying high-performance radio frequency (RF) and microwave transistors for these applications is only part of the challenge, as the company backs its devices with unparalleled capabilities in characterization, packaging, and applications engineering.

Freescale Semiconductor enjoys a rich heritage in fabricating and selling both discrete and integrated RF semiconductors. Last year, the company introduced its seventh generation of silicon RF laterally diffused metaloxide-semiconductor (LDMOS) in the form of the HV7 process, with the output power and linearity performance through 3.8 GHz needed for WiMAX infrastructure applications. They have also announced a high-voltage version of this process, operating at 48 V, for industrial, scientific, and medical (ISM) applications. Freescale has also extended its high-power GaAs PHEMT device performance to 6 GHz, for WiMAX amplifier applications.

More recently, the company announced the first two-stage radio-frequency integrated circuits (RF ICs) capable of delivering 100 W output power. When driven by Freescale's cost-effective MMG3005N general-purpose amplifier (GPA), the MWE6IC9100N and MW7IC181 00N RF ICs form a complete 100-W power-amplifier solution for wireless base stations operating at 900 and 1800 MHz.

While the performance levels of these discrete and integrated RF power devices are outstanding, putting the devices into the hands of their customers is only the beginning. Each shipped device is supported by the company's "service-in-waiting" personnel with diversified expertise in testing, modeling, packaging, and applications support.

RF Power Characterization
Load-pull (LP) measurement techniques have been increasing in popularity in recent years. RF power amplifiers are generally characterized using such techniques to determine the parametric values of, for example, peak output power, gain and efficiency under various complex load conditions presented at the device reference plane. The use of multiple complex modulated signals in the same measurement environment is also becoming increasingly commonplace. For a manufacturer of high power RF semiconductors, the difficulty in characterizing the products accurately is compounded by the fact that the device development must be carried out on large periphery devices, typically 60 mm, presenting terminal impedances in the sub-0.5-Ω region and with quality factors (Qs) in the range of 8 to 10.

For the past several years, the RF division of Freescale has developed several accuracy-enhancement methodologies and a multitude of automated custom measurement techniques. The division has well equipped high reflection (high gamma) load-pull labs capable of covering frequencies from 250 MHz to 8 GHz and power levels as high as 100 W CW (500 W pulsed) which service the company's GaAs, GaN and LDMOS device, modeling, applications, and other functional groups (Fig. 1). Freescale's systems are capable of performing advanced measurements on devices with impedances of 0.5 Ω and less. To enable such advancements, the company has developed a series of specialized test fixtures with optimum impedance transformation ratios transitioning a 50-Ω system characteristic impedance to the low impedances required for load-pull measurements of high power transistors.

In addition to the fixture-based systems, Freescale also uses on-wafer load-pull systems based on commercial wafer-probe equipment which is used mainly for device research and development, as well as modeling. The onwafer load-pull system features a unique three dimensional anti-vibration mechanism to minimize the effects of tuner vibration, thereby minimizing probe-to-wafer contact damage.

The accuracy of the Freescale Semiconductor load-pull systems typically shows transducer gain differential, ΔG_t, of less than 0.25 dB at maximum gamma (0.93 to 0.95 or the edge of the Smith Chart) and less than 0.1 dB inside the measurement region.¹ This level of accuracy is in part achieved by the use of precision 7-mm coaxial connectors at all measurement reference planes. These connectors exhibit typical VSWR of less than 1.008:1 at 2 GHz. The center contact resistance of less than 0.1 mΩ and excellent calibration characteristics, with unit-to-unit impedance variation of less than 0.1 percent and phase variation of less than 0.21 deg. at 18 GHz also contribute to excellent measurement accuracy.

A thru-reflect-line (TRL) calibration is used with the vector network analyzer (VNA) in conjunction with the load-pull test system to achieve source match of better than 45 dB.² In contrast to other VNA calibration approaches, such as the short-openload-thru (SOLT) method, a TRL calibration is not burdened by the parasitic circuit elements (inherent additional capacitances and inductances) of the calibration load standard at high frequencies.

Typically, 5000 to 6000 impedance points are characterized for each tuner to ensure a uniform distribution across the source and load impedance planes. A high density of points is required when evaluating large periphery unmatched devices, which are very sensitive to minimal impedance changes owing to their low terminal impedances. Such a high density may not be required in the assessment of the relatively high impedance production parts containing package matching elements. In this case, a sparse load-pull evaluation may be conducted.

A typical load-pull setup is shown in Fig. 2. Load-pull systems at Freescale are used to evaluate a device's peak pulse power compression, AM-to-AM conversion, AM-to-PM conversion, frequency response and large-signal device input impedance. The systems can also be used for measurements of complex signals to ascertain the average and peak power, adjacent-channel power (ACP), for two-tone and multi-tone testing of intermodulation distortion (IMD), and to assess the device behavior under different loading conditions with EDGE signals. Freescale also conducts complementary-cumulative-density-function (CCDF) analysis of device signal power. The CCDF testing is common to second-generation (2G) and third-generation (3G) wireless measurements. The requirement to perform measurements of CW, pulsed, and modulated signals comes from the fact that these signals exert different thermal loading on the device and, consequently, the optimum load impedance for each modulation format is also different, as shown in Fig. 3.³ In addition to this extensive measurement capability, Freescale has developed valuable data import and post processing tools to enable the user to analyze rapidly the behavior of the device under test (DUT) in two-dimensional or three-dimensional planes (Fig. 4).