In this article, we discuss MMIC mixers designed using high performance GaAs Schottky diodes. Compared to fixed Schottky diode options already on the market, we describe, for the first time, GaAs diodes with forward voltages ranging from 0.29 to 0.9 V. These diodes enable new circuit capabilities previously not possible in GaAs diode circuits. A state-of-the-art MMIC mixer has been developed and new performance characteristics demonstrated, including low LO drive for power sensitive applications and high LO drive for high linearity applications.
The “fabless” model is pervasive in the semiconductor integrated circuit (IC) industry. In this model, IC designers rely on the expertise of external foundries to support ongoing production as well as next generation device and process development. In this partnership, the fabless company is free to develop new designs efficiently while the foundry is able to amortize the costs of running the factory over numerous fabless or “fab-lite” customers and achieve better economies of scale. As it is well known in the industry, a foundry must achieve high wafer run rates in order to maintain high yields and high profitability. Most of the nimble, aggressive and smaller IC design firms cannot afford to run an internal fab, so they instead invest in the modeling, simulation, packaging and testing of the devices. This often implies that the fabless company understands the fine intricacies of a foundry process as well, or potentially better, than the foundry itself. Moreover, the foundry can often provide superior technical solutions to what is considered standard, but they may not develop these solutions without sufficient persuasion (i.e., economic, technical, etc.). By communicating the relative strengths and weaknesses of a given process openly, technical breakthroughs are achieved that benefit both firms. To use an analogy, the foundry builds racecars and the fabless designers are the drivers. The driver has no expertise to build the car, but he or she is well qualified to critique the performance and suggest improvements.
In the spirit of this synergistic designer-foundry relationship, Marki Microwave and GCS have recently collaborated to solve the major technical challenge of offering high quality GaAs Schottky diodes in a commercial foundry setting, with diode forward voltages (Vf) from 0.29 to 0.9 V. As we will describe, this new foundry process overcomes a previous technological boundary, where GaAs Schottky diodes were only offered with 0.7 V turn-on voltage. By engineering the diode processing and epitaxial layer design, GCS achieves variations in the Vf that were traditionally only possible in Si Schottky diodes. With this new type of device, Marki Microwave designers achieve a variety of previously impossible performance metrics in a GaAs MMIC platform. We focus on double balanced mixer designs because they benchmark well with classical Si-based designs. This GaAs process can be applied to any circuit that typically uses Schottky devices from DC through millimeter wave.
SCHOTTKY DIODES IN MIXERS
Historically, mixers based on diode technology have split into two camps: hybrid mixers using discrete diodes and MMIC mixers using integrated diodes. Generally, hybrid mixers are thought of as higher performance, broader band and more flexible in being optimized for specific performance specifications. MMIC mixers, on the other hand, overcome the limited flexibility by offering much smaller form factors at significantly lower cost. Generally, hybrid and MMIC mixers are so different in performance that suitable applications for the two technologies are distinct and non-overlapping. In other words, if highest performance and/or tunability is needed, hybrid mixers must be used; if cost and size are most important, MMIC mixers must be used. With few exceptions, these truisms have existed for at least two decades.
The “hybrid/MMIC divide” is explained by many physical differences:
- Hybrid mixers use suspended substrate balun structures on low dielectric material
- MMIC mixers are most commonly built on GaAs, which is a grounded substrate with high dielectric constant
- Hybrid mixers use discrete diodes that are hand assembled, limiting size reduction
- Hybrid mixers can be designed on almost any material platform, giving the designer many options to modify performance characteristics
- MMIC mixers have strict design rules for the stack-up, which limits design flexibility
- Hybrid mixers can use any kind of diode (e.g., low barrier Si, GaAs, beam lead, flip chip) while MMIC mixers must use the diode device offered by the process.
This final point regarding diode options is essential to understanding why hybrid mixers have survived and prospered in the marketplace for over a half century, despite higher cost and larger size. GaAs devices have been restricted to applications where a barrier potential of 0.7 V is acceptable, while hybrid mixers (mostly based on Si Schottky diodes ranging from 0.25 to 0.9 V) offer performance tailored advantages for applications where very low or high Vf is necessary.
THE SCHOTTKY DIODE PROCESS
While it is true that MMIC mixers can be fabricated using familiar GaAs PHEMT and MESFET processes, these technologies are generally not ideal for achieving high quality Schottky diodes. For example, mixers fabricated using PHEMT processes demonstrate worse performance because of the inferior I-V characteristics and device options of drain-to-source shorted PHEMT diodes. A cursory market survey of commercially available MMIC mixers reveals that mixers made in PHEMT and MESFET processes regularly have inferior conversion loss characteristics compared to hybrid diode mixers. It is normal to expect 10 dB conversion loss in mixers fabricated in PHEMT, where bona fide Schottky-based mixers regularly achieve 6 dB conversion loss. The poor performance of PHEMT diodes is because PHEMT devices are primarily intended for use in amplifiers, and PHEMT processes are designed to optimize the transistor ft and fmax – albeit to the detriment of the diode characteristics.
In 2010, GCS recognized this limitation in PHEMT solutions and released a new GaAs process optimized for Schottky diode-based ICs. The diodes produced by the optimized epi-layer and junction design offer significantly better I-V characteristics than PHEMT diodes, with RC time constants measured in THz. The process includes Schottky diode and passive components (thin film resistors, MIM capacitors, inductors and transmission lines) that can be integrated on a single chip.
Until now, all commercial GaAs processes (PHEMT, Schottky, etc.) have featured Schottky diodes with turn-on voltages near 0.7 V. This has been attributed throughout the literature to the pinning of the Fermi level to a fixed energy at the metal-semiconductor interface in III-V materials.1 It has been shown repeatedly that the GaAs Schottky forward voltage is essentially independent of the metal work function. Si Schottky diodes, by contrast, do not experience this Fermi level pinning phenomenon to the same extent and can more easily be engineered to a variety of barrier heights. Hence, one finds Si diodes from many suppliers with Vf below 0.2 to greater than 1 V. GaAs diodes lack this variety.
Recently, GCS succeeded in developing a modification to its traditional 0.7 V Schottky process to enable alternative diode levels in GaAs. This breakthrough was achieved by careful engineering of the epi layer and Schottky contact on the GaAs substrate. The resultant diodes exhibit nearly identical RC properties to the standard 0.7 V diodes and with the added benefit of altered junction potential. The diode forward and reverse I-V characteristics are shown in Figure 1, and the DC parameters of a 1.6 × 8 µm2 Schottky diode are summarized in Table 1. All diodes offer impressive performance characteristics, including an ideality factor (n) of approximately 1.1 with THz bandwidth capability (as calculated by the RC time constant). By engineering the epi-layer and fabrication processes, forward voltages range from about 0.29 to 0.9 V (@ 1 mA). In principle, other diode characteristics can also be achieved.
The goal is to offer a high quality GaAs diode that compare favorably with existing Si diodes in terms of Vf, C and R. Owing to the obvious speed advantages of GaAs versus Si, if GaAs diodes can be made with virtually the same Vf as Si, the GaAs alternative can offer significantly superior RC characteristics. For example, a low barrier Si diode (~ 0.3 Vf) for high frequency use might have C = 100 fF and R = 15 ohms. A comparable GaAs diode would have C = 30 fF and R = 3 ohms. This RC improvement directly impacts circuits like mixers, multipliers and detectors since, generally speaking, diode resistive losses are unwanted and high frequencies are limited by diode capacitance. In Si Schottky diodes, diode resistance and reverse breakdown voltage are often traded to realize low capacitance. By comparison, GaAs diodes have an outstanding reverse breakdown of ~10× the Vf and a resistance ~2.5 to 3 ohms.
7 to 26.5 GHz MMIC MIXER
A common band for diode mixers is 7 to 26.5 GHz. Below 6 GHz, many solutions exist for frequency conversion, including hybrid mixers, Si IC mixers (CMOS and SiGe) and FET mixers. Moreover, analog-to-digital (ADC) and digital-to-analog (DAC) converters and digital processing are becoming sufficiently fast so that many of the lower frequency mixer “slots” are being replaced by digital solutions. Generally, digital solutions are preferred over analog if the dynamic range and DC power dissipation requirements can be met. This trend will continue.
Above 6 GHz, fewer options exist. Si IC solutions generally require high volume applications, so microwave solutions are not usually cost effective. Second, ADCs and DACs do not offer sufficient dynamic range, so analog mixing with diodes is usually necessary. Therefore, a 7 to 26.5 GHz mixer with a DC to 10 GHz IF response has been designed on the GCS diode process. (The mixer actually performs well to 38 GHz; the performance data was truncated by the 26.5 GHz bandwidth of the test setup). The mixer is based on the standard double balanced architecture, colloquially referred to as the “horseshoe” topology (see Figure 2). The mixer was designed using HFSS™ (for the FEM analysis of the balun structures) and the Microwave Office™ harmonic balance engine, using a previously published design flow.2 The authors have demonstrated that if the diode models are accurately generated, near perfect agreement with measurement and simulation can be achieved. Although the simulations are not shown in this article, excellent agreement was obtained. This confirms that the diode models supplied in the process design kit (PDK) from GCS are very accurate and suitable for computer-aided design, optimization and tape-out of the masks.
The ICs were mounted into a test fixture with SMA connectors and wire bonded on all input and output ports (see Figure 3). The measured linear performance is shown in Figure 4. All the mixers exhibited nearly identical isolation and VSWR on all ports, regardless of the forward voltage. This is expected because the diode level does not impact the linear performance of a double balanced mixer if the LO drive signal is sufficiently large to switch the diodes. Figure 4a shows that 6 dB conversion loss is measured for all the diode types, the only differences due to the higher LO drive on the higher Vf ICs.
Where high linearity mixing is required, higher barrier diodes are used. When the diode Vf is higher, incoming small-signal RF tones are less likely to intermodulate the I-V characteristics of the diodes, limiting unwanted higher-order spurious tones. When the Vf is low, the incoming RF voltage more easily perturbs the I-V characteristics, leading to lower P1dB, lower IP3 and higher spurious content. These trends are confirmed by the measured data in Figure 5 and Table 2. The down-conversion spurious values are averaged over the entire 7 to 26.5 GHz RF range, with a fixed IF of less than 100 MHz. The variation in spurious levels is typically ± 5 dB. Measuring spur levels better than 100 dBc are limited by the dynamic range of the measurement equipment. Using the same mask set and different processing to change Vf, Figure 5 shows how the nonlinear performance varies with Vf. As expected, the low barrier diodes yield the worst nonlinear performance, and the high barrier diodes yield the best performance, albeit at the expense of high LO drive power.
25 to 67 GHz MMIC MIXER
A second mixer design was fabricated to highlight the high frequency capability of the process. Here, the lowest barrier process was combined with the low capacitance (~15 fF) diode design to create an extremely broadband and high frequency mixer that covers 25 to 67 GHz, with an IF response of DC to 30 GHz.
With prior GaAs MMIC mixers, the normal diode Vf of 0.7 V requires an LO drive level greater than +15 dBm. With increasing frequency, circuit losses increase, requiring higher LO drive – possibly up to +20 dBm. Therefore, using a fundamental mixer at millimeter wave frequencies requires high LO drive, with few options to generate such power over a broad bandwidth.
However, it is possible to design a power efficient millimeter wave mixer with the low barrier process that only requires a modest +10 to +15 dBm LO drive. The conversion loss for such a design is shown in Figure 6. If conversion loss is not critical, the mixer can be operated “backwards” (i.e., driving the LO into the RF port, rather than the LO port) to make use of the higher efficiency RF balun. In this power efficient but lossy mode (Configuration B in Figure 6), the mixer can reliably be operated with only +9 dBm at 67 GHz. Additional measurements show excellent operation in configuration B with the LO as low as +6 dBm for lower frequency designs. Because this mixer design uses the horseshoe topology, the IF response is flat within 2 to 3 dB from DC to 30 GHz.
For over half a century, mixer designers exploited the broad range of Si diode Vf to penetrate many different applications. For the first time this flexibility is available in a commercial GaAs IC process. The RC advantages of GaAs make this process especially compelling for Schottky circuits where either higher linearity or lower power are required. Conveniently, designs with different diode levels do not require separate mask sets – the biggest cost driver for IC development – merely a change in the starting wafer at the beginning of fabrication. Various applications can be served as long as the mixer passive circuitry (i.e., the balun and magic tee) is designed properly. The circuit results described in this article were obtained with only one mask set.
Marki Microwave has recently released a broad range of catalog GaAs MMIC mixers ranging from 3 to 67 GHz. GCS is offering access to the process through their foundry service.
- S. M. Sze and Kwok K. Ng, “Physics of Semiconductor Devices,” 3rd Edition, John Wiley and Sons, 2007.