CMOS-on-Sapphire RF Switches Enable Multi-Band Cellular Handsets
By Dylan J. Kelly, Peregrine Semiconductor Corporation

The need for integration of GSM, EDGE, and WCDMA in so-called WEDGE cellular handsets has disrupted the PIN diode and GaAs switch markets due to performance, size, and cost requirements. UltraCMOS™ CMOS-on-sapphire technology used for switches is monolithically solving the difficult performance challenges posed by this integration requirement.

Mobile telephone service is reaching nearly three billion subscribers worldwide, covering more than 80% of the world’s population. The evolution of multiple band systems requires the liberal use of high selectivity filters in handset front ends in order to achieve the necessary range. As the frequency band count continues to increase, handset designers have also needed to incorporate switching elements in order to optimize the link budget and maximize range. Adding to the challenges, mobile telephony has become the highest volume consumer electronics product in the world, so cost, performance, form factor, and a long-term roadmap have become critical factors in any technology’s success in this market.

In the early days of GSM-only handsets, the industry was well served with PIN diodes to handle switching functions due to their high performance and low cost. However, when the industry moved to converge GSM/EDGE and WCDMA, PIN diodes no longer met the size and performance requirements of the next generation of handsets. Requiring long quarter-wave transmission lines and large forward bias currents to operate, PIN diodes fell into disuse with the introduction of quad-band GSM. In response to the new need, IC-based switching devices manufactured using UltraCMOS or GaAs stepped in to fill the technology gap created by the multi-band requirement (Figure 1). Designers quickly found that these technologies readily solved the multiple implementation problems with PIN diodes for multi-band operation. As a result, they have essentially displaced PIN diodes in mobile handset designs.

The addition of WCDMA to GSM/EDGE handsets has been heralded as the start of the 3G era, and it also introduced a new technology gap with a challenging third-order intercept (IP3) requirement of +68 dBm on the front-end switch in order to ensure network robustness. To put it in perspective, this requires that the switch does not generate a distortion signal larger than three trillionths of the WCDMA transmit signal (the equivalent of one silicon atom in a half-mile string of atoms). Because it meets this difficult linearity requirement, UltraCMOS has emerged as the leading solution for RF front-end switching applications due to the benefits of a low-loss, low-capacitance sapphire substrate combined with the fundamentally exceptional linearity of an intrinsic MOS device. In fact, UltraCMOS is the first CMOS technology to reach the antenna of a handset. The PE42692 SP9T switch from Peregrine Semiconductor, for instance, demonstrates 0.60 dB insertion loss at 1GHz, and it is specified with a third-order intercept point of +68 dBm and third-order intermodulation distortion of –111 dBm.

Complex Switching Needs
The switching needs of mobile handsets continue to grow in complexity. Converged 3G handset shipments supporting WEDGE exceeded 150M units in 2007, with projected volume (Figure 2) exploding at a 50% CAGR in 2008 to beyond 500M units in 2011.

In 2007, the majority of 3G handsets shipped with quad-band GSM/EDGE and single-band WCDMA, which requires a single-pole 7-throw (SP7T) switch at the antenna. However, due to increasing 3G demand, the integration of three WCDMA bands is now required, increasing the switch complexity to SP9T. In GaAs, this need is typically met by combining an SP5T and SP4T. In UltraCMOS, it can be done in a single chip including two transmit ports that can be used for GSM/PCS/EDGE, three transmit/receive ports (TRX1, TRX2 and TRX3) that can be used for either WCDMA or as receive ports, and four symmetric receive ports. This trend of increasing switch complexity is expected to continue as there are now nine WCDMA bands defined, and that number is expected to rise.

Meeting the Specs: Performance, Voltage, and Footprint
To support WCDMA and GSM/EDGE simultaneously, the RF front-end switch needs to be the most linear element in the handset and, it is actually the most linear solid-state element of any high-volume application in the world. Because of the insulating gate of CMOS technology and the ability to natively incorporate mixed-signal design techniques, UltraCMOS ICs can meet demanding linearity performance requirements in a monolithic solution.

Table 1 summarizes typical SP9T performance of the two technologies. Insertion loss performance is approximately equal, while linearity performance of UltraCMOS far exceeds that of GaAs. The wide VDD range of UltraCMOS aligns with existing handset supply voltages, and bias generators are in development for direct operation from battery voltage or +1.8 V DC supplies.

Although nominal VDD is specified for 2.75 V, integrated level shifting allows for 1.8 V and lower control logic to interface with scaled CMOS transceivers on the market, such as Texas Instrument’s LoCosto platform. This example shows a simple 4:16 decoder, but Serial Peripheral Interface (SPI)-based serial programming is also available.

Since the transistors on sapphire are dielectrically isolated from one another, they can be placed in series, or stacked, to tolerate the very high voltages levels present in an antenna switch. Although CMOS is a low-voltage technology, peak-to-peak voltage handling of 50 V is readily tolerated. Additionally, on-chip bias generation provides for optimized performance and eliminates the need for external DC blocking capacitors.

ESD tolerance of UltraCMOS switches is exceptional due to the integration of ESD protection devices. With Class 2 (2000 V) HBM tolerance on control pins and 1500 V HBM tolerance on RF pins, UltraCMOS switches are highly robust against ESD damage. This reduces module fallout during manufacturing, and also dramatically reduces the required ESD circuitry at the antenna to meet the stringent IEC61000-4-2 requirement, recovering low temperature co-fired ceramic (LTCC) area and insertion loss.

By using standard flip-chip chip-scale packaging technology, UltraCMOS products significantly reduce the footprint of handset front-end modules (FEMs). Usually fabricated in costly LTCC, FEMs will benefit from any reduction in footprint, which will directly improve the total solution cost. In Figure 3, migration from wirebonding to flip-chip in UltraCMOS reduced the LTCC substrate area consumed by 43% while simultaneously increasing the supported frequency band count from five to seven. A typical GaAs SP9T switch implementation requires two chips and 29 wirebonds, while an UltraCMOS SP9T flip-chip device is 83% smaller and requires no wire bonds.

Easing Implementation
The UltraCMOS process allows the inclusion of shunting FETs on all ports, thanks to complementary logic. This greatly simplifies the job of antenna switch module (ASM) designs and improves their performance. By controlling the impedances of the isolated ports at the plane of the switch ports, the module designer does not have to optimize all paths simultaneously, which is an intractable multi-variable problem. Instead, the designer can focus solely on the active path. The shunt FETs also provide for very high isolation performance, relaxing the spurious requirements on transceivers. (The PE42692 switch achieves transmit-receive path isolation of 43dB at 1GHz.) They also ensure protection of receive SAW filters from high RF power and ESD events.

A WEDGE SP9T implementation comparison between UltraCMOS (the PE42692 SP9T switch from Peregrine Semiconductor) and GaAs is shown in Figure 4. Note the large difference in die sizes, with the UltraCMOS die measuring 1.43 mm2, which is approximately half the die size of the GaAs SP9T at 2.85 mm2. With the fine design rules of aluminum metallization and the flexibility to place FETs in any orientation, the UltraCMOS RF switches can be implemented more compactly than in GaAs. The complementary devices, analog control capability, and MOS capacitors allow for control of both series and shunt RF FETs with a simple four-wire interface and low current drain. Figure 4 also shows the implementation areas of the 4:16 decoder of the two designs. The triple-metal process provides for placing flip-chip solder balls directly above circuitry in the UltraCMOS SP9T, conserving die area as compared to the SP9T GaAs switch.

Employing flip-chip technology has an added benefit because the switch can now be directly placed as a surface mount device (SMD) component. Since all other components in the FEM are SMDs, the need for any wirebonding equipment is eliminated. Shipped in tape-and-reel form, these flip-chip switches are placed on the module with standard placement machines, resulting in the lowest manufacturing cost. Additionally, at less than 250 mm mounted height, flip-chips reduce the overall module thickness, which is critical to meeting the demands for handset slimming.

Using CMOS also readily allows for the use of mature advanced electronic design tools to accurately predict performance. Before first silicon arrives, multi-port S-parameter files can be generated from a simulation for module design, reducing the number of module design spins required and accelerating time to market.

As a nearly lossless electrical insulator, sapphire is also ideally suited for passive integration. For instance, by depositing thick-film cupper as part of the flip-chip processing, very high-Q inductors can be created. Figure 5 shows an example of passive integration with a switch. This SP6T switch is integrated with dual transmit lowpass filters, so it monolithically integrates the functionality of a traditional LTCC-based quad-band GSM antenna switch module, but it does it in only a 2.3 mm2 area.

High Volume Production
Technological advantages have culminated in rapid growth of UltraCMOS for RF switch applications in handsets. Figure 6 shows the exponential growth of UltraCMOS switch shipments into the handset market.

With more than 300M units shipped to date and a run rate of more than 500K units per day, UltraCMOS is making significant inroads into the growing 3G segment of the handset switch market.

Currently, UltraCMOS technology is enabling the growth of 3G applications using legacy 0.5 um technology. In addition, UltraCMOS switch products at the 0.35 um technology node are now ramping to production with improved performance and reduced die size. By following the scaling roadmap indicated in Figure 7, UltraCMOS is applying Moore’s Law to the RF front-end, reducing area required per function and enabling further integration.

CMOS fabrication facilities are widely available at the legacy technology nodes shown in Figure 7, and CMOS-on-sapphire is processed on the same equipment without modification. UltraCMOS is currently in mass production in two world-class fabrication facilities and will be in process qualified in two additional Asian foundries by early 2009.

Designers are finding that UltraCMOS RF switches provide tremendous value to handset front-ends, particularly in WEDGE applications functions where very high IP3 is required. Without a doubt, 3G and LTE handsets will continue to increase in complexity, requiring scaling to higher throw counts and even the integration of multiple switches on one die. These anticipated developments fully exploit the integration, performance, and footprint advantages of UltraCMOS.

Peregrine Semiconductor Corporation
www.psemi.com
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