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Design Benefits of Silicon MMICs
Monolithic microwave integrated circuits produced using double-polysilicon technology that provides several advantages over typical GaAs RF devices are featured on this month's cover
Design Benefits of Silicon MMICs
Monolithic microwave integrated circuits (MMICs) were developed originally to deliver cost-effective, low voltage and high performance RF solutions to the mobile communications market. Until recently, common belief held that MMICs needed to be manufactured from GaAs. However, as a result of advances in the double-polysilicon process, this belief has been shattered.
Based on small-scale integration, MMICs typically reduce the number of components in a receiver's front end from 30 to six. This reduction effectively lowers the overall materials bill, cost of manufacture and logistics, and size and weight of the mobile telephone. Silicon devices deliver good RF performance (operating to and beyond 2.4 GHz) while featuring optimized passive components on chip that automatically compensate for temperature and process variations. Thus, the cost/performance ratio of silicon MMICs is unrivaled today.
Several advantages accrue to designers and manufacturers of mobile telephones using silicon MMICs. Given the integration of on-chip biasing and temperature compensation, these smart RF silicon transistors eliminate much detailed circuit design. This functionality is squeezed into a package the same size as a single discrete RF transistor, resulting in a 75 percent reduction in board size. By providing small building blocks rather than integrating down to a single device, issues of cross talk and parasitics are minimized. This approach delivers the inherent advantages of discretes -- flexibility, ease of customization, very small surface-mount device packaging and good price/performance ratios -- while incorporating difficult-to-design functions on chip.
Initially, the double-polysilicon technology represented a processing breakthrough in the production of silicon RF transistors with transition frequencies of better than 25 GHz. Today, optimized for 2.4 to 3 V operation, the process allows for battery packs (the largest and heaviest items in mobile telephone design) to be made smaller and lighter, requiring only two cells instead of three. Double polysilicon reduces a telephone's weight without compromising standby and talk times. High gain, high power-added efficiency (PAE) silicon transistors have a substantial price/performance ratio advantage over competing devices such as GaAs MESFETs.
An Unbeatable Combination
The double-polysilicon process makes use of advanced, self-aligned transistor technology, which offers advantages over existing bipolar technologies. Figure 1 shows a structure comparison between an existing advanced bipolar transistor and a self-aligned, double-polysilicon buried npn transistor. One polysilicon layer is used to diffuse and connect the emitter while another polysilicon layer is used to contact the base region. The collector is brought to the top of the die via a buried layer. Advantages over conventional bipolar technology include higher transition frequencies, higher power gain, reduced feedback capacitance, lower noise operation, improved heat dissipation, simpler matching to preceding driver stages and higher integration for MMICs.
Although the impetus to develop the technology is attributable to low noise use in amplifiers, mixers and power amplifier circuits operating at 1.8 GHz and higher for cellular and cordless applications, double polysilicon is also well suited in such high performance RF applications as pagers and satellite television tuners. Wherever small-signal RF performance is required, double-polysilicon transistors and MMICs offer an unbeatable combination. Double-polysilicon MMICs can be designed into pagers, set-top boxes, community antenna television amplifiers, RF power modules for mobile telephone base stations, radar detectors, RF toll road applications and other applications requiring high gain (in the region of 20 dB) at 2 GHz and low noise (< 1.2 dB) at high frequencies.
To produce transistors with cutoff frequencies of 25 GHz to operate at VCEs of 3 V or less, it is necessary to achieve base widths of approximately 100 nm. The double-polysilicon process delivers these base widths where polysilicon is used for both base and emitter connections. Steep doping profiles of the base and emitter regions create these very narrow base widths required for a high cutoff frequency, while submicron emitter widths of typically 0.5 mm (possible because of the process' self-alignment features) ensure a high fmax and low base resistivity. Minimization of base resistance is essential to meet the low noise figures (typically < 1.2 dB) required in low noise receiver applications.
Lateral connection to the base region by polysilicon reduces base resistance and minimizes collector-base capacitance. Large-area metallizations for the emitter allow transistors to handle the high emitter current densities necessary in medium-power types. However, the performance of silicon high frequency MMICs is potentially at risk if bond-wire and lead-out arrangements in the device package are not designed carefully.
In practice, bond-wire inductance and collector-base capacitance CBC are extremely important factors that limit the final transistor's high frequency gain. For small-signal transistors such as the type BFG410W, the major contribution to collector-base capacitance is made by the bonding pads rather than the intrinsic transistor itself. If the transistor die is mounted conventionally (with the substrate forming the collector connection), bond-pad capacitance is unacceptably high. The solution is to fabricate the double-polysilicon transistors using a buried collector layer and to mount the transistor die collector up. This technique maintains a low value of overall collector-base capacitance, minimizes the length of emitter bond wire and, hence, reduces emitter inductance. Figure 2 shows this mounting technique vs. a conventional bipolar mounting configuration.
Once polysilicon and MMICs were combined, several benefits emerged (in addition to the cost benefits and ease of use), including high gain, which translates into fewer transistors required to meet a high degree of signal amplification. For small types, a very low noise figure (typically 1 dB) allows mobile telephones to pick up very weak signals from base station transmitters. For medium-power types, very high PAE (typically 60 percent) minimizes the amount of battery power wasted in the handset's RF power amplifier and reduces the amount of heat dissipated.
Figures 3 and 4 compare typical power gain and noise figure performance with previous bipolar techniques. For medium-power types designed for use in mobile telephone RF transmitters, critical specifications include power output capability, power gain Gp and PAE. At a given output power, the most important factor in maintaining high power gain is a high IC(max)/CBC ratio. The self-aligning double-polysilicon process already achieves low CBC values, while a high IC(max) specification is achieved by increasing collector-doping levels. Submicron emitter widths prevent current crowding effects, keeping base resistance low to prevent power gain degradation. Conduction of heat via the emitter lead frame into the PCB provides optimal heatsinking conditions. Operating at 1.8 GHz from a 3.5 V supply, power gains greater than 11 dB with PAEs as high as 60 percent have been demonstrated.
Four factors drive the mobile telephone market: size, weight, usability and cost. Capable of operating at supply voltages as low as 3 V, high performance silicon transistors enable designers to use battery packs with fewer cells. The transistors' excellent performance easily provides 2 GHz operation and their high operating efficiency reduces wasted battery power.
The new MMIC family includes the types BGA2001 and BGA2003 MMIC low noise general-purpose amplifiers targeting GSM, Digital Enhanced Cordless Telephony (DECT) and digital communication systems (DCS) applications with a low component count. Incorporating an internal bias circuit, these RF transistors feature internal compensation for temperature and current gain spread. Both units are housed in a plastic four-pin SOT343 package.
Offering very high power gains (19 dB for the BGA2001 and 21 dB for the BGA2003) and very low noise figures at 2 GHz of 1.5 and 1.9 dB, respectively, the BGA2001 device features a fixed bias current of 4 mA at a 2.5 V supply, while the BGA2003 MMIC is adjustable up to 30 mA via a control pin. Collector current can be switched using a bias control circuit requiring very low current without disconnecting the transistors from the power supply. The devices are suitable for low noise amplifiers and mixers in wideband applications such as analog and digital cellular telephones, radar detectors, satellite television tuners and high frequency oscillators.
The BGA2022 device is a low power, low voltage silicon MMIC mixer optimized for high gain and high linearity. The unit is designed for use in the receive chain of GSM, TDMA and CDMA portable telephones. Given that its performance is largely determined by external components, the device is extremely flexible and can be used for frequencies up to approximately 2.5 GHz. Applications include use in the receiver side of wireless systems that require high conversion gain and high linearity at low supply current (such as CDMA). Emphasis is on intermodulation performance (input referred) at a reasonable gain. To date, competitive products are only offered in GaAs. Supplied in a six-lead plastic SOT363 package, the device is smaller (2.00 mm ¥ 1.25 mm) when compared with competitive devices that measure 2.9 mm ¥ 1.3 mm.
The BGY2031 device is a MMIC variable-gain amplifier in double-polysilicon technology that functions as a general-purpose, variable-gain amplifier for low voltage and medium-power applications. Due to its high output power and good linearity, the unit also is a driver for power amplifiers in systems that require good linearity (such as CDMA) in both the cellular (850 MHz) and PCS (1.9 GHz) bands. As a result of the BGY2031 amplifier's features, mobile telephones adjust the power used as a function of base station distance, requiring less battery consumption because the amplifiers use less current. The device is specifically designed to optimize thermal and electrical performance and is housed in a small SOT551 package (1.80 mm ¥ 1.15 mm ¥ 0.90 mm).
A Breakthrough in Manufacturing
When manufacturing devices, 10 fully coupled processes are performed in a five-hour cycle time, taking whole wafers through to final parts with no manual intervention. The breakthrough in manufacturing (BIM) procedure incorporates integrated back-end equipment, including an online vision system that screens out assembly defects.
The shop floor control system, a fully computerized production environment, provides online access to all relevant production data such as order status and yield, while a line-monitor system monitors the equipment status throughout the factory. Process controls are paperless with control charts and documents displayed via a local area network. The BIM line features zero defects at parts-per-million levels. A defect rate within this manufacturing environment must be measured in parts per billion.
Increased Performance through Optimized Packaging
Substantial advances in packaging have been designed specifically to enhance the performance of silicon MMICs. The SOT551 package used to house the BGA2031 device has been specially developed to optimize electrical performance as well as thermal resistance. Since two ground pins are combined, the parasitic inductance to ground has decreased substantially, resulting in higher gain. The SOT551 package offers exactly the same dimensions and footprint as the SOT363 (the packaging for the BGA2022 MMIC).
The new four-pin SOT343 package features a small, optimized lead frame designed to reduce emitter inductance and base-collector capacitance. The package's specially designed lead frame and heat sinking through the emitter make it ideal for use in the BGA2001 and BGA2003 low noise general-purpose amplifiers.
Double-polysilicon technology is being utilized to produce MMICs that feature a superior cost/performance ratio for mobile telephone applications to 2 GHz. These devices provide a cost advantage over typical GaAs RF devices used to date in wireless communications applications. The MMICs feature high levels of integration, small package sizes and low voltage operation, making them ideal for use in mobile telephone handsets. Additional information can be obtained from the company's Web site at www.semiconductors.philips.com.
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