RF Wideband Silicon MMICs

Philips Semiconductors
Sunnyvale, CA

The recently developed Double Poly process for manufacturing wideband silicon RF transistors and MMICs has been enhanced to include inductance loops, capacitors and resistors on chip. The result is a new family of MMICs that help reduce costs and design time for mobile telephone manufacturers.

The new MMICs typically reduce external components in a mobile telephone's receiver front end from 30 to six. Fewer components reduce the overall material costs as well as the size and weight of the telephone, resulting in easier and quicker design and more reliable use.

The Double Poly Process
Double Poly technology involves the manufacture of silicon RF transistors with transition frequencies fT greater than 23 GHz. The process is optimized for 2.4 to 3 V operation, making the devices suitable for the new, light-weight, battery-operated cellular telephones that utilize two cells rather than three. Producing these transistors requires the fabrication of transistor base widths on the order of 100 nm. These base widths are achieved using a double-polysilicon process, as shown in Figure 1 .

Polysilicon is a layer of polycrystalline silicon that is deposited onto the semiconductor wafers through a low pressure chemical vapor deposition process. Because this process allows precise thicknesses of silicon to be grown over underlying layers of silicon or silicon dioxide and is self aligning, vertical transistor structures can be fabricated. In the Double Poly process, two separate layers of polysilicon are used. The steep doping profiles of the base and emitter regions create the narrow base widths required for a high cutoff frequency while submicron emitter widths ensure a high fmax and low base resistivity. Low base resistance is essential for low noise figure performance (typically less than 1.2 dB).

The Top-side Collector Construction
High frequency performance may be destroyed in transistor chips produced using this process if the bond wire and lead-out arrangements in the device package are not designed carefully. Bond wire inductance and collector-to-base capacitance are the most significant factors that limit the final transistor's high frequency gain. For small-signal transistors, the dominant contribution to the collector-to-base capacitance comes from the bonding pads rather than the intrinsic transistor. If the transistor die is mounted conventionally with the substrate forming the collector connection, the bond pad capacitance is unacceptably high.

To overcome this problem, the double-polysilicon transistors are fabricated using a buried collector layer and the transistor die is mounted collector up. This configuration maintains a low overall collector-to-base capacitance and minimizes the length of the emitter bond wires, reducing emitter inductance. This top-side collector construction is shown in Figure 2 .

Smart Transistors
The most recent enhancement to the Double Poly process is the ability to produce discrete transistors such that inductance loops, capacitors and resistors can be included on chip. The resulting MMICs can be created without extra mask stages or process steps. Thus, passive components can be integrated on active discrete devices to generate smart transistors.

These MMICs feature built-in temperature-compensation circuitry and biasing, and process spread compensation on chip. They are referred to as smart because they compensate for process and temperature variations automatically. Despite the devices' increased complexity, the overall die size is minute. In many cases, the same packaging is used for the MMIC as for the single transistor.

New MMIC Devices
Several new MMIC devices have been developed using the described process that are aimed specifically at today's mobile telephone applications. These devices include the models BGA2001/2/3 MMIC amplifiers, the model BGA2051 MMIC power amplifier, the model BGA2021 mixer MMIC and the model BGY241 UHF amplifier module. These devices allow designers to utilize more of a discrete circuit design technique due to the extremely small MMIC packages, thereby minimizing problems with parasitics and crosstalk often found in more integrated circuitry. The building block approach also offers the flexibility of tailoring the optimum solution for each individual application.

The models BGA2001/2/3 MMIC amplifiers feature an NPN Double Poly RF transistor combined with an integrated temperature-compensation bias in a plastic, four-pin, SOT343 package. These amplifiers offer very high power gains (19 dB for the model BGA2001 and 21 dB for the models BGA2002/3) with very low noise figures at 2 GHz of 1.5 dB (model BGA2001) and 1.9 dB (models BGA2002/3). The BGA2001 and BGA2002 amplifiers have a fixed bias current of 4 and 10 mA, respectively, using a 2.5 V supply. The BGA2003 amplifier's bias current is adjustable to 30 mA by means of a control pin.

These general-purpose RF amplifiers combine the advantages of integration with the high performance of discrete transistors. The devices are used for low noise amplifiers and mixers in wideband applications, such as analog and digital cellular telephones, cordless telephones, radar detectors, satellite tuners and high frequency oscillators.

The BGA2051 MMIC power amplifier, shown in Figure 3 , is a two-stage device that delivers an output power of 400 mW at a 3.6 V supply voltage. The output can be controlled via a pulsed DC switching voltage for time-division multiple access (TDMA) applications. The amplifier is supplied in a plastic, eight-pin, surface-mount package. It offers high gain (23 dB) and is suitable for 1.88 to 1.92 GHz cordless telephone applications.

The model BGA2021 mixer MMIC, shown in Figure 4 , is intended primarily for applications on the receiver side of wireless systems. The unit operates from 0.5 to 2.5 GHz; features an enable switch pin; and offers high gain, low noise figure and high third-order intercept point. The internally balanced structure ensures good isolation. The mixer is available in a six-pin SOT457 surface-mount package.

The model BGY241 UHF amplifier module, shown in Figure 5 , is a three-stage amplifier with 35 dBm output power offered in an SOT482A leadless package. The amplifier comprises one NPN silicon planar transistor die and one bipolar monolithic IC mounted on a metallized ceramic substrate with matching and bias circuitry. The device is suitable for use in TDMA digital cellular radio systems in the 880 to 915 MHz frequency range, such as those used in the Global System for Mobile communications.

Conclusion
Enhancements to the Double Poly process and its associated new devices add a new dimension to RF and microwave circuit design. In an era when higher levels of circuit integration seem to be the trend, these new devices allow the circuit designer to accomplish his or her task using miniature discrete components that are low cost, high performance and more versatile. Small-scale integration enables smaller, lighter and less expensive mobile telephones and quicker time to market. More information is available at http://www.phillips.com.

Philips Semiconductors, Sunnyvale, CA
(800) 447-1500, ext. 1477.