Sizing up Discrete Devices Against Integrated Circuits

Sizing up Discrete Devices Against Integrated Circuits

Discrete transistors require more extensive design efforts than integrated circuits—albeit with much greater flexibility for realizing an amplifier or other component with special features.

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Integrated circuits (ICs) may best symbolize the electronic technology of the present day, at times packing the equivalent functions of a system into a single semiconductor chip or package. ICs have made our current convenient world of portable computers and mobile communications devices possible, and at affordable prices.

But for many applications and reasons, discrete RF/microwave components are still to be preferred over ICs, even with their larger sizes and higher costs—and sometimes because of their larger sizes. While this industry may not use as many vacuum tubes as it did three decades ago, it still enjoys healthy demands for discrete RF/microwave components versus ICs, and for a number of different reasons.

While it is possible to pack a great deal of circuit functionality within a small, packaged IC, developing such a circuit typically requires longer time and greater expense than designing and assembling a circuit with similar functions using discrete active and passive components on a printed-circuit board (PCB). The small device geometries of the semiconductors and passive circuit elements in an IC enable the construction of extremely compact analog, digital, and mixed-signal circuits. That being said, those small device dimensions will also limit the amount of RF/microwave power that can be produced from any amplifiers within those ICs.

Fig. 1
Many IC components are available in chip versions as well as in packaged form, such as this 4-W power amplifier IC, for flexibility in circuit design and layout. (Photo courtesy of MACOM)

Put simply, in solid-state circuitry, size equates to power. Larger semiconductor devices are capable of higher energy output levels. With an amplifier, for example, larger transistors can produce larger output power levels. Larger devices will also consume more power than smaller devices to achieve those higher output levels. Still, there is no way to truly compare an RF/microwave transistor fabricated in a monolithic-microwave-integrated-circuit (MMIC) with a discrete transistor. This holds true even when both are based on the same semiconductor substrate—e.g., silicon, gallium arsenide (GaAs), or gallium nitride (GaN).

Even if a discrete transistor was fabricated with the same dimensions as the monolithic transistor, it would provide the potential for higher output power levels for a given bias supply, since all of the power supply is being directed to the transistor. In the MMIC, energy is consumed by the surrounding circuitry, including the active device’s input and output impedance-matching networks.

Of course, the additional circuitry provides additional benefits compared to a discrete transistor, depending on the design of the MMIC. It may include power-supply circuitry and even electromagnetic-interference (EMI) filtering on chip, so as to avoid the chore of adding those circuits to an amplifier designed around a discrete device.

At the small-signal power levels at which they can be compared (such as 1 W or less), an IC amplifier, for example, will have input and output ports matched to 50 Ω for ease of installation in a circuit or system layout. In contrast, a discrete-device amplifier can be constructed using a wide choice of PCB materials, selecting materials for optimum characteristics—such as permittivity and coefficient of thermal expansion (CTE)—to enable the highest amount of output power and greatest power-added efficiency from a discrete power transistor.

In addition, while they add to the size of the discrete power amplifier, thermal management materials and heat sinks can be included to vent heat away from the power transistors. This allows continuous operation at higher power levels than are possible with a monolithic amplifier designed for small size and lacking thermal management materials.

In spite of the many circuit functions available in IC form, many circuit designers still start with discrete transistors and diodes. According to Tim Boles, distinguished fellow of technology at MACOM, there are many benefits to be realized from the use of discrete components: “Historically, RF discrete circuits have been displaced by MMIC solutions at low power levels because the required functionality increased in complexity, and the volume requirements of the marketplace applications ensured an adequate ROI despite the dramatic increase in associated developmental costs.  

“This migration path was and continues to be true for the wide variety of technology platforms that are utilized for RF applications, including silicon, GaAs, and most recently GaN,” Boles continues. “However, MMIC solutions do have limitations. The first is in the realm of very high power. MMIC power integration continues to increase, but discrete RF components have increased much further, yielding discrete RF devices capable of delivering over 1 kilowatt of RF energy in the microwave range.

“Second, as mentioned earlier, the development costs and complexity associated with MMICs are much higher than the costs and effort of expanding the performance of an RF discrete component. In order to economically justify this development effort and expense, the final market application must require a large enough unit volume to provide a reasonable ROI. Thus, for small-volume RF circuit requirements, discrete makes the most sense in terms of cost-effectiveness.”

For any high-volume applications requiring excellent unit-to-unit repeatability, ICs can achieve levels of repeatability in amplitude and phase responses with frequency that can be quite challenging to achieve with discrete component designs. The repeatability possible with a semiconductor process and IC components requires a great deal of broadband testing and hand tuning of a discrete component design, with the associated higher costs noted by Boles for discrete device designs.

Boles’ firm, MACOM, supplies both ICs and discrete devices based on silicon GaAs, and GaN substrate materials. As he notes, “MACOM has been a leader in providing high-performance RF discrete power components, including silicon bipolar transistors, PIN diodes, Gunn diodes, GaAs HBTs, and GaN HEMTs for the RF and microwave marketplaces. We continue to build on this high frequency power component base, and we continue to advance our discrete RF offerings while using these building block components to enhance MMIC performance in the areas of GaAs pHEMT power amplifiers, high power PIN diode MMIC switches, and GaN modular power integrated circuits.”

IC-based components such as amplifiers are typically available in miniature packages, such as surface-mount-technology (SMT) housings, as well as in die form for designers to directly mount the IC onto a PCB without the package. As an example, the model MAAP-011139-DIE is a 4-W IC bare die amplifier for use from 29 to 31 GHz for very-small-aperture-terminal (VSAT) applications. It can also be supplied in an SMT package (see figure).

Based on GaAs pseudomorphic-high-electron-mobility-transistor (pHEMT) semiconductor technology, this device provides reasonable output power for its small size (4 W saturated output power), although higher power levels are possible using discrete transistors and associated amplifier circuitry, with the tradeoff being larger size.

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