Transistors Depend On Thermal Flow

Thermal management is a key phrase tossed about by solid-state amplifier designers. With the increasing power levels possible from solid-state RF devices, heat dissipation becomes a main limiting factor in the size of an amplifier, since too much heat can dramatically shorten a transistor's lifetime.

Earlier this year, for example, Freescale Semiconductor ( introduced its model MRF6VP3450H, a 50-V silicon LDMOS field-effect transistor (FET) capable of 90 W average output power and 450 W peak envelope power (PEP) at UHF. The company also offers the 50-V model MRF6V14300H LDMOS device for L-band radar applications, with 330-W pulsed output power from 1200 to 1400 MHz. Silicon bipolar transistors have become all but forgotten in the wake of such high-power LDMOS devices but, still, with the higher output levels comes thermal management issues.

Placing a transistor in a package with good thermal conductivity can help with thermal management. Materials developers are looking beyond conventional formulations to create enhanced thermal solutions. The firm sp3 Diamond Technologies (, for example, has developed silicon-on-diamond (SOD) wafers using a proprietary chemical-vapor-deposition (CVD) process.

In addition to surrounding a heat-generating RF power transistor with thermally conductive materials, it helps to know ahead of time the effects of a given transistor's heat generation, on the device, surrounding components and printed-circuit boards (PCBs). For that purpose, several companies offer effective thermal modeling software, including Ansys (, Flomerics (, and Thermoanalytics ( Ansys Icepak, for example, can predict air flow and heat transfer at the component level, at the PCB level, and even at the system level.

Flotherm from Flomerics might be one of the best-known thermal modeling tools for electronic components. The three-dimensional (3D) modeler allows users to create models of devices, components, and circuits and perform full thermal analysis of heat flow.

RadTherm from Thermoanalytics predicts the steady-state and dynamic thermal behavior of components and circuits, showing complete temperature distribution across a design as well as heat transfer rates for radiation, conduction, and convection thermalmanagement approaches. As with the other software tools, it is designed to optimize the thermal behavior of an electronic component, circuit, or system early in the design cycle, so that expensive modifications are not needed later in the design process.

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