Several active device semiconductor technologies are available today to amplify pulsed and continuous-wave (CW) signals across narrow or wide bandwidths from HF/VHF/UHF to L-, S-, C-, and X-band frequencies and beyond. But while more recent power-transistor technologies such as gallium-nitride (GaN) on silicon-carbide (GaN-on-SiC or GaN/SiC) high-electron-mobility-transistor (HEMT) power transistors are dominating new system designs, knowledge and insight on well-established, legacy semiconductor technologies such as silicon bipolar junction (Si bipolar or Si BJT), silicon laterally diffused metal oxide semiconductor (Si LDMOS), and silicon vertically diffused metal oxide semiconductor (Si VDMOS) transistors are still invaluable to any high power amplifier (HPA) designer today.

These technologies are depended on routinely in certain radar system HPA applications where the performance (and often cost) advantages of these device types are still critical to the radar's system-level performance and bill-of-materials (BOM) at-large.

This article provides insight on positions in certain RF/microwave power amplifier block diagrams where silicon technology still rules the roost. Amplification of CW communications signals, for instance, requires "always-on" power from the amplifier along with Class AB or other biasing schemes to create optimum device linearity. And when amplifying pulsed signals, the range of signal conditions can become very complex, such as defined by your unique pulse width and pulse duty cycle requirements.

Thermal management becomes the common denominator between the two, and although different types of RF/microwave power transistors are capable of high-power efficiency, no power transistor is 100% power efficient, as some DC and RF power supplied to a power transistor will always be lost as heat. (Which also must be dissipated). Amplifying CW signals, or long-pulse length and/or high-duty-cycle pulses, will result in more heat from one transistor technology than another and will vary when compared to handling short pulses or low-duty-cycle pulses. So, in many cases, certain proven silicon devices are still the most ideal choice.

So, while the silicon bipolar junction transistor (Si BJT) might be recognized as legacy technology in comparison to today's more advanced gallium nitride on silicon carbide (GaN-on-SiC) RF power semiconductor technology, it is by no means an obsolete technology. For example, Si BJT amplifiers have the smallest and lowest-cost circuits and only need a single positive supply voltage.

Integra's IB1011S1500 is an example of a popular Si bipolar transistor designed for IFF/SSR applications. This device delivers typically >1400 W output power at either 1030 or 1090 MHz with >9.8 dB gain and 48% efficiency with 10-μs, 1% duty-cycle pulses.

The extremely broadband model IDM500CW300 is a Si VDMOS transistor from Integra that is capable of 300 W CW output power from 1 to 500 MHz. This device requires no elaborate biasing or power-supply arrangements other than a single 28-V supply and it provides as much as 65% efficiency. It is only somewhat "bipolar-like" in its gain performance, however. Delivering 9 dB typical power gain across the frequency range, additional input gain stages may be needed in certain conditions.

Si LDMOS is a newer technology than Si bipolar and VDMOS, and has found widespread use in high-linearity communication applications, as well as in broadband CW amplifiers. It is also a great choice for pulsed applications up into L-band. Si LDMOS is well-suited to long-pulse and/or high-duty-cycle applications because of its very low thermal resistance per watt, which also contributes to its excellent VSWR-withstanding characteristics.

ILD1011L950HV from Integra is an excellent example of a state-of-the-art Si LDMOS transistor for L-band avionics applications. This transistor typically delivers 1100 W at 1030 MHz under the demanding Mode S ELM waveform (48 x {32 μs on, 18 μs off}, 6.4% long-term duty cycle) with 16 dB gain and 55% efficiency. Unlike similar devices from other RF transistor manufacturers, a unique feature of this device is that it is a single-ended rather than push-pull transistor. Consequently, it requires a smaller, less-expensive, and simpler circuit since no balun is required. (This type of added benefit is something RF power system designers should keep an eye out for.)

To learn more about the tradeoffs of Si bipolar, Si VDMOS, Si LDMOS, and GaN-on-SiC RF power transistor technologies, download the Tech Brief: "Zero-in on the Best RF Transistor Technology for Your Radar's High Power Amplifier Designs".

To review a complete library of legacy silicon bipolar devices, and to download data sheets, visit our RF Power Silicon Transistors product page.