Transistors are workhorses in both analog and digital circuits. In modern high-power applications, they have almost completely replaced the vacuum tube. Since the transistor was invented in the middle of the 20th century, these devices have continuously evolved. Today, for example, many high-power-transistor makers are making the switch to gallium nitride (GaN). Compared to more traditional technologies, such as silicon LDMOS and gallium arsenide (GaAs), GaN transistors promise to deliver better linear power and efficiency over wider bandwidths. Although GaN is the focus of many of today's headlines, it will not replace some of the more traditional technologies. Instead, it will give amplifier manufacturers a wide transistor selection that they can depend on to meet the plethora of applications that they are serving.

Currently, for example, more and more companies are choosing GaN for its ability to deliver higher frequency and higher output power. At June's International Microwave Symposium (IMS) in San Francisco, CA, Toshiba America Electronic Components (TAEC), Inc. (Irvine, CA) introduced a GaN power field-effect transistor (FET) capable of 174 W output power at 6 GHz. To achieve this impressive performance enhancement, TAEC optimized the epitaxial layer and chip structures for 6-GHz-band operation. It also adopted a four-chip combination structure to minimize heat buildup. The resulting GaN power FET boasts eight times the power density of a GaAs FET. Toshiba plans to refine this technology to manage heat-dissipation issues. It will then develop a series of GaN products for applications in satellite and terrestrial point-to-point communications. Additional applications for this technology include radar, homeland security, and medical. In an unusual application, for example, a customer has shown that RF heating can be used to treat tumors.

Nitronex Corp. (Raleigh, NC) also is known for its GaN work. Back in 2001, the company made headlines when it announced that it was producing GaNbased high-electron mobility transistors (HEMTs) on 4-in. rather than 2-in. silicon wafers. The company has continued its GaN developments ever since. This past March, it released a 50-W, 28-V GaN HEMT that is designed for WiMAX applications with frequencies from 3.3 to 3.8 GHz. For V_ds = 28 V, I_dq = 750 mA at a center frequency of 3.5 GHz. To provide broadband performance, the device offers input-only matching and no output matching. The PNPT35050 was built using the SIGANTIC GaN-on-Si platform technology.

Last month, RF Micro Devices (Greensboro, NC) made its first foray into GaN with the announcement of a high-power transistor family. These HEMTs offer power density of up to 4 W per mm. They provide 28-V bias operation and up to 16 dB of gain. The transistors are at the heart of a 120-W chip set for WCDMA and WiMAX applications (Fig. 1). The amplifiers promise to deliver peak drain efficiency of up to 67 percent at UMTS and up to 60 percent at the WiMAX frequency bands. They also vow to deliver 1000-hr. high-temperature reliability. Thanks to their high linearity, these products can deliver efficiency by operating as close to peak power as possible.

The HEMTs rely on a 0.5-µm GaN HEMT process. In a time when many other companies have gone fabless, RFMD is again leveraging its commercial wafer factory to manufacture these devices.

The transistors target the following applications: UMTS wireless infrastructure, 2.5-and 3.5-GHz WiMAX infrastructure, and pre-drive and driver stages for Class A/B operation. Currently, the company has a 48-V GaN device in the lab, which it is planning to release next quarter.

At the end of May, Cree also introduced a GaN HEMT. It is the first in a series of packaged GaN products that the company plans to offer for the broadband and WiMAX markets. This 15-W GaN transistor is dubbed the CGH35015. It is optimized for the broadband-wireless-access (BWA) applications that operate between 3.3 and 3.9 GHz. Typically, this transistor produces 2.5 W of average output power and 20 percent drain efficiency over that frequency range. Under orthogonal-frequencydivisionmultiplexing (OFDM) modulation, it features 11 dB of small-signal gain and 2-percent error vector magnitude (EVM) when operated at 28 V.

A 180-W GaN-HEMT from Eudyna (San Jose, CA) also targets WiMAX. The ES/EGN26A180IV features high-voltage operation of VDS = 50 V (Fig. 2). Typically, it delivers +53.0 dBm output power at 3-dB compression (P3dB). Its efficiency is usually 55 percent at P3dB, while its linear gain is 14.0 dB at 2.6 GHz. The GaNHEMT promises to provide ease of matching, superior consistency, and broad bandwidth for high-power L-band amplifiers with 50-V operation. The device targets both low-current and wideband applications for high voltage. The 180-W ES/EGN35A180IV is well-suited for those applications as well. Although many of its specifications are the same as the EGN26A180IV's, it differs in that it offers 50 percent typical efficiency at P3dB and 12.0 dB of linear gain at 3.5 GHz.

The company also offers the 90-W ES/EGN26A090IV. This GaN HEMT operates at a drain-source voltage of +50 VDC. It typically delivers +50.0 dBm at P3dB. The device boasts 55 percent typical efficiency at P3dB and 14.0dB of linear gain at 2.6 GHz. Its sibling, the ES/EGN35A090IV, offers the same level of operation and high power. Yet its efficiency is typically 50 percent at P3dB. In addition, this device's linear gain is 12.0 dB typical at 3.5 GHz.

The evolution of this multitude of GaN transistors will undoubtedly be driven by advances in materials and semiconductor processes. This past December, for example, Fujitsu Laboratories Ltd. (Kawasaki, Japan) announced the development of a GaN HEMT with insulated gates that are capable of practical output levels of 100 W or higher. This achievement was realized by using a GaN layer for the semiconductor surface instead of an AlGaN layer. By cutting out the aluminum, the company avoided the performance degradation that is derived from the easy oxidation of aluminum.

Essentially, Fujitsu modified the surface structure of the GaN HEMT. The transistor comprises insulated film of silicon nitride, which is deposited on the outermost layer of a GaN HEMT crystal. That crystal has a thin film of ntype doped GaN. A GaN layer is used for the semiconductor surface instead of an AlGaN layer (Fig. 3). Through this approach, Fujitsu succeeded in reducing gate leakage current to 0.1 mA or less. It claims that this amount is less than oneone millionth that of conventional technologies, which do not use insulated gates. Because the transistor targets basestation amplifiers, the company also verified that it was possible to bring the value of output-power leakage into adjacent channels down to practical levels.

On the heels of this announcement, Fujitsu announced the development of a GaN HEMT amplifier that achieves power output of 174 W at 63 V. The amplifier demonstrates drain efficiency of 40 percent, fulfilling WCDMA system requirements for base stations. Such amplifiers should help the industry achieve the goal of making smaller and more energy-efficient base stations for 3G mobile networks. By delivering output in excess of 150 W, it also meets the requirements for realistic application in 3G mobile systems.

Although many companies are achieving high-power industry firsts with GaN, impressive developments continue to be realized with other types of transistor technologies. For example, take a look at the novel plastic packages from Freescale Semiconductor (Austin, TX), which also aim to satisfy basestation amplifiers. The 2-GHz high-power-RF transistors, which are housed in over-molded plastic packages, vow to deliver performance that is comparable to air-cavity flange packages (Fig. 4). The devices will be based on the company's high-voltage, seventh-generation (HV7), RF laterally diffused metal-oxide semiconductor (LDMOS) technology. The first device, dubbed the MRF7S19120N (a model number that harks back to Freescale's Motorola heritage), delivers a minimum of 120 W output power at 1-dB compression (P1dB) and 36 W average power. Typical performance is 18 dB gain, 32 percent efficiency, and −37.5 dBc linearity at PAR = 6.1 dB (tested with a single-carrier WCDMA signal with PAR = 7.5 dB at 0.01 percent probability on CCDF). A corresponding family of 2.1-GHz products is planned for release in the third quarter of this year.