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IEDM Highlights Include SiGe HBTs Operating At 350 GHz

Results presented at last December's IEDM raised the bar for device performance, writes Tim Whitaker. Key announcements included a 350 GHz SiGe HBT, AlGaN/GaN FETs providing 2.3 W at 30 GHz and 100 Gbit/s logic ICs fabricated in InP-based HEMT technology.
As usual, the International Electron Devices Meeting (IEDM), held December 8-11 in San Francisco, gave a snapshot of the state of the art in advanced semiconductor technology. Most of the papers were related to mainstream silicon CMOS, although technologies such as strained silicon and SOI are becoming increasingly prevalent. A small number of compound semiconductor papers were also presented, notably in the field of nitride electronics.

SiGe HBTs reach 350 GHz IBM Microelectronics continued to increase the performance of its SiGe HBT process, reporting an ft of 350 GHz. This is the highest reported ft value for any silicon-based transistor and the highest for any bipolar transistor, including those fabricated using III-V materials. At last year s IEDM, Ida et al. from Fujitsu reported an InP/InGaAs DHBT with an ft of 341 GHz.

IBM s device had an fmax of 170 GHz and BVCEO and BVCBO values of 1.4 and 5 V, respectively. The structure (see figure 1) features shallow and deep-trench isolation, while a buried subcollector layer and an n-epitaxial layer form the collector region, along with a selectively implanted collector pedestal. A SiGe base layer doped with boron and carbon was grown by non-selective UHV CVD, and a B-doped raised extrinsic base was formed self-aligned to an in situ P-doped emitter. Simultaneous optimization of ft and fmax resulted in values of 270 and 260 GHz, respectively.

Infineon has also made great strides with SiGe. This year, Böck et al. reported a dynamic 1:2 frequency divider operating at 99 GHz, which the authors claimed to be the highest reported divider operating frequency for any transistor technology. The total power consumption of the circuit was just 290 mW. The SiGe transistors have a double-polysilicon self-aligned emitter base configuration with a SiGe base (doped with B and C), which is integrated by selective epitaxial growth. The devices had an ft of 155 GHz at BVCEO = 1.9 V, an fmax value of 167 GHz and a 4.7 ps gate delay. The analog performance of the technology was also investigated by fabricating a 19 GHz LNA, which had a 50 Ω noise figure of 2.2 dB and a gain of 26 dB; these are record values for silicon LNAs at this frequency.

IHP of Germany reported a novel SiGe HBT structure without deep-trench isolation and with low-resistance collectors formed by high-dose ion implantation after shallow trench formation. The technology yielded an ft value of 190 GHz at BVCEO = 2.0 V, as well as HBTs with ft x BVCEO products above 360 GHz. Ring oscillator delays of 4.3 ps and a high yield of 4000-transistor arrays were also demonstrated. The high-speed HBT module was integrated into a 0.25 µm CMOS platform, leaving CMOS device characteristics unchanged.

100 Gbit/s logic ICs using InPResearchers at NTT Photonics Laboratories described what they believe to be the fastest logic operation of a transistor-based IC. The group reported error-free multiplexing and demultiplexing functionality at 100 Gbit/s using InAlAs/InGaAs/InP HEMT technology (see figure 2). NTT fabricated two ICs using a 0.1 µm gate length InAlAs/InGaAs/InP HEMT process, which provided ft and fmax values of 175 and 350 GHz, respectively. A 100 Gbit/s selector IC contained two data buffers, a clock buffer and a 2:1 selector core circuit that was designed to directly drive the external 50 Ω load without the need for output buffers.

In order to confirm the error-free operation of the 100 Gbit/s selector IC, it was necessary to build a second IC capable of demultiplexing the 100 Gbit/s signal to 50 Gbit/s. This allowed the output waveform to be recorded using a 50 GHz bandwidth digitizing sampling oscilloscope. Error-free operation with a bit error rate of less than 10-10 was confirmed for both the 100 Gbit/s multiplexing function of the selector IC and the 100 Gbit/s demultiplexing function of the second IC. Power dissipation was 3.2 and 4.7 W, respectively.

Nitrides provide power and linearityA number of papers described record performance for AlGaN/GaN HEMTs, which are attractive for high-power and high-frequency applications. Kasahara et al. from NEC described the first successful watt-level Ka-band power operation of an AlGaN/GaN HFET on SiC. A device with a gate length of 0.25 µm and a gate width of 0.36 mm had a maximum CW output power of 2.3 W at 30 GHz, together with a PAE of 38% and a linear gain of 8.8 dB at Vds = 30 V (see figure 3). The authors believe that the output power of 2.3 W and the power density of 6.4 W/mm are the highest values reported to date for GaN-based devices at Ka-band frequencies. The device had an fmax of 120 GHz and a drain current density of 1 A/mm. The electrode layout and surface passivation layer were optimized to enable operation at a bias voltage of 30 V. GaAs transistors operating at a bias voltage of 5-8 V require passive circuitry to combine the output power of multiple transistors, resulting in larger chip and package sizes, increased power loss and narrower frequency bandwidths. NEC believes that the nitride-based power amplifier will be suitable for operation in all the main millimeter-wave communication bands at 22, 26 and 38 GHz.

Meanwhile, R Quay and colleagues from the Fraunhofer Institute of Applied Solid-State Physics reported AlGaN/GaN-on-SiC HEMTs operating at up to 40 GHz. The devices had a gain in excess of 6 dB and a maximum output power of 0.3 W, equating to a power density of 1.23 W/mm for a gate width of 8 x 30 µm (0.24 mm). The CW PAE was 10% at a Vds of 26 V, while the peak PAE was 16% at Vds = 15 V.

Linearity is also an important characteristic of AlGaN/GaN HEMTs targeted at applications such as wireless base stations. Nagahara et al. from Fujitsu reported excellent linearity characteristics of AlGaN/GaN HEMTs at Vds = 30 V, and class AB opera-tion at 1.9 GHz. The devices, grown on SiC, exhibited a third-order intermodulation distortion (IM3) of -34.7 dBc for an output power level of 26 dBm (backed-off 8 dBm from the saturation power). The RF performance is attributed to suppressing current collapse under a high applied voltage, using an optimum n-AlGaN donor layer thickness and SiN passivation.

Yi-Feng Wu and colleagues from Cree Lighting also reported on the excellent linearity of AlGaN/GaN HEMTs at 4 GHz, including an IM3 of -30 dBc and a PAE of 40% with only -2.6 dB back-off. The results can be compared with the -30 dBc IM3 and 45% PAE achieved by InP-based HBTs at 10 GHz.

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