Although the market is much smaller than that for silicon based devices, III-V devices have been developed for high performance, high frequency applications from UHF to millimeter-wave frequencies. In addition to the discrete devices, the past several years have seen significant advances in MMIC based on III-V materials. The most common baseline materials are GaAs and InP. GaAs metal Schottky FETs (MESFETs) are quite mature and are available off the shelf up to X and Ku band applications. More advanced high electron mobility transistor (HEMT) devices are the preferred choice for higher frequency and higher performance applications with lower noise figures for low noise amplifiers. InP HEMT provides better performance for higher millimeter wave frequencies as demonstrated by Daimler-Chrysler in Fig. 5.6.
Fig. 5.6. Daimler-Chrysler InP-based HEMTs.
TRW recently demonstrated the world's first 190 GHz InP HEMT low noise amplifier (LNA), which exhibited a gain of more than 7 dB from 160 - 190 GHz, with a peak of 9.6 dB, while the noise figure is 6 dB at 170 GHz (see Fig. 5.7). This device has a 70 nm e-beam defined T-gate on a 2-mil substrate with a 25 µ backside via. TRW also has demonstrated an InP HEMT MMIC power amplifier with an output power of 427 mW with PAE of 19% and an associated gain of 8.2 dB (see Fig. 5.8). The state of the art devices exhibit fT > 300 GHz and fmax > 500 GHz. The expected future performance would be fT > 500 GHz, fmax > 1 THz, and an LNA with a gain of 10 dB at 260 GHz, as well as a PA with an output power of 0.25 W at 260 GHz.
Fig. 5.7. TRW 190 GHz InP HEMT low noise amplifier.
Fig. 5.8. TRW W-band In-P HEMT power amplifier.
For applications at somewhat lower frequencies, GaAs HEMT (PHEMT or pseudomorphic HEMT) is a very practical and readily available device. In fact, Dr. Honjo of NEC believes that wireless applications up to 100 GHz do not require the use of InP devices. For future mobile wireless communications for multimedia, a high speed (> 20 GHz) and low power phase lock loop (PLL) is required. NEC has developed a CPW-based MMIC using a 0.1 µ GaAs E/D-HEMT. The salient feature of this device is the use of a two stage mushroom gate (see Fig. 5.9).
Fig. 5.9. NEC two-stage mushroom gate HEMT.
A new type of microwave device that has drawn a considerable research attention is III-V heterojunction bipolar transistor (HBT). This device tends to have a lower phase noise than FET type devices. At the moment, the usable frequency range is lower, however. NEC uses selective regrowth to reduce the base resistance to one fourth (see Fig. 5.10). At the same time, pseudomorphic InGaAs graded base was used so that the regrowth-performed GaAs-based HBT achieved low RbCbc leading to an fmax comparable to those of high-performance InP-based HBT that has a higher fT. Some of their devices include (1) a 26 GHz power amplifier module made of two device power combined with 3.63 W and PAE of 21%, (2) a 1W 35 GHz HBT power amplifier with PAE of 29% and a 60 GHz dynamic frequency divider, and (3) a low phase noise 38 GHz HBT MMIC VCO.
Fig. 5.10. NEC high fmax HBT technology.