18 English

High speed and low power consumption are vital in ICs for high-speed optical communications systems. Some of the competing IC elements in this field include: Si-BJT (Silicon Bipolar Transistor), Si-CMOS (Silicon Field Effect Transistor), SiGe-BJT (Silicon Germanium Bipolar Transistor), GaAs-HBT (Gallium Arsenide Heterobipolar Transistor), GaAs-FET (Gallium Arsenide Field Effect Transistor), and GaAs-HEMT (Gallium Arsenide High Electron Mobility Transistor), amongst others. Fig. 1 shows the range of bit rates covered by these various devices, and it shows that the compound semiconductor, such as GaAs, is generally more suitable as a material for high-speed operation than Si. The reason for this is that the electron velocity travelling in the compound semiconductor, such as GaAs, is higher than that in Si. Around 1990, prior to any of our competitors, our department at Oki started to concentrate on the development of high-speed GaAs FET and HEMT device technology as well as applying this technology to digital and analogue ICs for optical modules used in 10 Gb/s optical communications systems (STM64 or OC-192). The front end module of an optical transmission system converts between optical signals and electrical signals, and as shown in Fig. 2, the transceiver consists of LD (laser diode), EA (electro-absorption) modulator, and modulator driver, whilst the receiver consists of PD (photo-diode), transimpedance amp, and limiting amp.1)