Aiming at an application to ultra-high speed and large bandwidth optical communication systems, electronic and optoelectronic devices employing III-V compound semiconductors such as GaAs and InP are studied. As for devices, we investigate (1) ultra-high frequency transistors such as HEMT's and HBT's, (2) ultra-high speed optical receivers such as MSM-PD's and APD's, and (3) opto-electronic circuits integrated with devices mentioned above, i.e. OEIC's. In these devices, the quantum well fabricated with heterostructure semiconductors is used. We also investigate theoretically the quantum states based on the rigorous quantum theory and experimentally the optical properties using PL measurement method.
We have fabricated HEMT's with a strained InGaAs, whose mole fraction is close to that of InAs, as the channel and characterized high-frequency performances using a network analyser with a bandwidth of 50 GHz. HEMT's with the gate length of 0.1 μm exhibited a current cut-off frequency research ranging over 200 GHz. This is because the strained InGaAs has a relatively high drift velocity of electrons. In this way, an application of these HEMT's to future high-speed IC's is expected.
We have fabricated MSM-PD's with a strained InGaAs, whose mole fraction is close to that of InAs, as the channel and characterized the optical response using a fiber laser with a bandwidth of 400 femt-seconds. MSM-PD's with a L&S of 0.2/0.6 μm exhibited a pulse width less than 20 psec. Like HEMT's, this is because the strained InGaAs has a relatively high drift velocity of electrons. In addition, these MSM-PD's exhibited a responsivity more than one regardless of the channel width as thin as 10 nm, where the responsivity corresponds to an optical sensitivity. In this way, ultra-high speed OEIC's can be realized by simultaneously fabricating MSM-PD's and HEMT's on the same epitaxial wafer. Therefore, an application of these MSM-PD's to high-speed OEIC's for use in broad-band optical communication systems is expected.
We have established a method for analyzing the quantum state of the two-dimensional electron gas (2DEG) on the basis of the rigorous quantum theory. For a semiconductor whose bandgap energy is narrow like InAs, the non-parabolicity of band should be considered. For the first time, we have succeeded in taking account of the effect of the non-parabolicity in the 2DEG theory. This enabled us to easily analyze characteristics of strained InAs-HEMT's and then design their IC's. From PL measurements for strained InAs/InGaAs quantum well structures, the effect of strain involved in the epitaxial layers on the energy-band structure was made clear experimentally. In this way, it was possible to pursue an optimum design for strained InAs/InGaAs quantum well structures together with the theory mentioned above.