A 5Gb/s GaAs MSM/PHEMT Monolithic Optoelectronic Integrated Circuit (OEIC) Receiver Front End

Monolithic Optoelectronic Integrated Circuits (OEIC) partially or fully integrate the optoelectronic devices, such as lasers or photodetectors, and the electronic devices or circtuits, such as drivers, preamplifiers, limiting amplifiers, clock and data recovery circuits and so on, onto a single substrate to minimize the adverse influences on the performances of the integrated unit due to the interconnections between devices, which will also benefit the reliability of the integrated unit and the realization of miniaturization and light-weighting, and so, OEIC will be very widely used in the fields of long haul and large capacity optical fiber communication, free space optical communication, optical access network, large scale parallel optical interconnection, optical switch, optical storage, satellite-borne optoelectronic system, microminiaturized optoelectronic sensor, and so on. Considerable interests has been focused on monolithic OEIC some developed countries such as the United States of America, Germany, Japan, and so on, and the outcomes are very advanced, some of which have already been commercialized.Due to the potential applications of monolithic OEIC and the very large gap between the researches in our homeland and those in the countries mentioned above, three projects about monolithic OEEC optical receiver front end have been carried out in our laboratory. Finally, we get some innovative results, which set a new record in our country and lay good foundations for future researches aiming at higher performances and practical uses. The main achievements and relative contents are listed below:1) The first 5Gb/s monolithic optical receiver front end chip in our country has been developed, which consists of a Metal-Semiconductor-Metal (MSM) photodetector and a distributed preamplifier based on Pseudomorphic High Electron Mobility Transistors (PHEMT) on a GaAs substrate. The material epitaxy and device fabrication are fully based on domestic technologies.2) As the key problem to monolithic OIEC device fabrication, the compatibility between mesa process and GaAs PHEMT MMIC process has been resolved, which consists of the formation of MSM photodetector mesa with precise height and smooth working face by Reactive Ion Etching, interdigital electrodes metallization and lifting off, and the interconnections between mesa and planar devices.3) The first distributed preamplifier with 20GHz bandwidth based on 0.5μm GaAs PHEMT technology in our country has been demonstrated. The ratio of bandwidth to PHEMT characteristic frequency reaches about 2/3, and the minimal noise figure and the average equivalent input noise current density keep as low as 3.03dB and 14.6pA/(Hz)^1/2, respectively. The output 1dB power compress point is 13.7dBm, and the preamplifier has a good functional mode under the input signal of 10Gb/s NRZ pseudo-random bit sequences.4) As the key technologies to 0.5μm T gate GaAs PHEMT, ohmic contact and Schottky barrier processes are emphasized, the fabricated PHEMT die has a gain of 12 4dB in the range of 50MHz～26.5GHz, with the characteristic frequency and the max oscillation frequency of 32GHz and over 80GHz, respectively.5) The noise performance of the PHEMT device is investigated in detail. The dependences of minimum noise figure F_min, equivalent noise conductance g_n and optimum input impedance Z_opt on the intrinsic and parasitic parameters of PHEMT have been discussed, and the low noise working point is also determined, which is used for the design of distributed preamplifier. The final measured results show that the noise analyses mentioned before are valid.6) The thermionic emission model of MSM photodetector are studied in detail, and the fundamental working mode of GaAs MSM photodetector are explained, resulting to three important conclusions: a) the dark current is determined by the hole barrier at the metal-semiconductor interface and bias. b) The working speed of MSM photodetector is strongly influenced by bias, and the normal bias should be in the range of flat-band voltage and breakdown voltage. c) The fact that the flat-band voltage is determined by the dopant concentration and interdigital electrode space is important to practical application.7) The on-wafer measured results of MSM photodetector are analyzed in detail, resulting to the conclusion that the dark current mechanism at low electrical field is ohmic conductance, while at high field, the dark current comes from the Fowler-Nordhem tunneling of traped electrons in SiN dielectric film. Based on this result, the somewhat larger noise current in monolithic OEIC front end can be explained roughly, which is important to the optimization of relative process for a better noise performance.