Study OF Ultra-High Speed Heterojunction Bipolar Transistors with a Heavily Carbon-Doped Base

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Heterojunction bipolar transistors (HBTs) have attracted much attention as high-speed devices. In this research, it was strongly proposed that for further improved device performance of HBTs, reduction of base resistance is greatly important. In other words, heavily doped base layr with the well-confined base dopant is one of key structural parameters for realizing high-speed HBTs.From these points, effects of reduced base resistance on static and dynamic characteristics of InGaP/GaAs HBTs with an ultra-high doped base were studied by solving the basic device equations and the hybrid-pi type equivalent circuit model. By increasing the base hole concentration to 1.5x10^<21>cm^<-3>, current gains were estimated to be over 10 by assuming the minority carrier lifetime gamma_n of 10-100ps, and current-gain cutoff frequency f_T of 110 GHz and maximum oscillation frequency f_<max> of 160 GHz were predicted. Switching characteristics were also studied by using a SPICE simulator, and propagatio … More n delay time t_<pd> of l ps/gate was achieved in that device structures.Based on this consideration, heavy impurity-doping technique is developed by using novel crystal growth technique : metalorganic molecular beam epitaxy (MOMBE) . Instead of conventional p-type dopants such as beryllium (Be) and zinc (Zn) for base layr in HBTs, carbon (C) is proposed as a novel base dopant because of excellent stability and capability of heavy doping. Heavily carbon-doped p-type GaAs and InGaAs were developed as base materials by MOMBE with trimethylgallium (TMG) , and carbon-doping characteristics and electrical, optical and structural properties were studied in detail. Furthermore, phosphide materials such as InGaP and InP are also proposed in this research as novel emitter materials of the HBTs because of the tendency of low surface recombination velocity, and MOMBE growth of these compounds is investigated with new metalorganic phosphorus precursor : tertiarybutylphosphine (TBP) .In the growth of high quality materials, the catalysis by means of heated tantalum (Ta) inside the cracking cell was important to effectively decompose the TBP as compared to simple pyrolysis. Controllability of Si-doping in In_<0.5>Ga_<0.5>P suitable for HBT application was also studied by using cracked Si_2H_6. In the growth of InP by MOMBE with TBP,it was revealed that carbon from the TBP is incorporated and acts as well-activated donor.By combining these MOMBE growth techniques, InP/InGaAs HBTs with carbon-doped n-type InP emitter and carbon-doped p-type InGaAs base are proposed and fabricated for the first time in the world. In common-emitter static characteristics, relatively large commonemitter breakdown voltage BV_<CEO>above 6 V was achieved, and small signal current gain h_<fe> of 20 and d.c.current gain h_<FE> of 11 were obtained at collector current density J_C of 0.15kA/cm^2 for a device with emitter area A_E of 80*80mum^2.Furthermore, InGaP/GaAs HBTs having an ultrahigh carbon-doped base with a hole concentration of the order of 10^<21>cm^<-3> are realized for the first time. By using that device structure, improved high-frequency performance is strongly expected because base resistance could be extremely decreased due to the ultra-high doping in the base. In fact, h_<fe> of 16 and h_<FE> of 12 were obtained for devices with a base thickness of 15 nm. In the reliability measurements, a significant degradation of current gain was not observed in the range investigated, indicating a perfect stability of carbon as a p-type dopant. In conclusions, successful realization of HBTs with unique original structures ; Inp/InGaAs HBTs with carbon-doped InP emitter and carbondoped InGaAs base and InGap/GaAs HBTs with an ultra-high carbon-doped base (p=1.5*10^<21>cm^<-3>) , was studied for the first time. Less