The performance of AlGaAs/GaAs heterojunction bipolar transistor (HBT) are improved by using carbon-doped base and selective metalorganic chemical vapor deposition (S-MOCVD).
The characteristics of carbon tetrachloride ($CCl_4$)-doped GaAs and AlGaAs layers grown by atmospheric-pressure MOCVD are investigated by varying the growth parameters. The hole concentrations as high as $7\times10^{19}cm^{-3}$ are obtained and the hole mobility of the carbon-doped GaAs is higher than that obtained from zinc-doped GaAs at high hole concentrations. The dependence of GaAs lattice contraction on the hole concentration is determined from X-ray diffraction at high doping levels ($1\sim7\times10^{19}cm^{-3}$). For carbon-doped base HBT, it is found that the hole concentration of base layer can be nondestructively measured by using double crystal X-ray diffractometer from the relation between the hole concentration and the strain. AuGe/Ni/Au metallization to carbon-doped GaAs is performed in comparison with zinc- and beryllium-doped GaAs and good ohmic contact with specific contact resistance of $3\times10^{-6}Ω㎠$ is achieved. Carbon-doped base AlGaAs/GaAs HBT is fabricated and compared with zinc-doped base device. No carbon diffusion at the emitter-base junction is observed in secondary-ion mass spectroscopy (SIMS) profiles. DC characteristics of the carbon-doped base HBT are better than the zinc-doped base device. For the carbon-doped base HBT the common emitter current gain of 30 and the base current ideality factor of 1.4 are obtained. The current gain cutoff frequency $f_T$ of 27GHz and the maximum oscillation frequency $f_{\max}$ of 16GHz are achieved at the emitter size of 5Ⅹ50μ㎡.
Selectively grown AlGaAs/GaAs HBT with a reduced base-collector capacitance is proposed and fabricated by using S-MOCVD. The proposed HBT features a triangular void over $SiO_2$ stripe. The extrinsic base-collector capacitance is significantly reduced due to the isolation of the extrinsic collector region from the collector metal electrode by the void. The reduction of the extrinsic base-collector capacitance is confirmed by capacitance-voltage measurement. The DC current-voltage characteristics of the selectively grown HBT are similar to those of the conventional one. However, the microwave performance is improved due to the reduced base-collector capacitance in the selectively grown HBT. It is demonstrated that improved microwave performance is caused by greatly reduced extrinsic base-collector capacitance from extracted parameters of small signal equivalent circuit. For two types of HBT with two 3Ⅹ10μ㎡ emitter fingers, the selectively grown HBT has the $f_{max}$ which is 1.4 times higher than that of the conventional device, and the $f_T$ of 43GHz and the $f_{max}$ of 20.5GHz are achieved.
Finally, a new BiFET structure co-integrating of selectively grown HBT and GaAs junction-gated floated electron channel field effect transistor (J-FECFET) using S-MOCVD is proposed and fabricated for implementation to a variety of monolithic microwave integrated circuits (MMIC). Fabrication procedure of proposed BiFET is quite simple and has a single growth step. Inter-device isolation is achieved by $SiO_2$ stripes and AuGe/Ni/Au metallization is used for both n- and p-type ohmic contact. In the fabricated BiFET structure, J-FECFET does not operates because of a gate leakage but it can be solved by adjusting the mask layout.
