Analog to digital convertor studies

Call number: LD2668 .T4 EECE 1988 H45Master of ScienceElectrical and Computer Engineerin

Self-aligned gallium arsenide heterojunction bipolar transistor using refractory metallisation

Improvements in epitaxial growth and processing technologies have revived a great deal of interest in the heterojunction bipolar transistor (HBT). In this project, AIGaAs/GaAS HBTs have been fabricated using a new self-aligned process which exploits the characteristics of some refractory metals deposited by sputtering to obtain a T-shaped contact structure for the emitter. wet and dry etching techniques were used to fabricate the T-shaped contact. A refractory metallisation system consisting of sequentially sputtered layers of Ge/Mo/Ni was investigated for contacting the emitter of the transistor. After alloying in a thermal furnace at 750°C for 30 minutes in a nitrogen atmOSPhere, a low specific contact resistance of 2 x 10-6 ohm-cm was measured by standard transmission line model (TLM) for measurement of contact resistance. A metallisation system consisting of sequentially evaporated AU/Zn/Au was used for the base and Ni/AuGe/Ni/AU was used for the collector. Alloying with the same condition1 as above gave specific contact re1istances of 1.2 x 10-6 ohm-cm for the base and 8.6 x 10 -6 ohm-cm for the collector. AS an alternative to ion implantation, zinc diffusion was used as an alternative technique to dope the base contact region. The acceptor concentration profile of the diffused region was studied by 'Hall and Stripe' technique and a surface concentration of 1 x 10 20 cm -3 was measured. This highly doped base contact region can be used to achieve low ohmic contact to the base. Results show that for devices designed with similar dimensions for both processes, the new self-aligned process shows a net improvement in the frequency response of the devices (ft= 10.7GHZ and &nax=9.8GHZ for self-aligned and ft=8.0GHZ and for conventional 8um HBT)