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).

MPhil thesis. Collaborating institution: GEC Research Ltd.