InP based 77 GHz monolithic millimetre wave integrated circuits
The aim of this work was to design, fabricate and characterize InP high electron
mobility transistor (HEMT) based monolithic millimetre wave integrated circuits
(MMIC) which operate at 770Hz. To achieve this active and passive circuit
elements were designed, fabricated and characterized and accurate equivalent
circuit models extracted. All circuits were designed with coplanar waveguide
(CPW) as the transmission medium. Electron beam lithography was used for most
fabrication processes in this work.
A range of passive elements such as CPW discontinuities, series and parallel MIM
and interdigital capacitors of different sizes and NiCr resistors were designed
fabricated and measured. Equivalent circuit models of these elements were
extracted which were shown to be valid to 110 OHz.
Passive circuits such as branch-line coupler, rat-race coupler, Lange coupler and
Wilkinson divider were successfully demonstrated at W-band frequencies. In all
cases the circuits have equal power splitting characteristics with low insertion
losses and very good input and output match over large bandwidth. Equivalent
circuits of these circuits were extracted and were used in design of MMICs.
Active devices were fabricated on a lattice matched InAIAs/InOaAs InP HEMT
material structure. Two different 0.12 f..UD T -gate processes were used to make
these devices with a UVIIIIPMMA based process giving superior high frequency
performance when compared to a conventional CopolymerlPMMA based T -gate
structure. The end to end gate resistance of UVIIIIPMMA T -gate was comparable
to the lowest 0.1 J..lm gate resistance ever reported. The HEMTs fabricated in this
work have shown fT as high as 1930Hz and MAO of 13 dB at 940Hz.
Equivalent circuit models of these HEMTs were extracted and were valid up to
These passive and active circuit models were used to design MMICs, in particular
reactively matched single ended, balanced and balanced switching amplifiers at 77
OHz. Direct carrier modulators including BPSK, bi-phase amplitude modulation,
QPSK and QAM were designed, fabricated and measured at 770Hz. These
modulators are designed with reflection type topology to perform the different
modulation schemes. The Balanced BPSK modulator circuit was used to
' .. ~ .
demonstrate switching operation· with ON-OFF isolation better than 25 dB. The
two ON states showed 180±5° phase difference which is almost ideal for BPSK
modulation. For all states, the input and output reflections were measured to be
better than -17 dB at the design frequency. In the case of QAM and QPSK
modulation, the circuits showed non-ideal performance with high insertion loss and
phase errors but the input and out put reflections were better than -10 dB.