| Summary: | InP based InAlAs/InGaAs pseudomorphic High Electron Mobility Transistors
(pHEMTs) have shown outstanding performances, which makes them prominent in high
frequency mm-wave and submillimeter-wave applications. However, conventional
InGaAs/InAlAs pHEMTs have major drawbacks, i.e., very low breakdown voltage and high
gate leakage current. These disadvantages degrade device performance, especially in
Monolithic Microwave Integrated Circuit (MMIC) low noise amplifiers (LNAs). The
optimisation of InAlAs/InGaAs epilayer structures through advanced bandgap engineering
together with gate length reduction from 1 m into deep sub-μm regime is the key solution
to enabled high breakdown and ultra-high speed, low noise pHEMT devices to be fabricated.
Concurrently, device modelling plays a vital role in the design and analysis of pHEMT
device and circuit performance. Physical modeling becomes essential to fully characterise
and understand the underlying physical phenomenon of the device, while empirical
modelling is significant in circuit design and predicts device’s characteristic performance.
In this research, the main objectives to accurately model the DC and RF
characteristics of the two-dimensional (2D) physical modelling for sub-μm gate length for
strained channel InAlAs/InGaAs/InP pHEMT has been accomplished and developed in
ATLAS Silvaco. All modelled devices were optimised and validated by experimental
devices which were fabricated at the University of Manchester; the sub-micrometer devices
were developed with T-gate using I-line optical lithography. The underlying device physics
insight are gained, i.e, the effects of changes to the device’s physical structure, theoretical
concepts and its general operation, hence a reliable pHEMT model is obtained. The kink
anomalies in I-V characteristics was reproduced and the 2D simulation results demonstrate
an outstanding agreement with measured DC and RF characteristics.
The aims to develop linear and nonlinear models for sub-μm transistors and their
implementation in MMIC LNA design is achieved with the 0.25 m
In0.7Ga0.3As/In0.52Al0.48As/InP pHEMT. An accurate technique for the extraction of empirical
models for the fabricated active devices has been developed and optimised using Advance
Design System (ADS) software which demonstrate excellent agreement between
experimental and modelled DC and RF data. A precise models for MMIC passive devices
have also been obtained and incorporated in the proposed design for a single and double
stage MMIC LNAs in C- and X-band frequency. The single stage LNA is designed to
achieve maximum gain ranging from 9 to 13 dB over the band of operation while the gain is
increased between 20 dB and 26 dB for the double stage LNA designs. A noise figure of
less than 1.2 dB and 2 dB is expected respectively, for the C- and X-band LNA designed
while retaining stability across the entire frequency bands.
Although the RF performance of pHEMT is being vigorously pushed towards
terahertz region, novel devices such as Resonant Tunnelling Diode (RTD) are needed to
support future ultra-high speed, high frequency applications especially when it comes to
THz frequencies. Hence, the study of physical modelling is extended to quantum modelling
of an advanced In0.8Ga0.2As/AlAs RTD device to effectively model both large size and
submicron RTD using Silvaco’s ATLAS software to reproduce the peak current density,
peak-to-valley-current ratio (PVCR), and negative differential resistance (NDR) voltage
range. The simple one-dimensional physical modelling for the RTD devices is optimised to
achieve an excellent match with the fabricated RTD devices with variations in the spacer
thickness, barrier thickness, quantum well thickness and doping concentration.
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