THESIS
2013
xiv, 116 pages : illustrations ; 30 cm
Abstract
GaN transistors enable high-performance RF power amplifiers due to their unique
capability of simultaneously providing large voltage swing and high operation frequency.
Over the past decades, tremendous progress on high-speed GaN transistors has been
witnessed featuring operation frequency at several hundred GHz. Aiming to explore the
next-generation GaN technology to operate at unprecedented speed without penalty of short
channel effects, both novel epilayer structure and advanced transistor architecture need to be
engineered together.
To fabricate transistors featuring high transconductance, low parasitic loss, and excellent
reproducibility in process to facilitate mass production, several approaches have been
employed in this thesis. Compared with the conventional AlGaN/GaN...[
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GaN transistors enable high-performance RF power amplifiers due to their unique
capability of simultaneously providing large voltage swing and high operation frequency.
Over the past decades, tremendous progress on high-speed GaN transistors has been
witnessed featuring operation frequency at several hundred GHz. Aiming to explore the
next-generation GaN technology to operate at unprecedented speed without penalty of short
channel effects, both novel epilayer structure and advanced transistor architecture need to be
engineered together.
To fabricate transistors featuring high transconductance, low parasitic loss, and excellent
reproducibility in process to facilitate mass production, several approaches have been
employed in this thesis. Compared with the conventional AlGaN/GaN heterostructure, the
ultra-thin barrier AlN/GaN transistors grown on Si substrate hold great potential for
employment in the future deeply-scaled technology. In the first stage of this work,
high-performance AlN/GaN metal-oxide-semiconductors high-electron-mobility transistors
(MOSHEMTs) were fabricated using high-k Al
2O
3 utilized as both the gate dielectric and
passivation layer. The gate leakage current was reduced by four orders of magnitude to
7.6×10
-5 mA/mm and the current collapse was dramatically suppressed in comparison with
that of HEMTs counterpart. Regrowth techniques are then incorporated to achieve low contact
resistance and robust self-alignment process. Highly-doped n
+-GaN was selectively regrown as the source/drain to lower the contact resistance and access resistance. As a result, record
transconductance (G
m) of 509 mS/mm has been demonstrated for the Enhancement-mode
(E-mode) GaN HEMTs on Si substrate. However, the RF performance is greatly limited by
the large gate-to-source/drain overlap capacitance in this transistor. To minimize the parasitic
capacitance and enhance the f
T&f
max, gate-last self-aligned technology was finally developed
to separate the gate and source/drain contacts using a SiN
x supporting layer. In the gate-last
devices, the f
T has been improved to be around 40 GHz with a channel length (L
g) of 210 nm.
The gate-last process enables the scaling capability in both lateral and vertical dimensions,
and can also be adjusted to be compatible with the state-of-the-art Si CMOS fabrication
technology.
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