THESIS
2021
1 online resource (xv, 82 pages) : illustrations (some color)
Abstract
Thanks to GaN's high breakdown electric field, high electron mobility and saturation
velocity, GaN-based devices are considered promising candidates for power switching and RF
power amplifier applications. Among them, the highly conductive two-dimensional electron-gas
(2DEG) at the (In,Al,Ga)N/GaN heterostructure enables the power-efficient operation of
high-electron-mobility transistors (HEMTs). However, the inherent channel makes GaN
HEMTs normally-ON, while the Normally-OFF operation of GaN HEMTs is preferred for
various applications. Recess processes for fabricating normally-OFF devices generally
compromise devices’ performance. In this thesis, technologies for realizing recess-free
normally-OFF GaN HEMTs are developed.
Due to the piezoelectric nature of GaN, strain engineering can...[
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Thanks to GaN's high breakdown electric field, high electron mobility and saturation
velocity, GaN-based devices are considered promising candidates for power switching and RF
power amplifier applications. Among them, the highly conductive two-dimensional electron-gas
(2DEG) at the (In,Al,Ga)N/GaN heterostructure enables the power-efficient operation of
high-electron-mobility transistors (HEMTs). However, the inherent channel makes GaN
HEMTs normally-ON, while the Normally-OFF operation of GaN HEMTs is preferred for
various applications. Recess processes for fabricating normally-OFF devices generally
compromise devices’ performance. In this thesis, technologies for realizing recess-free
normally-OFF GaN HEMTs are developed.
Due to the piezoelectric nature of GaN, strain engineering can modulate the electric
performance of GaN HEMTs. The local strain engineering based on the SiN
x stress liner is
developed. The SiN
x-introduced compression in the gate region increases the threshold voltage
(V
TH) and reduces the Schottky gate leakage. As a result, the I
ON/I
OFF ratio of the Schottky gate
HEMTs with strain engineering reaches 10
8. Furthermore, the normally-OFF GaN HEMT using
strain engineering is realized, showing the potential application on class-AB or class-B RF
power amplifier applications.
Schottky gate is inappropriate for normally-OFF GaN HEMTs due to its severe leakage
under forward bias. Therefore, as the second part of the thesis, the ozone-based deposition of
HfO
2 for viable MIS gate structure is proposed. Replacing the conventional water oxidant with
ozone reduces the oxide bulk's oxygen vacancy-related defects, resulting in smaller oxide
leakage and higher critical electric field. Furthermore, the more abrupt HfO
2/GaN interface and
amorphous HfO
2 bulk form on the ozone pre-oxidized GaN surface. The improved interface
translates to the satisfactory electric performance of MIS HEMTs. The high-k nature is also
evidently helpful to develop the normally-OFF HEMTs, supported by an analytical model for
V
TH of MIS HEMTs.
The proposed HfO
2 gate oxide scheme is also integrated with an ultra-thin barrier
heterostructure. With the optimized annealing condition, the normally-OFF operation of MIS
HEMTs is realized. Also, the electron mobility in the gate region is enhanced after annealing.
The high electron mobility in the gate region together with the successful 2DEG recovery by
the SiN
x passivation enable the excellent output performance, benchmarked against recess-based
devices.
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