Abstract
This paper presents the following recent investigations of
transparent amorphous-oxide semiconductors (TAOS) from materials to devices
and circuits. 1) Composition of metals in TAOS are widely explored with the
aim of seeking semiconductors suitable for the channel layers of thin-film
transistors (TFTs) composing backplanes for flat-panel displays. It is found
in combinatorial approaches to the materials exploration that indium-based
ternary TAOS (In-<i>X</i>-O) and their TFTs show the properties and the performance as good
as those of the most popular material of amorphous In-Ga-Zn-O (<i>a</i>-IGZO) when <i>X</i>=Zn or Ge. 2) Defects and impurities in TAOS are investigated by
theoretical approaches. The first-principle calculation of the electron
states reveals that charge-neutral oxygen vacancy or interstitial forms the
density of states around mid-gap level and does not generate carriers
directly, while hydrogen impurity raises the Fermi level beyond the
conduction-band minimum and acts as a donor in TAOS. 3) Device structures of
TAOS-TFTs are also investigated extensively for better performance and
stability. It is found in channel-etch type TFTs with bottom-gate
inverse-stagger structures that the TFT characteristics and stability are
significantly improved by chemically removing the back-channel layer in a
wet-etching process. Coplanar homojunction (CH) structure is proposed as a
novel device structure where conductive <i>a</i>-IGZO regions work as the source and drain electrodes to the
channel region of semiconductor <i>a</i>-IGZO. The CH TFTs show excellent characteristics and stability,
with low series resistance without any difficulty in making good electrical
contact between metals and TAOS. 4) Circuits using TAOS-TFTs are
demonstrated. A ring oscillator composed of fifteen-stage inverters with a
buffer circuit operates as designed by circuit simulation with a TFT model
for hydrogenated amorphous Si TFTs. Pixel circuits composed of three TFTs
and one transparent capacitor successfully drive organic light-emission
diode cells without a planarization layer on a 2-in diagonal panel having 176x144x3 pixels.
© 2009 IEEE
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