“Research progress on p-type amorphous semiconductors has been notably sluggish,” according to the Pohang University of Science and Technology (PosTech). “Despite the widespread adoption of n-type amorphous oxide semiconductors, particularly those based on IGZO [indium gallium zinc oxide] in OLED displays and memory devices, the advancement of p-type oxide materials has been impeded by numerous inherent defects. This setback has hindered the development of CMOS.”
The team discovered that the charge of tellurium oxide increases in oxygen-deficient structures due to the creation of an acceptor level capable of accommodating electrons, enabling the material to function as a p-type semiconductor.
From this observation, transistors were designed using partially-oxidised tellurium thin-films modified with selenium – a ‘sub-oxide’ of the form Se0.25TeO1.44 – normally the oxides are SeO2 and TeO2
Results include hole mobility of 15cm2/V/s and an on-off current ratio of 106-107.
Why add selenium?
“Selenium can increase on-current and decrease-off current,” lead researcher Professor Yong-Young Noh (pictured) told Electronics Weekly. “Without selenium, TeOx showed a low mobility of 1-2cm2/V/s. After selenium was alloyed to the tellurium, a hole conduction channel is generated.”
The transistors are bottom-gate types, with a silicon dioxide gate insulator and a silicon gate. Source and drain contacts were nickel.
“These achievements nearly match the performance levels of conventional n-type oxide semiconductors such as IGZOs,” said PosTech, which went on to describe the transistors as having “exceptional stability under varying external conditions including fluctuations in voltage, current, air and humidity. Notably, uniform performance across all TFT components was observed when fabricated on wafers, affirming their suitability for reliable semiconductor devices applicable in industrial settings”.
Applications are foreseen in displays for OLED TVs, virtual reality and augmented reality devices, as well as CMOS and DRAM research – Samsung Display was among the project’s supporters.
Pohang University of Science and Technology worked with the Korea Research Institute of Standards and Science, and the Pohang Accelerator Laboratory.
The work is published in Nature as ‘Selenium alloyed tellurium oxide for amorphous p-channel transistors‘.