AMORPHYX APPROACH INCREASES MOBILITY
The Amorphyx approach - operating amorphous metal oxide semiconductors in bulk accumulation mode - increases like for like semiconductor performance by a factor of up to 6 times over typical bulk conduction mode operation, and our research is continuing. Here we explain why.
For nearly 20 years, the largest thin film electronics market in the world - the display industry - has focused on increasing mobility while decreasing transistor size - the same pursuit defined in the VLSI industry as “Moore’s Law”. In thin film electronics, efforts to advance Moore’s Law led the industry to first re-crystallizing sputtered amorphous silicon (so-called “low temperature polysilicon”) and then to compound metal oxide semiconductors (primarily Indium Gallium Zinc Oxide, or IGZO). This evolutionary path is similar to that in high performance analog ICs, where silicon gave way to doped silicon (silicon germanium) as well as compound semiconductors (gallium arsenide, indium phosphide).
While doped silicon and compound semiconductor proved successful paths for advancing transistor speed and physical size, LTPS and IGZO have returned limited benefits for over 20 years of research, development and commercial use. While most smartphone displays today are based on LTPS with a migration path to IGZO for cost reduction, nearly all LCD and OLED TV panels continue shipping amorphous silicon backplanes as the result of the costs of large-area LTPS fabrication and the materials-based issues with scaling IGZO TFT fabrication to large-area deposition.
To date, amorphous metal oxide semiconductor TFTs have been operated in bulk conduction mode (as demonstrated in the figure to the upper right.) The gate electric field - defined by the 150-200nm low dielectric constant silicon dioxide gate insulator - is sufficient to vary carrier concentration at the insulator-IGZO interface (A-A’), but has no effect on the semiconductor for the majority of its thickness to the back channel (B-B’). By underutilizing the IGZO’s total carrier content, the typical IGZO TFT wastes the carriers available for generating drain-source current - and therefore limits field effect mobility.
All TFTs operate on a principle similar to any MOSFET (metal oxide semiconductor field effect transistor): an electric field created by applying a voltage across the capacitance formed by the gate metal, the gate insulator and the semiconductor metal creates a conduction region in the semiconductor-oxide interface (commonly referred to as “the channel”) and a depletion region which acts as an insulating region within the semiconductor as it donates charge carriers to the channel.
The strength of the electric field in the semiconductor defines both the widths of the conducting channel and the depletion region; larger electric fields widen the channel, increasing transconductance and mobility for a given size TFT.
Hsu, C.C., Chen, H.P., Ting, W.C., “Correlation Between Carrier Concentration Distribution, I–V and C–V Characteristics of a-InGaZnO TFTs”, National Yunlin University of Science and Technology; IEEE Journal of Display Technology, Vol. 12, No. 4, April 2016. DOI: 10.1109/JDT.2015.2480871
Instead of focusing on increasing intrinsic semiconductor mobility as the path to Moore’s Law advances in TFT performance, Amorphyx chose to focus on increasing transconductance - the property defining the amount of channel conduction in response to the applied electric field - through increasing the gate electric field strength to achieve bulk accumulation mode.
The only path to increasing gate electric field strength is through increasing gate capacitance. Amorphyx dramatically increases gate capacitance by
• increasing the gate insulator dielectric constant through replacing silicon nitride and silicon dioxide with aluminum oxide; and
• reducing insulator thickness, which can only be supported through the use of an ultra-smooth amorphous gate metal..
In addition to increasing insulator dielectric constant, aluminum oxide does not interact with oxygen. This dramatically reduces leakage current into the gate, with the benefit of retaining that current in the conduction channel.
While searching for the next improvement to metal oxide semiconductor mobility continues to frustrate the display industry, Amorphyx has created a simple alternative: rethink the TFT though the use of more effective gate materials.
Removing the semiconductor performance impediment in TFT evolution, Amorphyx’s AMeTFT creates the shortest path to Premium TV image quality.