Overcoming the Limitations of Thin Film Transistors
The display industry is at the forefront of technology advances that free the display user experience and display manufacturing from the limitations of silicon-based TFTs.
Coupled with advances in organic LED deposition techniques and materials quality, Amorphyx’s TFT technologies lead a period of rapid change in users’ primary interface with technology - the display.
Flat-panel displays were invented at RCA Laboratories in New Jersey, USA in 1964. From simple seven-segment characters on watches and handheld calculators, flat-panel displays evolved into televisions as SHARP introduced the first LCD TV in 1988. While the OLED display was invented by Eastman Kodak in the US in 1987, the first commercial OLED display was produced by Japan’s Pioneer for automotive applications in 1997.
As the charts below show (from Display Supply Chain Consultants’ reports), display manufacturing has approached a crossroads. Historically LCD TV panels have dominated both display revenue share and area of processed glass (equivalent to wafers in semiconductors).
But the advent of the touch screen-centric mobile device has caused a shift away from LCD TV’s dominance and towards mobile devices. As mobile devices place an emphasis on both battery life and brightness in outdoor as well as indoor use, they drive fundamental change in the materials and technologies used to manufacture displays.
The transition to mobile phone-appropriate technologies has begun to expand from these smaller form factor displays to mid-sized (laptop, desktop monitor) and large-area (TV) displays - mostly with great difficulty overcoming the challenges of scaling new materials and TFTs from small-area display manufacturing.
Below, we examine the key drivers and materials demanding a new generation of high-performance thin film transistors to deliver their value to users.
Source for charts: DSCC Quarterly Flat Panel Forecast Report
Flat-panel displays began as liquid crystal (LCD). A TFT creates an electric field that controls the orientation of crystals suspended in a viscous liquid much like window shutters. A bright white light shines through the crystal shutters and is filtered by red, green or blue light filters to produce the image.
Organic light emitting diode (OLED) displays operate differently than LCDs. In an OLED display, the TFT creates an electric current that energizes the red, green or blue OLED material. The OLED pixel functions much like a dimmable light bulb - the amount of TFT current defines the brightness of each color pixel, producing the image.
The transition from liquid crystal displays (LCD) to light emitter-based displays incorporating organic light emitting diodes (OLED) is the defining event for elevating display performance to new levels of image quality and color richness with reduced power consumption.
But OLED comes with a new technical challenge. LCD pixel brightness is set using an electric field defined by a voltage created by the TFT. OLED pixel brightness, on the other hand, is defined by the amount of electric current created by a TFT to drive the OLED.
TFTs make voltages easily, suffering little degradation from operating stress over time. Making enough current to drive sufficient OLED brightness is a much more difficult task for the TFT. TFT current tends to vary slightly under operating stress; that variation translates to variations in pixel brightness. The larger the pixel dimensions, the larger the OLED material droplet, the more current the TFT has to produce, operating under increasing levels of stress. This is why OLED smartphones are achievable today, while OLED laptop displays and TVs are not.
(Side point: Many TV manufacturers sell “OLED TVs” today. None of them are actual RGB OLED pixel arrays, like those show in the image above. Most “OLED TVs” are actually white OLED backlights shining through red, green and blue color filters. The TFTs control the white OLED brightness. So what is called an “OLED TV” today is closer to LCD in implementation than to RGB OLED.)
For watching most types of images, 60Hz image refresh rate is satisfactory. But for dynamic images - say gaming, or sporting events - the picture of a racing car at different refresh rates points to a challenge. Fast-moving images tend to require more frequent pixel updates to produce a crisp image. (Next time you watch a soccer match on TV, watch the ball closely. When it moves quickly, the image of the ball is distorted.)
OK, simple enough problem to solve; increase the image refresh rate. This is one of the key benefits of the transition from LCD to OLED - LCD’s viscosity means it’s too slow to support increased image refresh rate. OLED, on the other hand, is more than responsive enough to changes in drive current to operate at high image refresh rate levels - 240Hz and beyond.
The image on any display is actually a series of rows of red, green and blue pixels (at left). Each row is updated sequentially once every 16.7 milliseconds (60 times per second, or 60 Hertz). At that refresh rate or higher, the human brain sees a uniform image - not a series of pixel rows changing. This is how a display presents motion.
The Amorphyx IGZO Amorphous Metal TFT (AMeTFT) technology is ideal for the demands of high image refresh rate displays. AMeTFT (green trace) provides the same amount of current to the OLED as does the only TFT technology capable of driving OLED today - the low temperature polysilicon (LTPS) TFT.
[The chart plots output current (drain-source current) against control voltage (gate-source voltage) for the AMeTFT and the latest production version of SHARP’s LTPS n-type and p-type devices.]
The transition to compound semiconductors in high-speed IC manufacturing ran into issues with speed and current/size tradeoff similar to those the display industry faces today with IGZO. While many attempted more complex semiconductors - adding additional metals to GaAs and InP and incorporating heterojunction semiconductor structures - the path forward for performance and manufacturability was found not in material science, but in device physics. Transitioning to new device physics opened up the path to fully realizing the performance potential of compound semiconductors. From mobile phone and WiFi networks to fiber optic communications, compound semiconductors paired with new device physics resulted in transistors that redefined communications physical layers.
This is how Amorphyx’s IGZO AMeTFT technology creates the critical link between OLED and high-performance, low power consumption, simple-to-manufacture displays. By applying high gate electric field energy to the IGZO semiconductor, AMeTFT transitions amorphous metal oxide FETs from bulk conduction to bulk accumulation, producing LTPS TFT-level drain-source current and LTPS TFT-level switching speed with IGZO’s orders-of-magnitude lower leakage current (see chart above). The result: a display backplane capable of supporting small-to-large-area OLED display image refresh rates in excess of 240Hz - ideal for the new range of applications demanding only the best in image quality and power consumption.
Increasing image refresh rate is the domain of the thin film transistor. The migration to OLED increased the electrical current demands on the TFT; larger TFTs generally produce more current than smaller TFTs. The silicon-based TFT that was the workhorse of the early years of flat-panel LCDs could not meet these demands. While laser annealing the silicon - one pixel at a time, with millions of pixels in a standard OLED smartphone display - increased both current and switching speed, it also dramatically increased the complexity of the TFT manufacturing process. Silicon-based semiconductors were insufficient for the transition to OLED.
The transition from silicon to compound semiconductors (gallium arsenide GaAs, indium phosphide InP, etc.) defined high-speed integrated circuit manufacturing in the 1990s and 2000s. Similarly, the display industry chose to pursue compound semiconductors - amorphous metal oxide semiconductors - for the transition to high image refresh rate OLED displays. Indium gallium zinc oxide (IGZO) has become the metal oxide semiconductor of choice for its combination of mobility, stability under stress, and low leakage current. Japan’s SHARP announced its first version of IGZO TFT backplane displays in 2014. Since then, IGZO TFTs have made a modest incursion into the premium segment of OLED smartphone displays. Limitations in switching speed and output current for a given TFT size have held back IGZO from broad market application.
