Around the year 1850, Rudolf Clausius and William Thomson (Kelvin) stated the First Law of Thermodynamics, that total energy is conserved -“Energy can neither be created nor destroyed, energy can only be transferred or changed from one form to another”
Energy conservation has to be among the most researched areas in today’s world. With the need for energy consumption increasing by the day, it is imperative that we figure out strategies, which not only help conserve energy, but also produce it. Automobiles, households and most importantly smart phones, are some areas where such strategies could be implemented. Focusing on Smart Phones and related communication devices like Smart Watches, this article talks about how display technologies are improving to reduce battery consumption.
Battery consumption is one of the most important factors that are looked into while buying a Smartphone. Smart phone displays are getting bigger and better, but so is their capacity to drain the battery life. The battery capacity for smart devices can only be increased to a certain limit, beyond which it can become hazardous for the user.
Apple recently launched its fourth generation Apple watch. An advanced communication system, along with revolutionary health capabilities, including a new accelerometer and a gyroscope are just a few of it's amazing features. What takes the crown is the new electrical heart rate sensor that is useful in taking an Electrocardiogram (ECG) using the new ECG app. In addition to that, the watch also got a 30 percent increase in the display size over the previous generation Apple watch. Besides the flashy specs, Apple brought in the new LTPO-TFT display technology, which is by far the most important upgrade to the watch. The company claims that this technology would help improve the power efficiency of the battery.
Fig 1: Apple Watch Series 4
Why Apple moved to LTPO
Apple uses LTPS-TFT technology in their iPhone x models. A major drawback of this technology is that it reduces the pixel density of the display. The pixel density of the iPhone X is 459 PPI, however it only works on 306 PPI which is lower than the 326 PPI of the Retina LCD screen of the iPhone. The reason for this is the PenTile pixel-aligned OLED screen of the iPhone X. Apple uses the LTPO-TFT technology for restoring the RGB subpixels alignment, so that the display panels give high-resolution image quality. LTPO technology can also improve the battery life for the future Apple smartphones. LTPO-TFT can save up to 5-15% more power when compared to LTPS-TFT, by decreasing the power consumption of the display screen. In addition to power savings, LTPO-TFT panels are easier to make. They also have higher pixel density (greater resolution) and refresh rates (240Hz or higher) than LTPS-TFT.
In the LTPS-TFT (Low Temperature Polysilicon Thin Film Transistor) process, a thin layer of amorphous silicon (~ 50 nm) is deposited on a glass substrate. This is annealed and crystallized using excimer laser of 300nm to convert the a-Si into the Polysilicon layer. The laser is scanned over an amorphous silicon layer, through which an amorphous silicon layer is melted by absorbing the ultraviolet radiations. Once the laser passes, the melted material solidifies and becomes a Polysilicon layer. This whole process occurs at a temperature lower or equal to 600 degree Celsius. LTPS-TFT has regular grain patterns which is better than the amorphous silicon TFT (previously used in electronic devices). Because of these regular grain patterns, the movement of electron is 100 times faster than amorphous silicon which results in high resolution and pixel quality.
Drawbacks of LTPS
The quality of LTPS-TFT decreases due to an increase in temperature when the transistor is on, which degrades the film by breaking the Si-H bonds in the material. This causes the device to suffer from drain breakdown and current leakage, mostly in small and thin transistors.
“IGZO” is an acronym for indium gallium zinc oxide. IGZO-TFT is used as an oxide transistor, as it reduces the need for backlight intensity, and improves the battery life of the display panel. IGZO-TFT also has higher electron mobility than LTPS. Smaller IGZO transistors also help increase the pixel density of display panels.
LTPO-TFT (Low Temperature Polysilicon Oxide Thin Film Transistor) is the combination of two different elements i.e. LTPS and IGZO, which gives high current when the transistor is on, and low power consumption when the transistor is off. LTPS Transistor is used because of its high switching speed and good drive current, whereas Oxide transistor (e.g. IGZO transistors) is used for its low leakage current.
LTPS transistors and Oxide Transistors are formed with different layouts. LTPS transistors have long gate lengths (L) and short gate widths (W) which ensure low ratios of W/L, and also reduce the high mobility of these transistors. This may cause LTPS Transistors to be inefficient for pixel layout. On the other hand, Oxide transistors are constructed with W/L ratios with smaller aspect ratios (4/4 for oxide relative to 3/30 for LTPS). Due to these layout efficiency considerations, it is preferred to use Oxide transistors as a drive transistor in a display pixel. LTPO achieves more efficient electron mobility, better stability, and scalability because of less leakage of the oxide TFT structure than that of LTPS.