Today, the flat panel display market primarily includes liquid crystal displays (LCD) and light-emitting diode (OLED) displays targeting end products from large information and television screens to tablets, smartphones, and micro-screens for augmented and virtual reality (AR/VR).
LCD screens use an inorganic LED backlight to emit light through an array of liquid crystals to generate images. OLED displays are self-emitting, and use organic compounds to emit light in response to electric current.
The emerging micro LED display technology promises more colors and higher brightness with lower power consumption compared to traditional displays. The first products made an impressive entry into the marketwith televisions and video walls provided by leading display operators, that promise the ultimate visual experience for entertainment, as well as business environments.
Micro LEDs are microscopic versions of traditional LEDs (>1 mm), measuring less than 50 µm. Whereas traditional LED lights are individually packaged, a large number of small LED lights are combined in an exposed mold to create displays. They are organized into sub-pixels, each containing small red, green, and blue LEDs to form a color display. The emitted color is tuned by the bandgap of the inorganic materials used to make the LED – for example, aluminum indium gallium phosphide (AlGaInP) for indium gallium nitride and red (InGaN) for green.
Large standard display opportunity
An array backplane transistor, which switches and drives individual display pixels, controls a small active matrix LED display. Small LED lights are divided into two categories based on the material of the back panel.
One uses silicon-based (Si-based) transistors, made with standard CMOS fabrication process flows. These transistors can be quite small, resulting in high pixel pitch back panels, which are ideal for high-resolution AR/VR applications or projectors. However, silicone back panels are relatively expensive, limited in size, and opaque.
The second category uses thin-film transistors (TFTs), which are made of amorphous Si, low-temperature polycrystalline Si (LTPS), or indium gallium zinc oxide (IGZO). TFTs can be processed on larger Si substrates and have the potential to achieve a lower cost per unit area when processed at larger sizes. This article focuses on small LEDs on TFT substrates.
One application is large modular displays intended for televisions or video walls for the home, cinemas, advertising, or conference and meeting rooms. Depending on the size and number of units, the finished screen can measure upwards of 200 inches. In this application, small LEDs over TFT displays are expected to outperform OLEDs over TFT displays because micro LEDs are more efficient, providing higher illumination at the same drive current. Also, since there are no organic layers, small LED displays do not require encapsulation, facilitating the path for a smooth transition from one unit to the next. With OLEDs, each unit must be encapsulated separately.
Unlike OLEDs, small LEDs intended for larger screens cannot be processed on the same substrate. Therefore, medium and large displays require other manufacturing methods, such as pick and place. In this approach, small LEDs are fabricated using three different baffles (for red, blue and green), cut into cubes, and converted into a TFT-reinforced panel using a high-speed pickup and mode system.
Small LED TFT Backlit Panel Design Considerations
High performance small LED displays present new challenges for backplane design. Various electronic approaches are followed – one starts from an active matrix OLED (AMOLED) design approach, and the other starts from a passive PCB-based design.
Each method comes with advantages and disadvantages in terms of gray level, flicker, pixel pitch, heat dissipation, and power consumption. Imec is leveraging its TFT circuit expertise into alternative pixel circuits that can develop micro LED displays. The following is an example of a new TFT circuit design for modular micro LED displays, developed with display/visualization technology company Barco.
6T2C pixel circuit for driving small LED screens
When developing a backplane circuit for a specific type of display, designers need to choose the optimal matrix architecture (active matrix driving vs passive matrix driving), the way the gray level is set (analog driving vs digital driving), and the LED programming mode (voltage vs. current programming). Imec researchers have compared and evaluated various recent approaches to develop a hybrid method that combines the best of different approaches into a new 6T2C pixel circuit capable of meeting the emerging challenges of small LED displays.
Active vs. Passive Matrix Architectures
Both passive and active matrix displays use horizontal and vertical lines, respectively, to define a line and apply corresponding image data to the columns to drive the pixels in that line. The lines are activated in a high-frequency order, so that the human eye perceives it as a flat frame rather than a linear scan.
In passive matrix driving, used in small LED walls today, pixels are turned off in unmarked lines. It only plays for a short time when the font is selected. In other words, there is only one line emitting light at that time.
In active matrix driving (comparable to AMOLED designs), all pixels have a storage element that allows them to remain active throughout the switching cycle until their values are updated again. Therefore, it also emits light when programming other fonts, reducing the requirements for pixel brightness and current levels.
Active matrix designs offer benefits in energy consumption, cost, and image quality. In passive matrix designs, the pixels are only energized over a short period of time, requiring much higher peak brightness and corresponding current through the LEDs to achieve the same overall brightness. This increases power consumption and the need for heat dissipation. For large modular displays with millions of emitters, an active matrix drive system is preferred over a passive matrix design.
Gray Level: Digital or Analog Driving?
The amount of current flowing through the LED determines the gray level (or brightness) of the individual emitter. The gray level of each individual LED made up of pixels contributes to the overall brightness of the small LED display.
The panel-based designs used for AMOLED displays typically implement analog driving technology: an analog voltage (or current) applied to a pixel results in a current through the OLED, which defines the gray level. In this approach, a higher current level results in higher light emission and therefore brighter pixels. But for small inorganic LEDs, changing the current to change the gray level also affects the wavelength of the emitted light, causing undesirable color change.
As a result, digital driving of a small LED display is preferred. This approach uses pulse width modulation (PWM) to determine the amount of current flowing through the small LED until a constant current level is applied to each LED (without changing color). But the average time an LED is on – or the duty cycle – can be varied to adjust for the average light emission and therefore the gray level of the pixel.
12-bit encoding table
There are multiple coding schemes for PWM programming to implement digital driving. These coding tables contain details of the exact time intervals that the small LEDs are on or off. Our R&D team has proposed a unique 12-bit coding table to achieve minimum dark time and improved optical appearance, severely reducing display flicker.
6T2C pixel circle
Designing a circle of pixels requires making several choices. For example, a designer can choose a conventional two-transistor one capacitor (2T1C) design, where one transistor selects the pixel and another transistor turns current through the LED by adjusting the voltage of the incoming data. However, any differences in transistor properties can lead to current shifts and color shifts, so a voltage-driven approach should be avoided.
Researchers from IMEC and Barco have developed a hybrid approach in which the constant current through the micro-diode is precisely tuned by a so-called current mirror, defined by transistors; PWM data is applied by means of voltage levels, which are enabled by switching transistors. These shunt transistors turn the constant mirror current on or off, following the coding table. The remaining two transistors of the 6T2C circuit define the pixel where the input current is updated.
Using this pixel circuit, the joint R&D team was able to implement the proposed hybrid design approach to drive the small LED display with optimal performance. Variations of the 6T2C pixel circuit have also been proposed to expand its application – eg, contrast to reduce the total area taken up by the pixel circuit, based on the current mirror-sharing concept, and a universal shutter design to improve synchronization with which subunits are updated to a modular display.
Moving towards mass manufacturing of TFT back panels
Innovative pixel circuit design enables small, high-performance, large-space LED displays while reducing display manufacturing costs. The proposed TFT-based active matrix design will reduce manufacturing costs when compared to active matrix Si CMOS or hybrid Si CMOS/TFT enhanced panel technologies because the TFT back panel design uses larger (compared to Si) substrates and increases the potential for mass production of TFTs. A unique foundry model has been invented and created with TFT manufacturers, which is similar to the existing multi-project chip services for Si CMOS technology.
To demonstrate these manufacturing capabilities, the imec and Barco team designed a prototype of the micro LED display with an enhanced LPTS panel, based on the described pixel circuit design, which is subsequently fabricated in a specific TFT foundry. LPTS was preferred over IGZO as a TFT material due to its ability to handle high currents to drive widths.
The hybrid approach to designing small LED TFT backboards combines the best of today’s approaches: a current PWM-driven programmable active matrix design. The new 6T2C pixel circuit facilitates a hybrid design approach to powering small LEDs with optimum optical quality. When manufactured in large volumes, TFT backplanes for driving small LEDs offer a low-cost alternative while maintaining high performance compared to traditional designs based on PCBs or Si CMOS embedded pixel technology.
* Imec offers R&D, prototyping, and manufacturing services such as those described in this article.
The results described in this article were provided by Kris Myny on behalf of imec on 21Street International Meeting on Information Display (IMID 2021).
Get to know our expert
Chris Mini He received his Ph.D. He received his PhD in Electrical Engineering from KU Leuven, Leuven, Belgium, in 2013. He is now a Principal Scientist at IMEC and professor at KU Leuven. Myny specializes in circuit design for flexible thin-film transistor applications. His work has been published in numerous international journals and conferences, including Nature Electronics and several ISSCC contributions. He was listed as one of Belgium’s Technology Pioneers by business newspaper De Tijd and in 2018 was awarded the European Young Researcher Award for his design on thin-film electronics. In 2016, Myny was also awarded a prestigious ERC grant from the European Commission to enable research into thin-film transistor circuits (FLICs). He is now a member of the Belgian Youth Academy (2019-2024), while serving as Head of the Track at IEEE FLEPs and VLSID Conferences and as a member of the New Foundation’s Editorial Board IEEE Journal on Flexible Electronics.