Author:
Yujie Bai, Senior Applications Engineer, Analog Devices
Date
08/26/2025
PDF
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Figure 1: TFT LCD display structure
To enhance the cabin experience, modern vehicles feature multiple types of displays: the instrument cluster, the center information display (CID), heads-up display (HUD), passenger display, smart e-mirror display, side mirror display, and rear entertainment display. The instrument cluster provides the driver with key information such as speed and fuel gauge status. The HUD projects crucial information onto the windshield. The rear seat entertainment displays and passenger displays are part of the infotainment system, allowing passengers to watch movies or engage in other entertainment activities. The digital camera monitor system (CMS) is replacing exterior rearview mirrors with two to three cameras, and the side mirror displays and e-mirror displays enhance the driver’s visual perception of the surroundings.
The TFT LCDs in Figure 1, OLEDs in Figure 2, and micro-LEDs in Figure3 represent three distinct technologies that revolutionize visual display capabilities.
TFT LCDs utilize liquid crystals sandwiched between two glass substrates. The bottom substrate is embedded with TFTs, while the upper substrate serves as a color filter. These liquid crystals align to modulate the rotation of light passing through them by controlling the current flow through the transistors, which causes changes in the electric field. Each pixel with a different color is generated by illuminating the color filter in varying proportions.
In contrast, OLED displays do not require a backlight due to their self-emissive capability. The basic structure of OLEDs consists of an organic light-emitting layer on indium tin oxide (ITO) glass. This organic light-emitting layer is sandwiched between two low work function metal electrodes: the upper cathode and the bottom anode.
When an external voltage is applied to the cathode and anode, the electron transport layer (ETL) and hole transport layer (HTL) inject electrons and holes into the organic light-emitting layer with controlled volume and speed. This process causes the OLEDs to emit light. Red, green, and blue light can be produced by using different chemical materials in the OLEDs. Consequently, OLED displays are thinner, more energy efficient, and offer superior color reproduction and contrast.
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Figure 2: OLED display structure
Micro-LED displays are a recent advancement that use arrays of microscopic LEDs as individual pixels. Typically, the chipsize of micro-LEDs is within 50 µm, making them hardly visible to the human eye. Due to their tiny size and advanced assembly technology, the illumination sources for red, green, and blue light can be integrated into a single pixel point, eliminating the need for color filters and liquid crystals in micro-LED displays.
Each micro-LED in the pixel emits its own light, offering high brightness, deep blacks, and excellent energy efficiency. These technologies represent significant strides in display innovation, each offering unique advantages in terms of structure and performance. Micro-LED displays are suitable for applications ranging from smartphones and televisions to augmented reality, wearables, and automotive displays.
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Figure 3: Micro-LED display structure
As TFT LCDs are a relatively mature technology with outstanding cost advantages, LCDs are currently the dominant flat panel display technology in the automotive industry. However, OLED displays and micro-LED displays are drawing more attention from car manufacturers.
OLED displays offer excellent display effects, low energy consumption,highflexibility,andultra-thinness.Micro-LEDdisplays are emerging as the next-generation display technology, enabling curved display designs with enhanced brightness and contrast, thereby adding flexibility to in-cabin screen designs.
However, OLED displays suffer from image retention, causing pixel degradation after displaying static images for a long time, and their lifespan is shorter than that of LCDs. Micro-LED displays are expensive due to the challenges in commercializing mass production.
The existing TFT LCD displays can be upgraded with a mini-LED (submillimeter light-emitting diode) backlight source and local dimming technology. Mini-LEDs are scaled-down conventional LEDs and serve as a bridge to micro-LEDs. LEDs with dimensions smaller than 200 micrometers are categorized asmini-LEDs, while LEDs under 100 micrometers are categorized as micro-LEDs.
Although mini-LEDs can primarily serve as a backlight source in LCD displays, they improve the thickness and contrast performance of LCD displays, while offering cost-effective solutions.
Pixel Drivers
Various colors are synthesized by mixing the three primary colors (red, green, and blue). The mixture of these three primary colors forms a pixel. Each pixel consists of three sub-pixels, which are managed and combined in one pixel.
In a TFT LCD display, the equivalent circuit of a sub-pixel, which controls the electric field across the liquid crystal, is shown in Figure 4. It comprises 1T2C (one transistor, one liquid crystal capacitor, and one storage capacitor). The gate driver provides a positive voltage, called voltage gate high (VGH), to turn on the TFT, and a negative voltage, called voltage gate low (VGL), to turn off the TFT. The picture information is transmitted to the source driver, which charges the liquid crystal capacitor (CLC). The storage capacitor (CST) acts as a buffer to prevent leakage current from the CLC.
Image retention or flicker in TFT LCDs is caused by parasitic capacitance (CGD) existing between the gate node and drain node of the TFT. When the picture content changes and the TFT turns off from the on state, a voltage drop on the CLC is caused by a capacitive voltage divider between CGD and CLC||CST. To improve panel performance consistency, the common backplane voltage (VCOM) is introduced and tuned to the center of the pixel voltage during the pixel transition time.
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Figure 4: A conventional pixel driver
The topology of popular pixel drivers in micro-LED and OLED displays is similar but more complex than in TFT LCD displays due to the fabrication process and integration of TFT circuits with LEDs on a glass or polyimide substrate. Consequently, LEDs in each pixel are driven individually with their own brightness.
As shown in Figure 5, a simple pixel driver called 2T1C (two transistors and one storage capacitor) sends the analog signal of LED emission is to TFT M1. The threshold voltage (VGS) is then stored in (CST), which is used to drive TFTM2 in the saturation region. The driving TFTM2 maintains the LEDs at a constant current with the positive voltage (VDD) and cathode voltage (VSS). The saturation operation driving method of this 2T1C pixel driver has the advantage of extending the LED’s lifetime compared to the linear region operation of the driving TFT.
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Figure 5: A2T1C pixel driver of OLED or micro-LED
There are, however, disadvantages to the 2T1C pixel driver, which include the Mura problem and threshold voltage shift under electric bias. The Mura problem is the uneven brightness in a display’s uniformity, which is mainly caused by variations in the manufacturing process, such as the density of the TFT layer, uniformity of LED forward voltage and threshold voltage, etc. These effects cause image quality issues. Although the best fabrication process can not overcome the threshold voltage shift, pixel circuits with voltage feedback methods and threshold voltage shift overcompensation methods have been proposed to improve image quality.
The 7T1C driving method is shown in Figure 6.This 7T1C pixel circuit has three operation stages, as shown in Figure 10: initialization, compensation, and emission. The TFT M4 is used for the diode connection ofdriving TFTM3. During compensation, the voltage stored in CST from the source driver maintains the LED emission. The TFTs M1, M6, and M7 are used to prevent the LED from turning on.
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Figure 6: Schematic of a 7T1C pixel driver
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Figure 7: Driving sequence of 7T1C compensation pixels: (a) initialization, (b) compensation, and (c) emission
Currently, the display backplane technology has been developed from hydrogenated amorphous silicon (a-Si:H) TFT to low temperature polycrystalline silicon (LTPS) TFF and low temperature polycrystalline silicon and oxide (LTPO) TFT constitute the next generation backplane technology for consumer electronics. The a-Si:H TFT has a low carrier mobility (1 cm2/Vs), which results in the large size of the backplane and leads to more power consumption. The LTPSTFT has superior carrier mobility (>50 cm2/Vs) so that it is applied in the OLED display. The LTPS TFT usually has a high off-current. However, the LTPO TFT has a low off-current. Thus, the hybrid pixel scheme combining the LTPS and LTPO TFTs is considered for use in OLED/micro-LED display backplanes.
