What Is Active Matrix Addressing?

Active matrix addressing is a method for driving flat-panel displays in which each picture element (pixel) is controlled by a dedicated switching transistor and a storage capacitor fabricated directly on the display substrate. The approach contrasts with passive matrix addressing, where pixels are defined by the intersection of row and column conductors without per-pixel switching devices. In an active matrix arrangement, the transistor at each pixel holds the applied data voltage on the storage capacitor for the duration of a full frame, rather than relying on the pixel to retain its state only while its row line is being scanned. This charge-holding mechanism eliminates the cross-talk and limited contrast that constrain passive matrix displays.

Active matrix addressing was first demonstrated with thin-film transistors (TFTs) at Westinghouse Electric Corporation in 1972 by T. Peter Brody and colleagues. It became the dominant technology for liquid crystal display (LCD) panels during the 1990s and subsequently underpins organic LED (OLED) and other flat-panel display technologies.

TFT Pixel Architecture

Each pixel in an active matrix display contains a TFT connected in series between the column data line and the pixel electrode, along with a storage capacitor connected between the pixel electrode and a common electrode or a dedicated capacitor line. The TFT gate connects to the row (gate) line. When the gate line goes high, the transistor turns on and transfers the data voltage from the column line to the storage capacitor. When the gate line returns to low, the transistor turns off and the capacitor holds the charge, maintaining the pixel voltage until the next frame. Amorphous silicon (a-Si), low-temperature polysilicon (LTPS), and indium gallium zinc oxide (IGZO) are the principal TFT semiconductor materials, each offering different electron mobility, uniformity, and process temperature trade-offs. The pixel circuit design space is examined in detail in MDPI's review of pixel circuit designs for active matrix displays.

Row-Column Scanning and Timing

In a full active matrix panel, gate driver circuits scan through the row lines sequentially, activating one row at a time. While a given row is selected, the column (source) driver circuits apply the target gray-level voltage to every column simultaneously. The time allocated to each row is the row select period, and it equals the frame period divided by the number of rows. For a 1080-line panel refreshing at 60 Hz, each row is addressed for approximately 15 microseconds. Gate and source drivers are typically integrated as peripheral circuits on the glass substrate using the same TFT process. Controlling the timing precision and minimizing feedthrough voltage at the gate-to-drain overlap of the TFT are key challenges in driver design, as described in the Springer chapter on active matrix LCD addressing techniques.

Charge Storage and Pixel Retention

The storage capacitor is the key element that distinguishes active matrix addressing from passive schemes. Its capacitance must be large enough to retain the written voltage over one full frame period despite leakage through the off-state TFT and through the liquid crystal or emissive material. TFT off-current, which must be minimized, is the dominant leakage path; LTPS and IGZO transistors achieve lower off-currents than a-Si TFTs, allowing the storage capacitor to be made smaller. The achievable contrast ratio, gray-scale accuracy, and uniformity of a display depend directly on how well pixel voltage is held between successive row selections. IEEE Xplore's overview of active matrix display technology documents the evolution of charge storage approaches across display generations.

Applications

Active matrix addressing has applications across a wide range of display and imaging technologies, including:

• Liquid crystal displays in laptop computers, desktop monitors, and television panels
• OLED and AMOLED displays in smartphones and wearable devices requiring per-pixel current control
• Medical flat-panel X-ray detectors, where each detector element is switched by a TFT to read out charge
• Electronic paper and e-ink displays that use TFT backplanes to address bistable electrophoretic elements
• Microdisplay panels in augmented reality and virtual reality headsets