# Pixel Circuit Designs for Active Matrix Displays
## 1 Introduction  
### Display Technology  
Since the 1950s, display technologies have evolved and active matrix liquid crystal displays (AM-LCDs), active matrix organic light-emitting diode displays (AM-OLEDs), and active matrix light-emitting diode displays (AM-LEDs) have become the dominant technologies for direct-viewing applications such as smartphones, televisions, and monitors [1, 2]. These displays typically have large panel sizes (ranging from 5 inches to 85 inches), glass substrates, and wide viewing angles (160\({}^{\circ}\) to 180\({}^{\circ}\)), as shown in Table 1. AM-LCDs are the most mature technology, which use a backlight unit that transmits light through the LCD panel and converts white light into red, green, and blue light through colour filters [3]. They are known for their low power consumption, high contrast, and low cost [3, 4]. AM-OLEDs utilise organic materials to emit light independently and offer high colour accuracy, flexibility, and higher brightness (2000-7000 cd/m\({}^{2}\)) compared to AM-LCDs [5, 6, 7, 8]. However, their lifetimes are significantly shorter under high brightness or high voltage conditions [9]. AM-LEDs, especially Micro-LEDs, are a promising technology due to their high brightness, high efficiency, and long lifetime. It employs micron-sized inorganic LEDs as light-emitting pixels with excellent brightness (500-1200 cd/m\({}^{2}\)) and long lifetime [10, 11]. Despite their-great potential, AM-LEDs face technical challenges such as complex fabrication processes and driver-integrated circuit design, which hinder their widespread application [12].  
Near-eye projection displays are a key technology for Augmented Reality (AR) and Virtual Reality (VR) applications, with high-resolution density LCDs, liquid crystal on silicon (LCoS), silicon-based OLEDs, and silicon-based LEDs being the main technologies [13]. These micro-displays are characterised by, among other things, small panel sizes (0.3-2.5 inches) and extremely high pixel densities (800-7000 PPI), which are critical for delivering sharp images when viewed at close ranges [14; 15]. High-resolution density LCDs have proven manufacturing processes and low cost for mass production, but their performance is limited by flicker effects, small aperture ratios, and colour mixing effects [16]. LCoS is known for its high-resolution density and fast response time, but its performance is constrained by the fringe field effect and finite voltage swing [17]. Silicon-based OLEDs have the characteristics of autonomous light emission, thinness, and flexibility, but their manufacturing process is complex, leading to yield and cost issues, and their lifetime issues limit their applications [18]. Silicon-based LED technologies have attracted attention for their ability to achieve high performance metrics, including fast optical response, low power consumption, and high contrast [19]. However, they also face unique challenges, such as giant transfer technology, full-colour technology [20], and driver technology. Due to the characteristics and size of micro-display devices, complex driving circuits are required to control the brightness and colour of each pixel, which places extreme demands on circuit design and fabrication. For example, variations in current can cause the central wavelength of micro-LEDs to shift, resulting in colour deviations [21]. Ultra-high resolution OLEDs and micro-LED displays have extremely small pixel pitches (ranging from tens of microns to less than ten microns) [22; 23]. The reduced pixel pitch increases the electrode resistance and contact resistance, leading to preferential current flow through the low-resistance path. This leads to an increase in local current density and triggers current crowding effects, which reduces efficiency and increases leakage current [24]. These issues make designing suitable pixel circuits more challenging but can be mitigated by advanced thin-film transistor (TFT) technology and sound pixel circuit design (e.g., dynamic compensation techniques)