A stable pixel circuit with constant current compensation for threshold voltage, mobility, and subthreshold swing variations

In this paper, we propose a novel active matrix organic light-emitting diode (AMOLED) pixel circuit using amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). The proposed pixel circuit, which comprises seven TFTs and two capacitors, is capable of comprehensively and simultaneously compensating for threshold voltage ( $${V_\textrm{TH}}$$ ), mobility ( $$\mu _n$$ ), and subthreshold swing (SS) variations. This circuit mitigates such variations by storing additional compensation voltage in the capacitor using a constant current that depends on a previously stored voltage. The compensation process is validated through the derivation of analytical equations. Additionally, the operation of the proposed circuit is demonstrated through HSPICE. The simulation results demonstrate that the proposed circuit can maintain accurate and stable OLED current despite the presence of various variations in the electrical characteristics of TFTs. Importantly, the circuit exhibits enhanced compensation accuracy in the low gray level region, which is critical for achieving high image uniformity in AMOLED displays.