On-Chip Smart Temperature Sensors for Dark Current Compensation in CMOS Image Sensors

Abstract

This paper proposes various types of on-chip smart temperature sensors, intended for thermal compensation of dark current in CMOS image sensors (CIS). It proposes four different architectures of metal–oxide–semiconductor (MOS)-based and bipolar junction transistor (BJT)-based temperature sensors inside and outside the CIS array. Both of the MOS-based temperature sensors make use of the thermal dependence of MOS transistors working in the subthreshold region with ratiometric currents and are quantized by the 14-bit first-order incremental delta–sigma analog-to-digital converters (ADCs). Fabricated using 0.18- $\mu \text{m}$ CIS technology and measured on four chips, the proposed temperature sensors are compared, on their resolution and process variability, as well as on their effects on the neighboring image pixels implemented on the same chip. Experimental results show that the MOS-based temperature sensors inside and outside the array consume a power of 36 and $40~\mu \text{W}$ , respectively, both achieving 3-sigma ( $\sigma$ ) inaccuracy less than ±0.75 °C on four different chips, over a temperature range from −20°C to 80 °C at a conversion time of 16 ms. The temperature sensors facilitate the digital thermal compensation of dark current in the CIS array, by at least 80%, in experiments.