Amorphous silicon active pixel sensor readout circuit for digital imaging

Abstract

The most widely used architecture in large-area amorphous silicon (a-Si) flat panel imagers is a passive pixel sensor (PPS), which consists of a detector and a readout switch. While the PPS has the advantage of being compact and amenable toward high-resolution imaging, reading small PPS output signals requires external column charge amplifiers that produce additional noise and reduce the minimum readable sensor input signal. This work presents a current-mediated amorphous silicon active pixel readout circuit that performs on-pixel amplification of noise-vulnerable sensor input signals to minimize the effect of external readout noise sources associated with "off-chip" charge amplifiers. Results indicate excellent small-signal linearity along with a high, and programmable, charge gain. In addition, the active pixel circuit shows immunity to shift in threshold voltage that is characteristic of a-Si devices. Preliminary circuit noise results and analysis appear promising for its use in noise-sensitive, large-area, medical diagnostic imaging applications such as digital fluoroscopy. Index Terms-Active pixel sensor (APS), amorphous silicon (a-Si), digital fluoroscopy, integrated pixel amplifier, medical imaging.

Noise analysis of a novel hybrid active‐passive pixel sensor for medical X‐ray imaging

“…6. The lower limit of the integration is usually determined from the frame rate of the imager [6]. From the noise spectrum in Fig.…”

“…Using the theory presented previously in this section, the contribution from various noise sources is presented in Table 1. In order to produce low-noise images for X-ray modalities like fluoroscopy throughout the dosage range, the total electronic noise should be less than the quantum noise, which is approximately 1000 electrons for a-Se photoconductors at electric fields of 10 V/µm at fluoroscopy energy levels [6]. Table 1 shows that the total input referred noise is below the quantum noise limit of the X-ray imager.…”

“…Since r oA (=1/λI dsA , λ = 0.04 typically for a-Si TFTs [6]) >> r oR we can simplify the above expression to yield, …”

Compensation technique for DC and transient instability of thin film transistor circuits for large-area devices

“…The time dependence of (1) can be calculated as [2] and (b) AMI pixel circuit [3] According to (2), the current of the GP pixel circuits increases as it ages. Moreover, the major effect is due to C g2 , which is representative of charge injection component.…”

“…It has, therefore, become attractive for low-cost large-area backplanes for active matrix organic light emitting diode (AMOLED) displays [1] and active matrix imagers (AMIs) [2]. Figure 1 shows two different pixel circuits proposed for AMOLED displays [1] and X-ray imaging [3]. In the AMOLED pixel circuit, the current flowing through the OLED is controlled by an a-Si:H TFT (T1) while in the sensor pixel circuit, T1 is used to amplify the signal.…”