A novel source follower design and analysis using tunneling mechanism for image sensor

Abstract

In the field of CMOS image sensors, the signal-to-noise ratio (SNR) problem at low light is predominantly affected by Source Follower (SF) noise, following the advent of 4T-Active Pixel Sensors. Previous research has studied the use of a Buried Channel (BC), which has been shown to improve 1/f noise by traps located in the silicon/gate-oxide interface [1]. However, it has also been found to cause degradation in voltage gain (Av). To address this issue, we propose a novel SF design, referred to as tunneling-field effect transistor (TFET)-SF, employing the Band-To-Band Tunneling (BTBT) mechanism. Our design is based on an analysis by TCAD simulation. Utilizing conventional TFETs for SF was challenging because the source voltage (VS) has to follow the gate voltage (VG). To overcome this challenge, we place a grounded area (GND) between the gate and source. The energy band raised under the influence of GND enables BTBT from the valence band of GND to the conduction band of the channel. The GND potential is controlled by VS for constant current as the gate bias is applied. In TFET-SF, GND also serves a similar role to the high work function gate of BC-SF in raising the energy level to satisfy the operating range. Simulation results considering the operating range show that VG-VS is around 0.3 V from VG= 1.2 V to 2.6 V, while the tunneling width and probability remain constant. TFET-SF has several advantages over Conventional SF (Conv-SF), including the ability to reduce Noise Spectral Density (NSD) and higher Av. The n-doped channel in TFET-SF helps to lower the flat band voltage and suppress the formation of an inversion layer, which is responsible for reducing NSD by forming BC. Moreover, a high drain voltage can further impact the n-type channel, increasing the potential of the deep substrate region relative to that of the interface. The analyzed NSD of TFET-SF was one-order of magnitude lower than that of Conv-SF. In terms of Av, TFET-SF shows higher transconductance than CMOS under a specific current condition [2], corresponding to the SF operation range, due to the influence of the extremely low subthreshold swing in the subthreshold region. The absence of channel length modulation also leads to an increase in output resistance. Both of these TFET characteristics contribute to higher Av. According to simulation results, TFET-SF demonstrates high Av of over 0.9 (V/V) within its operating range. In summary, we have proposed a novel SF structure employing BTBT that shows improvements in multiple characteristics. In addition to reducing the normalized NSD by 10 times, it is possible to increase Av to a high value exceeding 0.9 (V/V) over the operating range. Furthermore, a TFET has certain characteristics that result in a low capacitance, which means it has high conversion gain, high cut-off frequency, and broad bandwidth. This is because there is no inversion layer formed in the TFET. Thus, our proposed structure could be a strategic advancement for the next-generation image sensor technology.