Dark Current RTS-Noise in Silicon Image Sensors

Study of RTS noise and excess currents in lattice-mismatched InP/InGaAs/InP photodetector arrays

Random telegraph signal noise instabilities in lattice-mismatched InGaAs/InP photodiodes

To realize high-resolution pixels in the CMOS image sensor, it is necessary to reduce low-frequency noise, particularly random telegraph signal (RTS) noise of the source-follower transistor (SFT). To achieve less relative variation of drain noise current, ΔID/ID, a metal–oxide–semiconductor field-effect transistor structure without the lightly doped drain (LDD) for the SFT transistor is proposed. Then, a comparison of RTS noise characteristics between the proposed SFT structure without LDD and the conventional SFT structure with LDD was conducted. Although the RTS noise occurrence probability of the proposed SFT structure without LDD is somewhat greater than that of the conventional SFT structure with LDD, the amplitude of relative variation of drain noise current of the proposed SFT structure is significantly less than that of the conventional SFT. Despite changes in several factors in the proposed SFT, such as effective channel length, trap depth profile in gate oxide, and random dopant fluctuation (RDF), it is believed that the change of trap depth profile is a primary factor for the improved RTS characteristic. Therefore, the proposed SFT is highly desirable for the high-resolution CMOS image sensor.

The behavior of random telegraph signal (RTS) noise in metal oxide semiconductor field effect transistor (MOSFETs) with different oxidation processes and its effects on the readout circuit in complementary MOS (CMOS) image sensor were experimentally studied. Statistically, RTS noise in large number of devices have been measured both in the device with single oxide and dual oxide and we analyzed its amplitude and power spectrum density. Through the analysis, we found that there are significantly different behaviors of RTS noise between single oxide devices and dual oxide devices due to different characteristic of oxide traps. We not only provided information of traps in both single and dual oxide devices but also applied different RTS noise behavior of both two cases to readout circuits in CMOS image sensor.

The low-frequency (LF) noise behavior of metal-oxide-semiconductor field-effect transistors (MOSFETs) is studied when cycled between inversion and accumulation. On large-area devices the decrease of the LF noise is systematically found, and supports the observations by Bloom and Nemirovsky [Appl. Phys. Lett. 58, 1664 (1991)]. The random telegraph signal (RTS) noise observed in small (submicrometer) devices disappears when the transistor is cycled into accumulation. The drop in LF noise observed may thus be explained by the fact that most or all of the RTSs, which are caused by carrier trapping into slow oxide states, no longer contribute to the noise of the system. The method indicates a possibility to separate the contributions of different sources of 1/f noise in MOSFETs.

The low-frequency noise sources are investigated in as-prepared and aged GaN light-emitting diodes (LEDs). Accelerated aging is performed by thermal (300h at 240°C) and electrical forward-bias stressing (20 and 50mA for 2500h). At low currents I<IRTS, where IRTS is a critical current, the low-frequency noise is dominated by random telegraph signal (RTS) noise on top of the 1∕f noise. An explanation is given for the giant relative current jumps ΔI∕I≈50% and an expression for IRTS is derived. The RTS noise in our devices is a less-sensitive diagnostic tool for studying the results of accelerated aging. Two components of the 1∕f noise were observed: one is related to the quantum-well junction and the other is due to series resistance noise. The two 1∕f spectra have different current dependences. It was found that the junction 1∕f noise is not significantly affected by aging. However, a strong increase in series resistance noise, by a factor of 60–800 compared to unstressed devices, is observed after strong electrical and thermal aging. This high increase goes hand in hand with a relatively small increase in the value of the series resistance (13%–90%). This makes 1∕f noise a very sensitive reliability indicator for GaN LEDs after accelerated aging. We discuss the physical origin of LED degradation.

Temporal noise such as thermal and low-frequency noise (LF noise) in the CMOS imager readout circuit has been analyzed. In addition, the effect of correlated double sampling operation on the noise was included. We have derived an analytical noise equation for the specified readout circuit, and confirmed its validity by comparing it with the simulation result. Thermal noise model which is accurate in short-channel devices operating in saturation region was used. Since the in-pixel devices (source follower and selection transistor) of the readout circuit are relatively small in size, and thus exhibits random telegraph signal (RTS) noise, both 1/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> and RTS noise were considered for their LF noise. Based on the analyzed noise components, we presented the noise reduction method by adjusting the transistors sizes in the readout circuit.

Effects of shallow trench isolation on low frequency noise characteristics of source-follower transistors in CMOS image sensors

Random Telegraph Signal (RTS) noise in submicron MOSFETs showing a capture process, which deviates from the standard Shockley-Read-Hall kinetics, is analyzed using generation-recombination-tunneling model of current modulation in order to explain quadratic dependence of capture rate on current. Proposed model of two-step charge carrier quantum transitions involving secondary trap at the channel and gate oxide interface better represents observed complex switching phenomena in nanoscale devices, as is confirmed by presented experimental results.

Random telegraph signal (RTS) fluctuations with relative amplitude up to 50% are observed in forward and reverse gate current of unstressed and reverse-bias-stressed AlGaN/GaN high electron mobility transistors. Measurements of RTS amplitude and mean pulse widths as a function of forward gate bias indicate that the RTS is due to modulation of current along an intrinsic or stress-induced percolation path across the AlGaN-barrier by electron capture and emission on a trap within the barrier. Processes of electron capture from GaN to trap and subsequent tunneling to metal gate or electron exchange between GaN channel and the trap are considered.

This work investigates the role of the phosphorus doping element in the radiation-induced dark current in a CMOS image sensor (CIS) photodiode. The neutron and proton irradiations on shallow arsenic-based photodiode CISs and deep phosphorus-based photodiodes CISs have been performed. The results highlight the applicability of the same dark current increase and random telegraph signal (RTS) models. Already verified on other photodiode structures, these results further extend the universality of these analytic tools. Moreover, it emphasizes that the phosphorus element does not play a significant role either in the radiation-induced dark current increase or in the dark current RTS. The results on RTS after annealing reveal the same recovery dynamic than those already observed in irradiated image sensors, suggesting that the phosphorus element does not play a significant role after annealing. Therefore, this work is a piece of experimental evidence supporting the idea that RTS induced by displacement damage is principally due to defect clusters mainly constituted of intrinsic silicon defects such as clusters of vacancies and interstitials.

As device size shrinks smaller, random telegraph signal (RTS) noise, which is caused by the trapping and detrapping of a single carrier will become a serious issue. In this paper, we characterized four level RTS and extracted the characteristics of two independent traps. Once the position of the trap is found in the oxide (xT) and along the channel (yT) with respect to source, we extracted difference between the oxide conduction band energy (ECox) and trap energy (ET). Finally we introduce eight level RTS observed in some sample which indicates existence of three active traps.

InP/InGaAs‐based heterojunction phototransistors (HPTs) are promising for ultrasensitive short‐wave infrared (SWIR) detection due to their high internal gain, low excess noise, and low power consumption. This work demonstrates an HPT achieving a high optical gain of ∼1200 at −2 V bias, with dark current lower than state‐of‐the‐art avalanche photodiodes (APD). To further push its sensitivity to the limit for weak light/ultra‐long‐distance imaging, the noise mechanism in low frequency (LF) regime and its origins have been investigated, by which generation current in depletion region caused by the defect‐related traps is explored and studied. In particular, the LF noise power is found to be independent of the internal gain. A Hooge's constant of 2.1 × 10 −5 is reported for the first time in gain‐based detectors, comparable to HgCdTe devices. Remarkably, temperature‐dependent multi‐level random telegraph signal (RTS) analysis reveals two dominant origins of the generation current, corresponding to relaxation processes of two typical types of mid‐gap traps. One is Zn‐related extrinsic defects caused by Zn diffusion into the InP emitter, while the other is Ga‐related intrinsic defects in InGaAs collector. As the guideline for optimizing the HPT detector design, this work paves the way to further improve the sensitivity for low‐flux SWIR photodetection.

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Random telegraph signal (RTS) noise is of increasing concern for sub-100-nm Flash memories. To quantitatively study the shallow trench isolation (STI) edge effect, NMOS devices with and without STI edges in the channel area are designed and analyzed. The significant impact of STI on width scaling is demonstrated and quantified. It is shown that the noise induced by STI edges dominates the RTS noise for smaller device sizes and is caused by an increase in the number of trapping sites at the STI edges. By rounding the STI corner, the number of stress-induced traps can be significantly reduced. </para>

IV, noise and electroluminescence analysis of stress-induced percolation paths in AlGaN/GaN high electron mobility transistors