Ognian Marinov

Organic Thin-Film Transistors: Part I—Compact DC Modeling

A generic analytical model for the current-voltage characteristics of organic thin-film transistors (OTFTs) is derived. Based on this generic model, a TFT compact dc model that meets the requirements for compact modeling, including for computer circuit simulators, is proposed. The models are fully symmetrical, and the TFT compact dc model covers all regimes of TFT operation-linear and saturation above threshold, subthreshold, and reverse biasing. The empirical fitting parameters are mostly eliminated from the characteristic equations. The developed models are also in close correspondence to several physical, parametric, and limiting models for current-voltage and mobility characteristics. An essential practical feature of the TFT compact dc model is that the model is both upgradable and reducible, allowing for easier implementation and modifications and also simultaneously allowing for separation of characterization techniques. This allows for systematic fitting of experimental data with large scattering in the values, but at the same time, preserving consistently the OTFT behavior in the model.

Effect of forward and reverse substrate biasing on low-frequency noise in silicon PMOSFETs

Forward body biasing improves the low-frequency noise performance of p-channel metal-oxide semiconductor (PMOS) transistors by about 8 dB/V. Therefore, for analog design, forward body biasing may be preferred if noise is a concern. This is in agreement with the improvement of other MOSFET parameters such as the decrease of the threshold voltage (V/sub T/) or the increase of unity current-gain frequency (f/sub T/) on forward substrate- (or body)-source biasing (V/sub BS/). Also, forward V/sub BS/ is very attractive for low voltage supply (V/sub DD/ 0.8 V. Generally, it is confirmed that the spectral density S/sub I/ of the total low-frequency noise of the drain current I/sub D/ is proportional to the square of I/sub D/, i.e., S/sub I//spl prop/I/sub D//sup 2/, but it cannot be clearly ascribed to either number fluctuation or mobility fluctuation models. In addition, both models cannot accurately describe the dependence of the noise level on the body bias.

Low-frequency noise in polymer transistors

The low-frequency noise (LFN) properties of field-effect transistors (FETs) using polymers as the semiconducting substrate material are investigated and explained in terms of the nonstationary mobility /spl mu/ in the semiconducting polymer. In the frequency (f) range f<1 kHz it was found that 1/f noise prevails over other types of LFN in these polymer FETs (PFETs). The spectral density S/sub I/ of LFN of the drain current ID is proportional to the DC power V/sub DS//spl middot/I/sub D/ applied to the PFETs channel, from the ohmic to the saturation modes of device operation. In addition, S/sub I/ is affected by the carrier mobility /spl mu/ in PFETs channel, as /spl mu/ in organic FETs is dependent on the biasing. Thus, SI can have an additional sensitivity to I/sub D/, that is, S/sub I//spl prop/(V/sub DS//spl middot/I/sub D/)/sup 1-k/, where k/spl sim/0.1. In general, the 1/f LFN of PFETs follows the relations that have been obtained for crystal and inorganic FETs with minor correction for nonstationary mobility /spl mu/.

Organic Thin-Film Transistors: Part II—Parameter Extraction

A parameter extraction methodology and a verification of a generic analytical model and a thin-film transistor (TFT) compact dc model for the current-voltage characteristics of organic TFTs are presented. The verification shows that the proposed models meet the requirements for compact modeling and for computer circuit simulators. The models are fully symmetrical, and the TFT compact dc model is validated in all regimes of operation-linear and saturation above threshold, subthreshold, and reverse biasing. Suitable characterization techniques for parameter extraction of mobility, threshold voltage, and contact resistance are provided. Approaches are elaborated for the essential practical feature of upgradability and reducibility of the TFT compact dc model, allowing for easier implementation and modification, as well as separation of characterization techniques.

Effects of hot-carrier stress on the performance of the LC-tank CMOS oscillators

The effects of hot carrier stress on a fully integrated negative resistance LC-tank CMOS oscillator are investigated. The major effect is the decrease of the amplitude of the oscillation due to degradation in the I-V characteristics of the MOSFETs. The oscillator phase noise increases with stress duration since the amplitude of the oscillation decreases. A change in the biasing of the circuit due to the stress affects the parasitic capacitances in the circuit which in turn cause a slight change in the oscillation frequency.

Low-Frequency Noise in Bilayer MoS2 Transistor

Low-frequency noise is a significant limitation on the performance of nanoscale electronic devices. This limitation is especially important for devices based on two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs), which have atomically thin bodies and, hence, are severely affected by surface contaminants. Here, we investigate the low-frequency noise of transistors based on molybdenum disulfide (MoS2), which is a typical example of TMD. The noise measurements performed on bilayer MoS2 channel transistors show a noise peak in the gate-voltage dependence data, which has also been reported for graphene. To understand the peak, a trap decay-time based model is developed by revisiting the carrier number fluctuation model. Our analysis reveals that the peak originates from the fact that the decay time of the traps for a 2D device channel is governed by the van der Waals bonds between the 2D material and the surroundings. Our model is generic to all 2D materials and can be applied to explain the V, M and Λ shaped dependence of noise on the gate voltage in graphene transistors, as well as the noise shape dependency on the number of atomic layers of other 2D materials. Since the van der Waals bonding between the surface traps and 2D materials is weak, in accordance with the developed physical model, an annealing process is shown to significantly reduce the trap density, thereby reducing the low-frequency noise.

A new model for the low-frequency noise and the noise level variation in polysilicon emitter BJTs

Presents a new, physically-based model for the low-frequency noise in high-speed polysilicon emitter bipolar junction transistors (BJTs). Evidence of the low-frequency noise originating mainly from a superposition of generation-recombination (g-r) centers is presented. Measurements of the equivalent input noise spectral density (SIB) showed that for BJTs with large emitter areas (AE) S(I/sub B/) is proportional to 1/f, as expected. In contrast, the noise spectrum for BJTs with submicron A/sub E/ showed a strong variation from a 1/f-dependence, due to the presence of several g-r centers. However, the average spectrum has a frequency dependence proportional to 1/f for BJTs with large as well as small A/sub E/. The proposed model, based only on superposition of g-r centers, can predict the frequency-, current-, area-, and variation-dependency of with excellent agreement to the measured results.

High-Speed, Single-Photon Avalanche-Photodiode Imager for Biomedical Applications

The design of a low-light level pixel in CMOS technology for biomedical applications is described. This pixel is also suitable for very high-speed applications, such as fluorescence lifetime imaging (FLIM) used for drug discovery and/or minimally-invasive optical biopsy. In order to achieve high-speed imaging using single-photon detection, a detector with a very low dead-time is needed. The single-photon avalanche-photodiode (SPAD) discussed in this work uses a mainstream deep-submicron CMOS technology in order to achieve ultrahigh-speed operation and high pixel fill-factor, with in-pixel active quench and reset circuits. The paper also presents an innovative approach for reducing the deadtime of the detector and an attractive technique for simultaneous high-speed image acquisition by all the pixels of an array in parallel.

Compact modeling of charge carrier mobility in organic thin-film transistors

Finding the common points in theoretical models for mobility in thin-film transistors (TFTs), we demonstrate that there exists a generic analytical model for the mobility in organic TFTs (OTFTs), and the generic model is then converted into a TFT Compact Mobility Model, which is physically derivable from one perspective, and properly arranged to be suitable for compact modeling of OTFTs from another perspective, by separation and proper interfacing of temperature and bias dependence of the mobility, both significant for OTFTs, with the compact models for electrical current in OTFT. The proposed TFT Compact Mobility Model is verified theoretically and against experimental data, and the model is applicable even for high temperatures T>To, above the characteristic temperature To of the distribution of states in the organic material, a condition at which other models diverge in principle. The improvement is achieved by the identification of a temperature “shaping” function, which contains a diverging function...

Model for the injection of charge through the contacts of organic transistors

A compact model has been employed in organic thin film transistors (OTFTs) to study the electrical characteristics of the contacts, which are formed between the organic layer and source/drain electrodes of the OTFT. The model shows the importance of interrelating different physical phenomena: charge injection, redox reactions at the interface, and charge drift in the organic semiconductor. The model reproduces and explains several features that have been reported for current-voltage curves, ID-VC, at the contacts of OTFTs. The ID-VC curves are extracted from the experimental output characteristics by two techniques. One technique uses a set of transistors with different channel lengths and a simultaneous extraction of the ID-VC curve and the mobility of carriers in the channel of the transistor. When a set of transistors with different channel lengths is not available, we propose an iterative method for the simultaneous extraction of the ID-VC curve and the mobility by changing the gate bias voltages.