Performance Of Phototransistors Research Articles

Role of gradual gate doping engineering in improving phototransistor performance for ultra-low power applications

In this paper, an optimized ultra-low power phototransistor design based on gradual gate doping engineering is proposed. Using an analytical investigation and numerical simulation, an overall performance comparison of the proposed phototransistor design and conventional structure has been studied, in order to show the improved characteristics provided by the proposed design in terms of increased $$I_{ON}/I_{OFF}$$ION/IOFF ratio and superior photoresponsivity capability. The results obtained from our analytical investigation are validated by comparison with the numerical simulations, thus establishing the accuracy of our analytical investigation. Moreover, the developed analytical models are used to optimize the proposed design using a genetic algorithm (GA) based-computation. The advantages offered by the proposed design suggest the possibility to overcome the most challenging problem with the power requirements of the optical interconnect: power consumption in the light emitter and in the receiver. In this context, the proposed phototransistor owing to the high responsivity requires less optical power from the light emitter to achieve an acceptable signal-to-noise ratio compared to the phototransistor with conventional design.

Two Dimensional Optoelectronic Simulation Based Comparison of Top and Bottom Contact Organic Phototransistors.

This paper deals with two-dimensional optoelectronic simulation based study of organic phototransistors. The top and bottom contact device structures were simulated to compare their characteristics under dark and under light illumination conditions. It was observed that the performance of these phototransistors can be tuned by shining a monochromatic light of given intensity. Further, the photosensitivity of the top and bottom contact phototransistors was compared and it was concluded that the difference in their photocurrent can be attributed to their different carrier extraction areas, which also leads to the difference in their contact resistances. Our results clearly indicate that the top contact structure has higher photosensitivity compared to the bottom contact structure. Furthermore, we simulated the dependence of photosensitivity on gate voltage, light intensity and channel length of the two structures and found the presence of both photoconductive and photovoltaic effects governing performance of organic phototransistors. Finally by simply scaling the device channel length contact resistance in the bottom and top contact organic photo transistors were estimated under different light illumination conditions.

Benzodithiophenedione and diketopyrrolopyrrole based conjugated copolymers for organic thin-film transistors by structure modulation

Three copolymers (P1, P2, and P3) based on benzodithiophenedione and diketopyrrolopyrrole units were synthesized by Suzuki reaction. The thermal, optical, electrochemical, microstructure, charge transport and phototransistor properties of the copolymers were also investigated. The field-effect performances of the copolymers were optimized by tailoring their structure and using thermal annealing. The three copolymers exhibited ambipolar transport under vacuum, but hole characteristics in air. The optimized copolymer P3 exhibited a balanced hole and electron mobility of 0.022 and 0.028 cm2 V−1 s−1, respectively, under vacuum conditions, but hole transport in air with a hole mobility of as high as 0.034 cm2 V−1 s−1. Phototransistors based on P3 showed a photocurrent/dark current ratio of 6.4 × 103 under near-infrared light illumination (808 nm). This is the first investigation on the field-effect and phototransistor performances of benzodithiophenedione-based copolymers.

Edge illuminated SiGe heterojunction phototransistor for RoF applications

The first edge illuminated SiGe phototransistor based on the available commercial SiGe/Si BiCMOS technology for a low-cost detector or mixer in radio-over-fibre applications. Its technology and structure are described. Edge mapping of the phototransistor performances, so as to observe the fastest and the highest sensitive areas of the structure, is performed. The phototransistor exhibits a cutoff frequency of 890 MHz and low frequency responsivity of 0.45 A/W at 850 nm. This new SiGe/Si phototransistor structure provides a cutoff frequency double that of a top illuminated SiGe/Si phototransistor obtained using the same BiCMOS technology.

Photoconductive behaviors of difluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene

In this report, we demonstrate a photoconductive device based on difluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TES-ADT). The light intensity dependences of the transient photocurrent combined with theoretical analyses clearly show that a simple device architecture consisting of diF-TES-ADT and two Au electrodes patterned in parallel forms an efficient, trap-limited, gain-generating photoconductive device, resulting in high responsivity up to approximately 500A/W. Moreover, the efficient gain-generating nature of diF-TES-ADT enables efficient tuning of the charge carrier density in the channel region, resulting in unprecedentedly sensitive phototransistor performance with high current modulation exceeding 106 as well as hysteresis-free threshold voltage shifts.

Copolymers of benzo[1,2-b:4,5-b′]dithiophene and bithiazole for high-performance thin film phototransistors

A series of conjugated polymers based on benzo[1,2-b:4,5-b′]dithiophene and bithiazole were systematically investigated for applications of polymer field-effect transistors and thin film phototransistors. Grazing incidence X-ray diffraction and atomic force microscopy were employed to investigate the solid state molecular organization and film morphology. The solution processability, solid state molecular organization, film morphology, charge transport and phototransistor performance can be tuned by elaborately optimizing the polymer backbone and side chains synchronously. For the optimized polymer (P5), a charge-carrier mobility of 0.194 cm2 V−1 s−1, a threshold voltage of −6 V and an on/off current ratio of 106 were achieved. Phototransistors based on this polymer showed a highest photoresponsivity of 132 A W−1 and a photocurrent/dark current ratio of 2 × 105.

High responsivity of amorphous indium gallium zinc oxide phototransistor with Ta2O5 gate dielectric

This study investigates the electrical performance of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with a Ta2O5 gate dielectric under monochromatic illumination. The relationship between the phototransistor performance and oxygen partial pressure is determined. The oxygen content of the a-IGZO channel significantly affects the electrical and optical characteristics of a-IGZO TFTs. At applied gate biases of 0, 0, and 0.25 V, oxygen partial pressures of 0%, 0.1%, and 0.2% yielded measured device responsivities of 0.23, 0.44, and 4.75 A/W, respectively. Oxygen content can be used to control the mobility of TFTs, which can amplify photocurrent and enhance the responsivity of a-IGZO TFTs with a Ta2O5 gate dielectric.

The effect of the active layer thickness on the performance of pentacene-based phototransistors

In this paper, we discuss the fabrication and characterization of pentacene-based phototransistors using indium tin oxide as a transparent electrical gate and PMMA as a transparent dielectric gate. The photoelectric properties with different pentacene film thicknesses were characterized under ultraviolet (365nm) illumination. We observed that for the thinner pentacene films, the threshold voltage upon UV illumination was shifted from its initial value in the dark to a positive voltage of more than 16V, whereas the shift was only of 3V for thicker films. Thus, we obtained a higher photosensitivity of 6.5×104 for thinner pentacene films, which indicates that the organic thin film transistors could find use in photodetector applications.

Simulations of AlGaAs/GaAs heterojunction phototransistors

Abstract Heterojunction bipolar phototransistors, based on GaAs technology, are widely used in optoelectronic integrated circuits. One of the methods to improve phototransistor performance is applying a delta-doped thin base, which causes both a higher current gain and a better time response. There is a lack of information about this kind of device in the literature. Our work presents the results of computer simulations of AlGaAs/GaAs n-p-n phototransistors, with a bulk-doped and delta-doped base working as two- and threeterminal devices. The characteristics and device parameters obtained clearly show that phototransistors with delta-doped base have higher sensitivity and a better time response compared to the structures with a bulk-doped base. Based on simulation results, we modified the epitaxial phototransistor structure with a double delta-doped base that should improve device performance.

Effect of Gate Voltage in Organic Phototransistors Based on Polythiophene/Fullerene Bulk Heterojunction Nanolayers

Here we report the effect of gate voltage on the performance of organic phototransistors made using bulk heterojunction films of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM). To understand the illumination effect, a monochromatic light (520 nm) corresponded to the maximum absorption of present P3HT:PCBM blend film was employed. Results showed that the present device followed a p-type field-effect transistor in the dark and under illumination though the saturation trend of output curves became weakened under illumination. The responsivity exhibited a two-stage trend as a function of gate voltage. The maximum responsivity reached 0.13 A/W under illumination of green light at 81μW/cm2.

Organic Thin Film Photo-Transistors Based on Pentacene

In this article, we studied the influence of light exposure on the electrical properties of a pentacene-based transistor. While the charge carrier mobility is found unaffected at 2.10−2cm2V−1s−1 after 2 hours of constant ultraviolet illumination, the photo-excitation mainly perturbs the charge carrier density in the active region. Irreversible change of the mobility was only observable after 10 hours of light exposure. The corresponding phototransistor performance under UV show a maximal current gain (Idsillumination/Idsdark) at Vgs = 0V of 103. Moreover, light intensity dependent measurements revealed both a photovoltaic (in turn-on state) and a photoconductive (in turn-off state) effect in the device.

Dynamic Performance of Dual-Emitter Phototransistor as Electro-Optical Switch

The dynamic performance of a dual-emitter phototransistor (DEPT) was studied. In the static case of a DEPT under a 12.5 µW optical power, the emitter-induced gain effect from photocurrent modulation improves collector photocurrent (88 to 129 µA). For the DEPT as an electro-optical switch with a 2 V supply voltage, the stable electrical logic swing is 0.41 V for a 1 V clock signal. When a short-period clock signal is applied, a clear electrical logic swing larger than 1.2 V is achieved. The emitter-induced gain effect from both photocurrent modulation and junction-voltage modulation is also introduced to clearly describe DEPT dynamic performance.

Emitter-Induced Gain Effects on Dual-Emitter Phototransistor as an Electrooptical Switch

In this paper, an investigation of the static/dynamic performance of dual-emitter phototransistors (DEPTs) as an electrooptical switch was made. In the static case, the measured optical gains (the collector photocurrents) of DEPTs operated at a 12.5-muW optical power have been enhanced from 13.7 to 20.2 (88 to 129 muA) due to an emitter-induced gain high (EIGH) from photocurrent modulation. For DEPTs as an electrooptical switch, the EIGH from photocurrent modulation and the emitter-induced gain effect from junction-voltage modulation dominate the dynamic operation. In the case of continuous light input, good bistable-state output characteristics can be realized with an electrical clock signal. When the switch is operated with a 2-V supply voltage and a 10-kOmega load resistance, the stable electrical-logic swing (ELS) is 0.41 V for a 1-V/1-kHz clock signal. Electrical inputs of 5-, 10-, and 50-kHz clock signals are also employed to achieve outputs with increased ELSs of 1.18, 1.19, and 1.3 V, respectively. An optical inverter is realized. When two clock signals, an electrical input and an optical input, are employed, DEPTs are operated as a tristable-state switch.

RECENT DEVELOPMENT OF SB-BASED PHOTOTRANSISTORS IN THE 0.9- TO 2.2-μM WAVELENGTH RANGE FOR APPLICATIONS TO LASER REMOTE SENSING

We have investigated commercially available photodiodes and also recent developed Sb -based phototransistors in order to compare their performances for applications to laser remote sensing. A custom-designed phototransistor in the 0.9- to 2.2-μm wavelength range has been developed at AstroPower and characterized at NASA Langley's Detector Characterization Laboratory. The phototransistor's performance greatly exceeds the previously reported results at this wavelength range in the literature. The detector testing included spectral response, dark current and noise measurements. Spectral response measurements were carried out to determine the responsivity at 2-μm wavelength at different bias voltages with fixed temperature; and different temperatures with fixed bias voltage. Current versus voltage characteristics were also recorded at different temperatures. Results show high responsivity of 2650 AIW corresponding to an internal gain of three orders of magnitude, and high detectivity (D*) of 3.9×1011 cm.Hz 1/2/ W that is equivalent to a noise-equivalent-power of 4.6×10-14 W/Hz 1/2 (-4.0 V @ -20°C) with a light collecting area diameter of 200-μm. It appears that this recently developed 2-μm phototransistor's performances such as responsivity, detectivity, and gain are improved significantly as compared to the previously published APD and SAM APD using similar materials. These detectors are considered as phototransistors based-on their structures and performance characteristics and may have great potential for high sensitivity differential absorption lidar (DIAL) measurements of carbon dioxide and water vapor at 2.05-μm and 1.9-μm, respectively.

Performance enhancement of GaInP/GaAs heterojunction bipolar phototransistors using dc base bias

GaInP-GaAs heterojunction bipolar phototransistors grown by metal organic vapor phase epitaxy (MOVPE) and operated with frontside optical injection through the emitter are reported with high optical gain (<88) and record high frequency performance (28 GHz). Heteropassivation of the extrinsic base surface is employed using a depleted GaInP emitter layer between the nonself-aligned base contact and the emitter mesa. The phototransistor's performance is shown to improve with increasing dc base bias in agreement with predictions of a recently reported Gummel-Poon model. Experimental results are reported for devices with optical active areas of 10/spl times/10 /spl mu/m/sup 2/, 20/spl times/20 /spl mu/m/sup 2/, and 30/spl times/30 /spl mu/m/sup 2/, with peak measured cutoff frequencies of 28.5, 23.1, and 18.5 GHz, respectively, obtained at collector current densities between 2/spl times/10/sup 3/ and 6/spl times/10/sup 3/ A/cm/sup 2/.

The source-gated amorphous silicon photo-transistor

We discuss the effect of scaling the thickness of the a-Si:H layer on the performance of photo-transistors. We show that reducing the a-Si:H layer thickness to 0.05 /spl mu/m, in order to increase the speed of pixel-switching thin-film transistors (TFTs), causes a large drop in the photo-transistor's dynamic range. We then propose, fabricate and test the Source-Gated Photo-Transistor, a new a-Si:H photo-transistor which allows the use of thin a-Si:H layers while boosting the dynamic range of the phototransistor. Our experimental results show that the source-gated photo-transistor improves the dynamic range of photo-transistors with thin a-Si:H layers (0.05 /spl mu/m) by more than 30 times. This increased dynamic range is obtained by increasing the illumination current and reducing the dark current. Illumination current is improved by doubling the optical path and increasing the photo-generated carriers lifetime within the a-Si:H layer. Dark current is reduced by pinning the gap-region to a constant potential. Finally, we confirm the principle of photo-generated carriers separation in our source-gated photo-transistor by conducting optical and bias stress measurements on conventional and source-gated photo-transistors.

Multiple quantum-dot infrared phototransistors

A novel infrared photodetector — the quantum-dot infrared phototransistor — based on a multiple quantum dot structure is considered theoretically. An analytical model of the device is developed to evaluate its performance. The dark current and sensitivity are calculated. It is shown that the quantum-dot infrared phototransistor performance can surpass that of the quantum-well photodetectors.

Current crowding effects in GaAs/AlGaAs heterojunction phototransistors

The heterojunction phototransistor (HPT) has applications as an optical detector/amplifier in receivers for fibre optic communications systems. It is well known that the provision of an external bias, either optically or electrically, via a third (base) terminal, can enhance the performance of the HPT in terms of gain, speed of response and signal to noise ratio. This is due principally to an increase in emitter efficiency and a reduction in the RC time constant of the emitter base junction. In the paper we show that crowding of the bias current and the signal current can affect very significantly the high frequency performance of phototransistors and will limit the advantage given by the external bias in three-terminal devices but improve the performance of two-terminal devices. The implications for the design of high speed phototransistors are discussed.

An analysis of the performance of heterojunction phototransistors for fiber optic communications

The theory of operation of the heterojunction phototransistor (HPT) is reviewed and the limitations on gain and speed-of-response are examined in the context of fiber optic systems requirements. The response of the base potential is shown to depend on the input optical power, and this dependence results in a power-dependent gain-bandwidth product, f T . Model calculations assuming optimized device structures suitable for multimode and single-mode systems operating in the 1.3-1.55-µm spectral region are used to demonstrate the consequences of this dependence. The results indicate that the HPT can have sufficient gain and speed-of-response for particular applications if a dc bias (optical or electrical) is used. The noise sources, S/N , and sensitivity of the optimized devices are discussed to clarify system applicability.

Noise in phototransistors

The investigation reported here presents both analytical and experimental results on the noise performance of phototransistors. The noise performance of phototransistors depends on fluctuations traceable to two main sources: 1) the random fluctuations in the rate of photon arrival; and 2) mechanisms inherent in the device such as fluctuations in the generation of free carriers, diffusion and recombination fluctuations, and 1/f mechanisms, to cite a few. On the basis of corpuscular arguments, it is shown that the important fluctuations may be represented by simple, partially correlated shot noise current generators in parallel with the device junctions for a wide range of frequencies. The hybrid-pi transistor model and these noise generators have been used to derive an expression for the noise current to the short-circuited output of a phototransistor. Attention has been primarily focused on the low-frequency expression since correction terms due to high-frequency effects are negligible. The output noise spectra of commercially available silicon phototransistors and silicon units fabricated at the Microelectronics Laboratory of the University of Florida have been measured. The measurements are in good agreement with the model. The current gains and beta cutoff frequencies of these devices have been computed by interpreting the measured spectra with the aid of the analytical results. Independent ac and dc measurements of these parameters are in satisfactory agreement with values determined from noise measurements. It is thereby ascertained that noise measurements on phototransistors can be used to characterize them in terms of current gain and beta cutoff frequency.