Bipolar Phototransistor Research Articles

Enhancing the photoresponsivity of bipolar phototransistors for near-infrared detection

Higher photo-responsivity brings the advantage of reducing device area cost for photodetectors. This work demonstrates that the responsivity of a bipolar phototransistor can be greatly enhanced by reusing the parasitic photo-generated carriers in the substrate. The reuse of parasitic photo-carriers can be realized by strapping the base terminal and the local substrate terminal of a bipolar phototransistor via simple metallic interconnect. Test devices were implemented with a commercially available bipolar-CMOS (BiCMOS) process. Compared with the conventional bipolar phototransistor without reusing the substrate carriers, the proposed phototransistor showed at least one order of magnitude improvement in responsivity. The peak of spectral response shifted from the visible light range into the near-IR region, which is beneficial to near-IR detection.

An 850 nm SiGe/Si HPT with a 4.12 GHz maximum optical transition frequency and 0.805A/W responsivity

A 10 × 10 μm2SiGe heterojunction bipolar photo-transistor (HPT) is fabricated using a commercial technological process of 80 GHz SiGe bipolar transistors (HBT). Its technology and structure are first briefly described. Its optimal opto-microwave dynamic performance is then analyzed versus voltage biasing conditions for opto-microwave continuous wave measurements. The optimal biasing points are then chosen in order to maximize the optical transition frequency (fTopt) and the opto-microwave responsivity of the HPT. An opto-microwave scanning near-field optical microscopy (OM-SNOM) is performed using these optimum bias conditions to localize the region of the SiGe HPT with highest frequency response. The OM-SNOM results are key to extract the optical coupling of the probe to the HPT (of 32.3%) and thus the absolute responsivity of the HPT. The effect of the substrate is also observed as it limits the extraction of the intrinsic HPT performance. A maximum optical transition frequency of 4.12 GHz and an absolute low frequency opto-microwave responsivity of 0.805A/W are extracted at 850 nm.

Hybrid photo‐receiver based on SiGe heterojunction photo‐transistor for low‐cost 60 GHz intermediate‐frequency radio‐over‐fibre applications

The first results of an integrated hybrid photo-receiver based on a 850 nm two-terminal (2T) silicon germanium (SiGe) heterojunction bipolar photo-transistor (HPT) for low-cost radio-over-fibre (RoF) applications is presented. A hybrid module was realised with two cascaded low-noise amplifiers of 20 dB total gain and an SiGe 2T-HPT exhibiting a −15 dB opto-microwave gain at 5.15 GHz with a 1.6 GHz bandwidth. A 16% degradation of the error vector magnitude compared with back-to-back measurement for the transmission of a 2 GHz OFDM signal at 3 Gbit/s was measured based on the 60 GHz IEEE 802.15.3c standard transposed at the intermediate frequency (IF) of 5 GHz, according to an IF-RoF distribution mode.

ANALOG CONVERTER OF LIGHT ON THE BASE OF UNIJUNCTION TRANSISTOR

The principle of action of the analog converter of light based on the base of unijunction transistor oscillator with current driving element and the capacitor in the emitter circuit is shown. The output parameter is the frequency of oscillation as a function of light intensity. Maximum sensitivity is achieved when the element is used as a current driving bipolar phototransistor and the capacitance decreases with decreasing voltage.

High-speed bipolar phototransistors in a 180 nm CMOS process

Several high-speed pnp phototransistors built in a standard 180nm CMOS process are presented. The phototransistors were implemented in sizes of 40×40μm2 and 100×100μm2. Different base and emitter areas lead to different characteristics of the phototransistors. As starting material a p+ wafer with a p− epitaxial layer on top was used. The phototransistors were optically characterized at wavelengths of 410, 675 and 850nm. Bandwidths up to 92MHz and dynamic responsivities up to 2.95A/W were achieved. Evaluating the results, we can say that the presented phototransistors are well suited for high speed photosensitive optical applications where inherent amplification is needed. Further on, the standard silicon CMOS implementation opens the possibility for cheap integration of integrated optoelectronic circuits. Possible applications for the presented phototransistors are low cost high speed image sensors, opto-couplers, etc.

PNP PIN bipolar phototransistors for high-speed applications built in a 180 nm CMOS process

This work reports on three speed optimized pnp bipolar phototransistors build in a standard 180nm CMOS process using a special starting wafer. The starting wafer consists of a low doped p epitaxial layer on top of the p substrate. This low doped p epitaxial layer leads to a thick space-charge region between base and collector and thus to a high −3dB bandwidth at low collector–emitter voltages. For a further increase of the bandwidth the presented phototransistors were designed with small emitter areas resulting in a small base-emitter capacitance. The three presented phototransistors were implemented in sizes of 40×40μm2 and 100×100μm2. Optical DC and AC measurements at 410nm, 675nm and 850nm were done for phototransistor characterization. Due to the speed optimized design and the layer structure of the phototransistors, bandwidths up to 76.9MHz and dynamic responsivities up to 2.89A/W were achieved. Furthermore simulations of the electric field strength and space-charge regions were done.

SiGe heterojunction bipolar phototransistor for optics–microwaves interfacing

A new SiGe heterojunction bipolar phototransistor (HPT) based on a commercially available process was designed, realized, and experimentally characterized. Its internal characteristics, mainly the collector-to-base capacitance, vary significantly with the received light power, making it suitable as an active element of a light-controlled photo-oscillator. It can also be a key component of optical network-on-chip (ONoC). Its responsivity was improved and its transition frequency remains in the range of 30 GHz.

Modeling and analysis of the spectral response for AlGaAs/GaAs HPTs for short wavelength optical communication

Detailed spectral response (SR) modeling for heterojunction bipolar phototransistors (HPTs) is presented in this work. All the related physical parameters are taken into account for the resolution of photogenerated excess minority carrier continuity equations in the active layers of the HPT. The layer dependence of the optical flux absorption profile at near-bandgap wavelengths is also investigated and its generalization as a single-exponential has been refuted for HPTs based on GaAs material systems (InGaP-GaAs/AlGaAs-GaAs). The variation in the responsivity of the device with changing base width is analyzed at various wavelengths and a detailed experimental setup for optical characterization of HPTs is also provided. The measured results at 635, 780, 808, and 850 nm show good agreement to the modeled data, validating the newly developed theoretical model.

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.

Coupling electron-hole and electron-ion plasmas: Realization of an npn plasma bipolar junction phototransistor

Coupling e−–h+ and gas phase plasmas with a strong electric field across a potential barrier yields a transistor providing photosensitivity and voltage gain but also a light-emitting collector whose radiative output can be switched and modulated. This optoelectronic device relies on the correspondence between the properties of a low temperature, nonequilibrium plasma and those for the e−–h+ plasma in an n-type semiconductor. Hysteresis observed in the collector current-base current characteristics is attributed primarily to charge stored in the base, and the photogeneration of e−–h+ pairs at the base-collector junction. Extinguishing the collector plasma requires an emitter-base junction reverse bias of <1 V.

The phototransistor action of a reverse-biased Au/CaF2[3–7 ML]/n-Si(1 1 1) structure

Metal–ultra-thin insulator–semiconductor (MIS) tunnel structures with a high-quality ultra-thin epitaxial fluoride layer have been fabricated on n-Si wafers. Under the reverse bias, these structures have been experimentally shown to amplify photocurrent. They can therefore be treated as MIS tunnel emitter bipolar phototransistors. The gain value reaches 102–103, and is due to the asymmetry between conduction-band and valence-band tunneling. The importance of the valence-band component has been elucidated. The experimental data have been well reproduced by simulations considering the conservation of the transverse wave vector of tunneling particles. The current multiplication effect was observed within the voltage range up to the structure overload. After the insulator damage, the structure ceased to exhibit gain rendering a photodiode.

Enhancement of Optical Gain in Floating-Base InGaP–GaAs Heterojunction Phototransistors

Comparative studies on the performances of floating-base InGaP-GaAs heterojunction bipolar phototransistors (HPTs) with and without base contact metal were carried out. Both the fabricated HPTs with base contact metal (HPTs-WB) and without base contact metal (HPTs-WOB) exhibited collector dark current as low as 0.2 pA when they were operated in a floating-base configuration at VCE = 1 V. The HPTs-WOB demonstrated much higher optical gain (120 at an optical power of 1.1 μW irradiated by a 635-nm laser) than the HPTs-WB. The optical gain showed dependency on the illuminating optical power, increasing as the illuminating optical power was increased.

A bipolar PIN phototransistor with modulated base doping

A new CMOS-based p+np−p+ phototransistor utilizing a modulated base doping is presented. Furthermore, the collector of the device is formed by a thick low-doped p− region. Both measures lead to excellent responsivity values of 12.2, 46.5, 46.9 and 41.9 A/W at wavelengths of 405, 675, 785 and 855 nm, respectively. A detector with a light-sensitive area of 50 μm in square reaches a −3 dB bandwidth of 4.2 MHz at 3 V collector-emitter voltage at 850 nm.

Electrothermal model of optocoupler for SPICE

In the paper a new electrothermal model of the optocoupler for SPICE is proposed. The model is based on electrical models of the LED and the bipolar phototransistor with their parameters dependent ...

Large-Signal Charge Control Modeling of Photoreceivers for Applications up to 40 Gb/s

A charge control model was used to simulate the sensitivity and responsivity in a range of photodetector configurations including heterojunction bipolar phototransistors (HPTs), PIN-HBT, and APDs. Our simulations enabled for the first time a direct comparison of the performance between these photodetectors to be made. Simulations have been performed at bit rates from 2 to 40 Gb/s using various combinations of device design parameters (layer thickness, source resistance, and dc base voltage). For a BER=10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> at 40 Gb/s the best sensitivity of approximately -20 dBm was achieved using an optimized APD-HBT configuration, followed by sensitivities of approximately -14 dBm using optimized PIN-HBTs and HPTs. These results were found to agree well with published experimental data.

Electrothermal model of optocoupler for SPICE

AbstractIn the paper a new electrothermal model of the optocoupler for SPICE is proposed. The model is based on electrical models of the LED and the bipolar phototransistor with their parameters dependent on temperature, their thermal models including both the self‐heating phenomenon and the mutual thermal interactions between the LED and the phototransistor, and the dependences describing electrical power dissipated in these components. The final form of the electrothermal model of the optocoupler elaborated by the authors, dedicated to the d.c. and a.c. computations, was implemented in SPICE. The usefulness of the new model was experimentally verified for the optocoupler 4N25. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Low-Voltage and High-Responsivity Germanium Bipolar Phototransistor for Optical Detections in the Near-Infrared Regime

Germanium (Ge) bipolar phototransistors with high-responsivity performance are demonstrated using a low-temperature selective Ge epitaxy process. Large photocurrent and optical response enhancement are achieved over a conventional p-i-n Ge photodetector, which is predominantly attributed to the current gain induced by transistor actions. When illuminated with a photon wavelength of 1.55 mum , a Ge phototransistor shows a large responsivity of ~ 2.0 A/W for a low operating bias <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">VA</i> of 1.0 V. Optical measurement results further show that good photoresponse could be achieved for a spectral range of 1.31-1.62 mum, making such a device a very promising option for optical detections in the near-infrared (including <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> -band) wavelength regime.

Numerical analysis of SiGe heterojunction bipolar phototransistor based on virtual substrate

SiGe heterojunction bipolar phototransistors based on Si1−yGey virtual substrate have been proposed in this paper. The current gain and external quantum efficiency of the heterojunction bipolar phototransistors are calculated by solving one-dimensional diffusion equations. The computed results show that external quantum efficiency increases with the increase of Ge content of Si1−yGey virtual substrate and a high external quantum efficiency of 142 can be obtained in the heterojunction bipolar phototransistor with ten periods of Ge/Si0.4Ge0.6 multi-quantum wells on virtual substrate with Ge content of 0.6. The dependences of external quantum efficiency and current gain on structure parameters of the heterojunction bipolar phototransistors are investigated in detail. It is useful for design and optimization of the heterojunction bipolar phototransistors.

AlGaAs/GaAs heterojunction phototransistor with a double delta-doped base grown by AP MOVPE

Heterojunction bipolar phototransistors (HPTs), based on GaAs technology, are widely used in the optoelectronic-integrated circuit (OEIC) [H.T. Lin, D.H. Rich, A. Larsson, J. Appl. Phys. 79 (1996) 8015 [1]; Lin Chien-chung, M. Wayne, J.S. Harris Jr., F. Sugihwo, Appl. Phys. Lett. 76 (2000) 1188 [2]]. They exhibit a large optical gain and a negligible noise level at low bias voltages. The presented work describes the fabrication and device characteristics of n–p–n AlGaAs/GaAs heterojunction phototransistor with a double Zn delta-doped GaAs base region (2δ-Zn-AlGaAs/GaAs HPT). Such construction of the transistor allows to obtain higher current gain and lower power consumption. The electrical and optical properties of the epitaxial HPT structure were examined using capacitance–voltage (EC– V) measurements, photovoltage, photocurrent and micro-Raman spectroscopies. The current–voltage (dc I– V) characteristics as well as the results of time response measurements are also presented.

Preparation and properties of Zn delta-doped GaAs/AlGaAs heterojunction phototransistor

We have prepared, characterized and discussed the performance of AlGaAs/GaAs heterojunction bipolar phototransistor (HPT) including Zn delta-doped base. Due to the existence of δ-doped sheets located in the middle of undoped GaAs base the δ-doped HPT devices exhibit low dark current, nearly zero offset voltage, saturation voltage ∼0.4V, and rise and fall times in ns range at wavelength of 850nm up to 6V of applied voltage. Due to avalanche multiplication behavior at the collector junction, an increased optical gain G>10 can be reached for applied voltages in the range of 6–12V. For voltages higher than the device breakdown voltage (∼12V) switching and negative differential resistance (NDR) effect is measurable in the inverted mode of operation.