Heterojunction Bipolar Phototransistor Research Articles

Germanium-incorporated Si-Ge-Si heterojunction phototransistors for a high-limit of detection and wide linear dynamic range near-infrared light detection

This paper investigates overcoming the limitations of conventional silicon (Si) bipolar junction transistors (BJTs) for near-infrared (NIR) light detection. Silicon BJTs have inherent limitations in the NIR region due to silicon’s bandgap. To address this, the paper proposes high-performance silicon-germanium-silicon (Si-Ge-Si) heterojunction bipolar phototransistors (HPTs). The key innovation is introducing a thin germanium (Ge) layer at the base-collector junction and engineering its doping concentration. This Ge layer extends the BJT phototransistor’s optical response into the NIR region by enhancing optical absorption. The paper analyses the performance of the HPTs, demonstrating linear photoresponsivity of 432 mA/W over a wide optical power range, leading to an exceptional linear dynamic range of 159 dB and a very low dark current, which produces a limit of detection of -99.64 dBm. Additionally, the device exhibits a large photo-to-dark current ratio of 2.16 ×107 (at optical power of 5 µW) and a fast response time of 11.35 ps to modulated NIR optical signals within the linear dynamic range. This research paves the way for high-performance CMOS-compatible NIR phototransistors using Si-Ge-Si heterostructures.

Ultrahigh-performance heterojunction bipolar phototransistor enabled by bilateral piezoelectric charge modulation effect

Heterojunction photodiodes based on p-n junction with enhanced piezo-phototronic effect have been extensively studied. However, the photoresponsivity of conventional photodiodes is small due to the natural zero current gain. Heterojunction phototransistor usually has greater photoresponsivity due to the non-zero current gain. More importantly, heterojunction phototransistor owns two interfaces, offering the opportunity to introduce the bilateral piezoelectric charge modulation effect for further performance enhancement compared to the conventional single-interface piezo-phototronic effect in photodiode. In this work, a base-floating heterojunction bipolar phototransistor (FHPT) based on n-ZnO/p-Si/n-ZnO structure with a spectral detection range from ultraviolet (UV)–visible is demonstrated. The positive piezoelectric charge generated at the two interfaces of the collector junction and the emitter junction, combined with the optimal base width and low resistance base doping concentration of the FHPT, leads to an excellent photoresponsivity (18046 A/W, 365 nm), high detectivity (5.7×1012 Jones), and a wider modulation (7120 %). The physical mechanism leading to the ultrahigh performance is attributed to the cooperative positive piezoelectric charge at the two interfaces, simultaneously reducing the effective base width and lowering the emitter junction barrier. This work illustrates the regulation of heterojunction phototransistor performance by the bilateral piezoelectric charge modulation effect, providing insightful guidance to high-performance phototransistor design.

High-performance InP/InGaAs heterojunction bipolar phototransistors for optoelectronic applications

<span>Phototransistors are attractive devices for applications in optical fiber telecommunication systems. They are used for the detection of optical signals and the amplification of these signals. This paper presents an investigation of how the technological parameters of indium phosphide (InP)/indium gallium arsenide (InGaAs) heterojunction bipolar phototransistor can impact its responsivity at two wavelengths, 1310 nm and 1550 nm. Based on the results of this investigation, we proposed optimized structures for the studied phototransistor. In this work, we used the software technology computer aided-design (TCAD)-Silvaco to simulate the physical and the electrical behavior of the different structures. The proposed optimized phototransistors can be used for various optoelectronic applications.</span>

High-Performance Solar-Blind p-NiO/n-ZnO/p-Si Ultraviolet Heterojunction Bipolar Phototransistors With High Optical Gain

Ultraviolet (UV) photodetectors (PDs) have attracted significant attention for civil and military applications. Zinc-oxide (ZnO) and nickel-oxide (NiO) have been widely applied in UV-PDs owing to their wide bandgap (3.2–3.8 eV), transparency, excellent optical and electrical properties, and good chemical stability. However, UV signals are generally weak; hence, UV-PDs with high optical gain are essential. In this work, high-performance solar-blind p-NiO/n-ZnO/p-Si heterojunction bipolar phototransistors (HBPTs) were fabricated. The fabricated HBPTs exhibited a high responsivity of 9.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> A/W at a wavelength of 280 nm with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_CE</i> = –7 V, a high optical gain of 3.96 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , and a large detectivity of 3 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> Jones. In addition, the UV/visible rejection ratio was as high as 880. These behaviors indicate that the prepared HBPTs are good solar-blind photodetectors and suitable for the detection of weak UV signals. However, for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_CE</i> value below –7 V, (| <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V_CE</i> | > 7V), the optical gain decreased owing to the punch-through effect. Furthermore, band-diagrams showed that the photogenerated electrons were blocked by the potential barrier at the NiO/ZnO interface (base–emitter junction) owing to a large conduction-band discontinuity (Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E_C</i> ) of 2.7 eV, which resulted in a large optical gain in the prepared HBPTs.

Bilateral piezoelectric charge modulation as a perspective of piezo-phototronic effect in tri-/multi-layer structured optoelectronics

Mechanical stimulus produced piezoelectric charges could modulate the local energy band near interface to control the performances of electronics and optoelectronics, whereas almost half of the piezoelectric charges are electrostatically screened by the Ohmic contact and not effectively utilized. Multi-layer structured devices should be adopted to introduce multiple interfaces, so that most or even all of the piezoelectric charges could be effectively utilized. Such piezoelectric charge modulation on the multi-layer structured devices is proposed as Bilateral Piezoelectric Charge Modulation (BPCM). In this work, the concept of BPCM is illustrated and explained in detail and we further demonstrate the BPCM on the performances of a floating-base PNP heterojunction bipolar phototransistor (HBPT). The results indicate an interesting optimization modulation behavior. The working mechanism of BPCM in PNP HBPT is carefully illustrated and thoroughly discussed by analyzing the piezoelectric charges induced modulation of energy band near the two interfaces of PNP HBPT and its effects on photo-carriers’ dynamics. This work not only proposes the BPCM and further demonstrates it in a floating-base PNP HBPT successfully, but also presents the BPCM’s huge potential in advanced electronics and optoelectronics as a perspective of piezo-phototronic effect in multi-layer structured piezoelectric semiconductor devices.

AlGaN-based solar-blind UV heterojunction bipolar phototransistors: structural design, epitaxial growth, and optoelectric properties

A challenging development of an AlGaN-based solar-blind UV heterojunction bipolar phototransistor with a gain-type NPN structure.

Demonstration of an n-ZnO/p-Si/n-Si heterojunction bipolar phototransistor for X-ray detection

We fabricated an n-ZnO/p-Si/n-Si heterojunction bipolar phototransistor by the low cost RF magnetron sputtering method and characterized its photoresponse to both the 780 nm visible light and the X-ray in this paper. The device structure and the film transmittance were characterized by the SEM, XRD and transmittance spectra. The phototransistor shows high sensitivity, linearity and repeatable response to both the 780 nm visible light and the X-ray illumination indicating the promising potential of the n-ZnO/p-Si/n-Si detector. The sensitivity of the phototransistor to the X-ray is about 280 μC·Gy−1·cm−2 under the bias voltage of 10 V with the X-ray tube voltage of 30 kV. The phototransistor also shows a fast response with a decay time of about 7.75 μs, which corresponds to the lifetime of the sputtered p-Si film. The temperature dependent dark current of the device indicated the activation energy of 0.32 eV, which is probably caused by the deep-level defect in the high resistivity silicon substrate. The photo current of the phototransistor to the 780 nm visible light linearly increases with increasing the bias voltage in the saturation region indicating an Early voltage of about 5.6 V, from which the effective impurity density of the p-Si film was deduced to be ∼1 × 1016 cm−3 revealing the low activation rate of the sputtering method, and the unactivated dopant could act as recombination centers and responsible for the short carrier lifetime of the p-Si film and the fast response of the phototransistor. This paper demonstrated that the n-ZnO/p-Si/n-Si heterojunction bipolar transistor has potential for the development of an X-ray detector.

Impact of base bias current and incident optical power on the InP/InGaAs heterojunction phototransistor performances

In this article, the optical and electrical properties of NPN InP/InGaAs heterojunction bipolar phototransistor (HPT) are studied by two dimensional semiconductor simulation. The phototransistor is modelled using a numerical method, and the results are presented and discussed. This model is based on heterojunction bipolar transistor (HBT) model with the consideration of the optical beam effect. Responsivity of 12.62 A/W for 1100 nm light is achieved when the thickness of base layer is 50 nm. A good qualitative agreement of the numerical and analytical simulated value of both collector current and responsivity as a function of the wavelength with the existing experimental data was achieved. We present a comparative analysis of our obtained solutions at IB = 5 μA with that simulated at IB = 100 μA and we found that the value of optical power (Popt) giving high injection into a phototransistor at IB = 5 μA is three times higher than that at IB = 100 μA. The effects of both the base bias current and the incident optical power on the device performance are investigated. The simulation results, which are presented and compared at low, ideal and high base bias current, show that the highest photocurrent and optical gain are obtained when the base bias current reaches the ideal region. The HPT optical characteristics such as the electrical characteristics of HBT are affected by the high injection effect (Kirk effect).

Base-width modulation effects on the optoelectronic characteristics of n-ITO/p-NiO/n-ZnO heterojunction bipolar phototransistors

ITO/NiO/ZnO npn heterojunction bipolar phototransistors (HBPTs) with various base widths are fabricated using a radio-frequency sputtering system. The effects of base-width modulation on the optoelectronic characteristics of the prepared HBPTs are studied. The dark current of HBPTs decreases with increasing base width because the injected electrons from the emitter are recombined in the wide base region. The photocurrent increases with decreasing base width, which is attributed to higher emitter-base injection efficiency. The responsivity increases with the collector-emitter voltage (V CE) in the HBPTs with a 100 nm base width, whereas the responsivity sharply decreases at V CE > 4 V for the HBPTs with a thinner base width (80 nm) due to the punch-through effect. In contrast, the responsivity approaches saturation at large V CE for HBPTs with a thicker base width (120 nm). The responsivity and detectivity decrease with increasing incident light intensity, which is caused by an increase in the base recombination loss. The HBPTs with a base width of 100 nm exhibits the largest responsivity and detectivity; their detectivity is higher than that of HBPTs with base widths of 80 and 120 nm by approximately two and three orders, respectively.

All transparent high-performance solar-blind n-ITO/p-NiO/n-ZnO ultraviolet heterojunction bipolar phototransistor

All transparent high-performance solar-blind n-ITO/p-NiO/n-ZnO ultraviolet heterojunction bipolar phototransistors (HBPTs) were fabricated using a low-cost radio frequency magnetron sputtering system. In the HBPT structure, ITO, NiO, and ZnO were employed as the emitter, base, and collector, respectively. The applied voltage is across the emitter and collector, and the base is floating. The photocurrent increases with the collector–emitter voltage (VCE). The prepared HBPTs presented the highest optical gain of 7.4 × 104 and a responsivity of 1.67 × 104 A/W, at VCE = 4 V for the 280-nm illumination wavelength. As VCE exceeds 4 V, the optical gain and responsivity decrease owing to the punch-through effect. The prepared HBPTs have an ultraviolet (UV)/visible rejection ratio of more than three orders, allowing their use in practical applications as UV detectors for weak UV signals.

Electrical Compact Modeling of SiGe Phototransistor: Impact of the Distributed Nature on Dynamic Behavior

This article presents and analyzes an improved electrical compact model of SiGe/Si-double heterojunction bipolar phototransistors (HPT). This is a complete one, operating in dc, in nonlinear large-signal ac mode and in optomicrowave operating. This HPT was fabricated based on an existing industrial SiGe heterojunction bipolar transistor (HBT) Telefunken GmbH technology. Several deviations from the standard HBT electrical behavior are observed when the optical access on the top of the HPT was created. A compact, physically based model is then proposed to take into account these phenomena called “2-D electrical extension effect,” which explains the 2-D and distributed nature of charge carriers flow within the SiGe HPT. Parameter extraction, verification, and validation of the dynamic behavior model are done by RF characterization, through S-parameters and transition frequency curves, and by the optomicrowave gain of the optical characteristics over a wide frequency range.

SiGe Photo-Transistor for Low-Cost SSMF-Based Radio-Over-Fiber Applications at 850nm

This paper investigates the possibility of implementing SiGe Hetero-junction bipolar Photo-Transistor (HPT) within low cost and low power consumption Radio-over-Fiber (RoF) systems based on standard single mode fiber (SSMF) operating in the first optical window for wireless communication networks scenarios. The SiGe HPT under study has potential lower cost compared to standard PIN photo-detectors as it is fabricated through the consolidated SiGe bipolar transistor process technologies, allowing the possible integration with Si-based circuits. Various operation modes of the photo-transistor (such as three-terminals and two-terminals) are investigated, analyzing their impact on the SSMF-based RoF link in terms of gain and noise (including modal noise fluctuations) for a frequency range up to 2.5 GHz. A 20 MHz LTE transmission is finally demonstrated as example of possible applications of the studied RoF link.

Intrinsic Frequency Response of Silicon–Germanium Phototransistor Associated With 850-nm Multimode Fiber

The intrinsic frequency response of silicon–germanium heterojunction bipolar phototransistors (HPTs) at 850 nm is studied to be implemented in multimode fiber systems. The experimental analysis of an HPT with an optical window size of $\textsf {10} \times \textsf {10}\,\,\mu \text{m}^{\textsf {2}}$ is presented. An opto-microwave (OM) scanning near-field optical microscopy is performed to observe the variation of the HPT dynamic behavior versus the illumination location of the phototransistor. The photocurrent generated by the photodiode at the interface between the n++ subcollector and the p+ guard ring is analyzed, and its impact on the performance of the HPT is investigated. Then, we propose a technique to remove the substrate photocurrent effect on the optical transition frequency ( ${f}_{\textsf {Topt}}$ ): ${f}_{\textsf {Topt}}$ value of 4.1 GHz given by raw measurement results increases up to 6 GHz after removing the substrate response. The influence of the 2-D carrier flows on the HPT intrinsic OM behavior is also studied. Design aspects of SiGe/Si HPT structures are finally discussed as a conclusion.

Study of Lateral Scaling Impact on the Frequency Performance of SiGe Heterojunction Bipolar Phototransistor

The influence of the lateral scaling such as emitter width and length on the frequency behavior of SiGe bipolar transistor is experimentally studied. Electrical transistors of different emitter sizes are designed and fabricated by using a commercial bipolar transistor technology. The effect of peripheral current and collector current spreading on electrical bipolar transistor performances are analyzed in regards to the state of the art. Furthermore, the lateral scaling effect on SiGe phototransistor electrical and opto-microwave frequency behavior is studied. The impact of the lateral flow of photo-generated carriers toward the optical opening in phototransistor structure is investigated. Moreover, the 2-D carrier flow effect on the opto-microwave frequency behavior of the phototransistor is characterized through opto-microwave scanning near-field optical microscopy measurements, in the course of which the intrinsic parameters, such as transit time and junction capacitances are extracted over the surface of the phototransistor. An intrinsic optical transition frequency of 6.5 GHz is measured for $10\times 10\,\,\mu \text {m}^{2}$ .

Large-Signal Static Compact Circuit Model of SiGe Heterojunction Bipolar Phototransistors: Effect of the Distributed Nature of Currents

This paper presents a physically based opto-electrical compact model of a SiGe/Si heterojunction phototransistor (HPT). This static large-signal model is intended to model the static electrical and optical static operation of the HPT. The measured HPT was developed by extending the structure of an existing SiGe heterojunction bipolar transistor (HBT) from the Telefunken GmbH process technology. Unusual behaviors are observed compared to the standard HBT behaviors when the optical access on the top of the HPT is created. The proposed compact model then includes a phenomenon also called “2-D electrical extension effect” that explains the 2-D (vertical and lateral) charge carriers flow within the SiGe HPT. The validity and accuracy of the proposed model in the static mode of operation are confirmed through I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> -V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CE</sub> and Gummel characteristics, both under dark and light conditions. The dc responsivity of the HPT is also extracted from the model and agrees quite well to measurement.

A Distributed Extended Ebers–Moll Model Topology for SiGe Heterojunction Bipolar Phototransistors Based on Drift–Diffusion Hydrodynamic Behavior

This paper proposes a novel type of compact circuit model for silicon germanium (SiGe) heterojunction phototransistor (HPT) that is justified from the distributed nature of both its electrical and optical behaviors. The proposed model is based on a modified Ebers–Model structure and contains 28 different parameters. It is independent of the nature of the base bias (current or voltage), as opposed to existing models. The method to identify that the architecture of the model is original as it makes use of a drift–diffusion numerical simulation of a SiGe HPT adjusted to experimental data. A good fit of the model both in amplitude and phase is obtained through the nine optomicrowave S-parameters.

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.

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.