Part I: Physical Insight Into Carbon-Doping-Induced Delayed Avalanche Action in GaN Buffer in AlGaN/GaN HEMTs
01 Jan 2019-IEEE Transactions on Electron Devices (IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC)-Vol. 66, Iss: 1, pp 561-569
TL;DR: In this article, the effect of carbon-doping in GaN buffer on the performance of AlGaN/GaN HEMTs is discussed. But the authors focus on the degradation of the breakdown voltage, leakage current, sheet charge density, and dynamic ONresistance.
Abstract: Physics behind the improvement in breakdown voltage of AlGaN/GaN HEMTs with carbon-doping of GaN buffer is discussed. Modeling of carbon as acceptor traps and self-compensating acceptor/donor traps is discussed with respect to their impact on avalanche breakdown. Impact of carbon behaving as a donor as well as acceptor traps on electric field relaxation and avalanche generation is discussed in detail to establish the true nature of carbon in GaN that delays the avalanche action. This understanding of the behavior of carbon-doping in GaN buffer is then utilized to discuss design parameters related to carbon doped buffer. Design parameters such as undoped channel thickness and relative trap concentration induced by carbon-doping are discussed with respect to the performance metrics of breakdown voltage, leakage current, sheet charge density, and dynamic ON-resistance.
Citations
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TL;DR: In this article, the authors describe the physics, technology, and reliability of GaN-based power devices, starting from a discussion of the main properties of the material, the characteristics of lateral and vertical GaN transistors are discussed in detail to provide guidance in this complex and interesting field.
Abstract: Over the last decade, gallium nitride (GaN) has emerged as an excellent material for the fabrication of power devices. Among the semiconductors for which power devices are already available in the market, GaN has the widest energy gap, the largest critical field, and the highest saturation velocity, thus representing an excellent material for the fabrication of high-speed/high-voltage components. The presence of spontaneous and piezoelectric polarization allows us to create a two-dimensional electron gas, with high mobility and large channel density, in the absence of any doping, thanks to the use of AlGaN/GaN heterostructures. This contributes to minimize resistive losses; at the same time, for GaN transistors, switching losses are very low, thanks to the small parasitic capacitances and switching charges. Device scaling and monolithic integration enable a high-frequency operation, with consequent advantages in terms of miniaturization. For high power/high-voltage operation, vertical device architectures are being proposed and investigated, and three-dimensional structures—fin-shaped, trench-structured, nanowire-based—are demonstrating great potential. Contrary to Si, GaN is a relatively young material: trapping and degradation processes must be understood and described in detail, with the aim of optimizing device stability and reliability. This Tutorial describes the physics, technology, and reliability of GaN-based power devices: in the first part of the article, starting from a discussion of the main properties of the material, the characteristics of lateral and vertical GaN transistors are discussed in detail to provide guidance in this complex and interesting field. The second part of the paper focuses on trapping and reliability aspects: the physical origin of traps in GaN and the main degradation mechanisms are discussed in detail. The wide set of referenced papers and the insight into the most relevant aspects gives the reader a comprehensive overview on the present and next-generation GaN electronics.
141 citations
TL;DR: In this article, drain current transient spectroscopy (DCTS) and low-frequency (LF) output admittance (Y}_{{22}}$ ) dispersion techniques were used to identify trap locations and types.
Abstract: Deep-level traps in AlGaN/GaN- and AlInN/GaN-based HEMTs with different buffer doping technologies are identified by drain current transient spectroscopy (DCTS) and low-frequency (LF) output admittance ( ${Y}_{{22}}$ ) dispersion techniques. TCAD simulations are also carried out to determine the spatial location and type of traps. The DCTS and LF ${Y}_{{22}}$ measurements on Al0.25Ga0.75N/GaN HEMT (Fe-doped buffer) reveal a single electron trap at ${E}_{C} - {0.47}$ eV. On the other hand, an electron trap at ${E}_{C} -$ (0.53–0.59) eV and a deep hole trap at ${E}_{V} + {0.82}$ eV are detected in Al0.845In0.155N/AlN/GaN HEMT with unintentionally doped (UID) buffer, while a slow detrapping behavior is noticed at ${E}_{C} - {0.6}$ eV in Al0.83In0.17N/AlN/GaN HEMT with C-doped buffer. The DCTS and LF ${Y}_{{22}}$ measurements yield nearly the same trap signatures, indicating the reliability of the trap characterization techniques used in this article. The simulated LF ${Y}_{{22}}$ characteristics show that all these traps are acceptor-like states located in the buffer layer. The identified trap parameters in various buffers may be helpful to improve the crystalline quality of the epitaxial buffer layers.
36 citations
Cites background from "Part I: Physical Insight Into Carbo..."
...Equal fixed charge density is placed on either side of the barrier layer, but with opposite polarity to account for the polarization-induced charge [6], [35], [41]....
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...contacts are essentially ohmic [40], [41]....
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TL;DR: In this paper, the breakdown behavior of drain-connected field plate-based GaN HEMTs was investigated and the proposed vertical and dual-field-plate designs were proposed to alleviate the channel electric field by uniformly distributing it vertically into the buffer region.
Abstract: TCAD studies are performed to develop physical insights into the breakdown behavior of drain-connected field plate-based GaN HEMTs. Using the developed insights, to mitigate the performance bottleneck caused by the lateral drain-connected field plate design, we have proposed novel vertical-field-plate designs. The proposed designs alleviate the channel electric field by uniformly distributing it vertically into the GaN buffer region. As a result, the proposed vertical and dual-field-plate design offer ${2} \times $ and ${3} \times $ improvements in breakdown voltage, respectively, compared with the design without field plate. Similarly, compared with a design with a lateral field plate, a 50% improvement in the breakdown voltage was seen with dual-field-plate architecture. RF power amplifier (PA) performance extracted using load-pull simulations demonstrates an improved RF PA linearity at higher drain bias, improved output power, efficiency, and PA gain for HEMTs with dual- and vertical-field-plate designs.
35 citations
TL;DR: In this paper, a comprehensive review summarizes the current progress, understanding, and challenges in vertical GaN power devices, which can serve as not only a gateway for those interested in the field but also a critical reference for researchers in the wide bandgap semiconductor and power electronics community.
Abstract: Vertical gallium nitride (GaN) power devices are enabling next-generation power electronic devices and systems with higher energy efficiency, higher power density, faster switching, and smaller form factor. In Part I of this review, we have reviewed the basic design principles and physics of building blocks of vertical GaN power devices, i.e., Schottky barrier diodes and p-n diodes. Key topics such as materials engineering, device engineering, avalanche breakdown, and leakage mechanisms are discussed. In Part II of this review, several more advanced power rectifiers are discussed, including junction barrier Schottky (JBS) rectifiers, merged p-n/Schottky (MPS) rectifiers, and trench metal–insulator–semiconductor barrier Schottky (TMBS) rectifiers. Normally- OFF GaN power transistors have been realized in various advanced device structures, including current aperture vertical electron transistors (CAVETs), junction field-effect transistors (JFETs), metal–oxide–semiconductor field-effect transistors (MOSFETs), and fin field-effect transistors (FinFETs). A detailed analysis on their performance metrics is provided, with special emphasis on the impacts of key fabrication processes such as etching, ion implantation, and surface treatment. Lastly, exciting progress has been made on selective area doping and regrowth, a critical process for the fabrication of vertical GaN power devices. Various materials characterization techniques and surface treatments have proven to be beneficial in aiding this rapid development. This timely and comprehensive review summarizes the current progress, understanding, and challenges in vertical GaN power devices, which can serve as not only a gateway for those interested in the field but also a critical reference for researchers in the wide bandgap semiconductor and power electronics community.
31 citations
TL;DR: In this paper, a modified Si-doping profile in the GaN buffer is proposed to lower the Cdoping concentration near GaN channel to mitigate the adverse effects of acceptor traps.
Abstract: In part I of this paper, we developed physical insights into the role and impact of acceptor and donor traps—resulting from C-doping in GaN buffer—on avalanche breakdown in AlGaN/GaN HEMT devices. It was found that the donor traps are mandatory to explain the breakdown voltage improvement. In this paper, silicon doping is proposed and explored as an alternative to independently engineer donor trap concentration and profile. Keeping in mind the acceptor and donor trap relative concentration requirement for achieving higher breakdown buffer, as depicted in part I of this paper, silicon & carbon codoping of GaN buffer is proposed and explored in this paper. The proposed improvement in breakdown voltage is supported by physical insight into the avalanche phenomena and role of acceptor/donor traps. GaN buffer design parameters and their impact on breakdown voltage as well as leakage current are presented. Finally, a modified Si-doping profile in the GaN buffer is proposed to lower the C-doping concentration near GaN channel to mitigate the adverse effects of acceptor traps in GaN buffer.
31 citations
Cites background or result from "Part I: Physical Insight Into Carbo..."
...Furthermore, scaling of VBD with LGD is also observed, as Si-doping concentration is increased, which signifies electric field redistribution in the vertical direction, as discussed in part I of this paper [8]....
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...that an optimum concentration of donor traps is present [8]....
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...paper, revealed self-compensating nature of traps induced by C-doping to be the key parameter controlling the VBD [8]....
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...It was thus concluded that the required improvement in VBD can be achieved along with optimized dc performance of the device by reducing the acceptor trap concentration and maximizing the donor trap concentration [8]....
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...Clearly, as also seen in part I of this paper [8], the doping requirement for VBD improvement is higher compared to what is required to merely increase buffer resistivity....
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References
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TL;DR: In this paper, the effects of carbon on the electrical and optical properties of GaN were investigated using hybrid functional calculations, and it was shown that carbon substituting for N (CN) has an ionization energy of 0.90 eV.
Abstract: Using hybrid functional calculations we investigate the effects of carbon on the electrical and optical properties of GaN. In contrast to the currently accepted view that C substituting for N (CN) is a shallow acceptor, we find that CN has an ionization energy of 0.90 eV. Our calculated absorption and emission lines also indicate that CN is a likely source for the yellow luminescence that is frequently observed in GaN, solving the longstanding puzzle of the nature of the C-related defect involved in yellow emission. Our results suggest that previous experimental data, analyzed under the assumption that CN acts as a shallow acceptor, should be re-examined.
536 citations
"Part I: Physical Insight Into Carbo..." refers background in this paper
...proposed to behave as deep acceptor [3], [4], while other...
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...9 eV [4]....
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TL;DR: In this article, the ionization rates for holes and electrons in silicon have been determined over the following ranges of field: for holes, (2.5-6.0)\ifmmode\times\else\texttimes\fi{}${10}^{5}$ volts
Abstract: The ionization rates for holes and electrons in silicon have been determined over the following ranges of field: for holes, (2.5-6.0)\ifmmode\times\else\texttimes\fi{}${10}^{5}$ volts ${\mathrm{cm}}^{\ensuremath{-}1}$; for electrons, (2.0-5.0)\ifmmode\times\else\texttimes\fi{}${10}^{5}$ volts ${\mathrm{cm}}^{\ensuremath{-}1}$. The ionization rate for electrons is higher than that for holes. The results suggest that the field dependence of the ionization rate for holes and, probably, for electrons also, can be expressed by $a\mathrm{exp}(\ensuremath{-}\frac{b}{E})$, where $E$ is the field. The constants $a$ and $b$ are different for electrons and holes.
526 citations
Additional excerpts
...into account using the Chynoweth law [11]....
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TL;DR: In this paper, the impact of carbon impurities on the electrical and optical properties of GaN, AlN, and InN using density functional calculations based on a hybrid functional was investigated.
Abstract: Carbon is a common impurity in the group-III nitrides, often unintentionally incorporated during growth. Nevertheless, the properties of carbon impurities in the nitrides are still not fully understood. We investigate the impact of carbon impurities on the electrical and optical properties of GaN, AlN, and InN using density functional calculations based on a hybrid functional. We examine the stability of substitutional and interstitial configurations as a function of the Fermi-level position and chemical potentials. In all nitrides studied here, C${}_{\mathrm{N}}$ acts as a deep acceptor and gives rise to deep, broad photoluminescence bands. Carbon on the cation site acts as a shallow donor in InN and GaN, but behaves as a $DX$ center in AlN. A split interstitial is the most stable configuration for the C impurity in InN, where it acts as a double donor and likely contributes to $n$-type conductivity.
346 citations
"Part I: Physical Insight Into Carbo..." refers background or methods in this paper
...as deep [3] or shallow [1], [2], [5], [6] donor....
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...C in GaN can replace Ga or N and behave as either donor or acceptor trap site [3], respectively....
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...As C is widely debated to behave as acceptor trap in GaN [2], [3], C-doping in this section is modeled as deep acceptor traps with energy EV + 0....
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...2878770 of traps it introduces [1]–[3], [5], [6]....
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...proposed to behave as deep acceptor [3], [4], while other...
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TL;DR: In this paper, the advantages and limitations of the current-transient methods used for the study of the deep levels in GaN-based high-electron mobility transistors (HEMTs), by evaluating how the procedures adopted for measurement and data analysis can influence the results of the investigation.
Abstract: This paper critically investigates the advantages and limitations of the current-transient methods used for the study of the deep levels in GaN-based high-electron mobility transistors (HEMTs), by evaluating how the procedures adopted for measurement and data analysis can influence the results of the investigation. The article is divided in two parts within Part I. 1) We analyze how the choice of the measurement and analysis parameters (such as the voltage levels used to induce the trapping phenomena and monitor the current transients, the duration of the filling pulses, and the method used for the extrapolation of the time constants of the capture/emission processes) can influence the results of the drain current transient investigation and can provide information on the location of the trap levels responsible for current collapse. 2) We present a database of defects described in more than 60 papers on GaN technology, which can be used to extract information on the nature and origin of the trap levels responsible for current collapse in AlGaN/GaN HEMTs. Within Part II, we investigate how self-heating can modify the results of drain current transient measurements on the basis of combined experimental activity and device simulation.
320 citations
"Part I: Physical Insight Into Carbo..." refers background in this paper
...68 eV [15] and a constant concentration of 1....
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TL;DR: In this paper, the bulk trap-induced component of current collapse (CC) in GaN/AlGaN heterojunction field effect transistors is studied in drift diffusion simulations, distinguishing between acceptor traps situated in the top and bottom half of the bandgap, with Fe and C used as specific examples.
Abstract: The bulk trap-induced component of current collapse (CC) in GaN/AlGaN heterojunction field-effect transistors is studied in drift diffusion simulations, distinguishing between acceptor traps situated in the top and the bottom half of the bandgap, with Fe and C used as specific examples. It is shown that Fe doping results in an inherent but relatively minor contribution to dispersion under pulse conditions. This simulation is in reasonable quantitative agreement with double pulse experiments. Simulations using deep-level intrinsic growth defects produced a similar result. By contrast, carbon can induce a strong CC which is dependent on doping density. The difference is attributed to whether the trap levels, whether intrinsic or extrinsic dopants, result in a resistive n-type buffer or a p-type floating buffer with bias-dependent depletion regions. This insight provides a key design concept for compensation schemes needed to ensure semi-insulating buffer doping for either RF or power applications.
270 citations
"Part I: Physical Insight Into Carbo..." refers background in this paper
...Although the impact of C-doping, modeled as traps, on the dc-RF dispersion of the device has been well explored [2], [5], [6], the mechanism behind delayed avalanche action or VBD improvement with C-doping is not well understood....
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...11 eV [1], [5], [6]....
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...as deep [3] or shallow [1], [2], [5], [6] donor....
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...Doping of Fe/C is also known to introduce trap sites in GaN buffer [1]–[6]....
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...2878770 of traps it introduces [1]–[3], [5], [6]....
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