Reverse Bias Characteristics Research Articles

Analysis of barrier inhomogeneities in Ti/p–type strained Si0.95Ge0.05 Schottky diodes using reverse current-voltage characteristics

Extracting electrical parameters such as barrier height inhomogeneities (BHi) from the forward bias can be very challenging in metal/semiconductor (M/Sc) contacts characterized by low barrier height and high series resistance. In this work we demonstrate that the reverse bias characteristics can be used as an efficient alternative method to investigate the inhomogeneity of low barrier height in M/Sc contacts. In particular, the BHi in Ti/p-strained Si0.95Ge0.05 Schottky barrier diodes (SBD) has been investigated using reverse current–voltage-temperature (IR-VR-T) characteristics over the temperature range of 100–300 K. The temperature dependence of the measured barrier heights (ΦRBp) using reverse-bias is successfully explained in terms of a thermionic emission (TE) current transport associated with Gaussian distribution of the barrier height due BHi. The mean homogeneous barrier height (Ф‾RBp) and its corresponding standard deviation (σRs) were determined for different reverse voltages. A decrease in Ф‾RBp and an increase in σRs with increasing reverse bias are observed, indicating a large barrier height distribution upon high applied reverse bias. The deduced average zero-field barrier height (ФFBp≈0.59eV) from IR-VR agrees well with the corresponding value determined from the forward characteristics. Moreover, the reverse bias extracted values for Richardson constant A* based on BHi model, are found in fair agreement with the reported theoretical value of 32 A cm−2 K−2 for our p-type strained Si0.95Ge0.05 alloy. Finally, the results obtained on Pd/n-type Si0.90Ge0.10 structure shown in this work fully support the utilization of such reverse bias method.

Fabrication and Electrical Characterization of GaAs/GaN Junctions

GaAs/GaN heterojunctions are likely to be an ideal platform for high-frequency/high-output-power electron devices because of excellent electrical properties of GaAs and GaN as well as the similarity in their coefficients of thermal expansion. We have to note, however, that heteroepitaxial growth of GaAs/GaN junctions is quite difficult because of difference in crystal structures (in case of GaAs/wurtzite GaN junctions) and large mismatch in lattice constants (GaAs/zincblende GaN junctions). In this work we fabricate p+-GaAs/n-GaN heterojunctions by bonding GaAs and GaN epi wafers to each other using surface-activated-bonding technologies [1,2], selectively etching off the GaAs substrate used in the epitaxial growth, making ohmic contacts on the exposed surface of GaAs layer, and forming GaAs mesas. Ohmic contacts on the backside of GaN epi wafer are formed before bonding. The junctions are annealed at 400 ℃ in contact formation on GaAs surfaces. The temperature is much lower in comparison with the temperature in a wafer-fusion process of GaAs and GaN (750 ℃), which were reportedly applied for fabricating group-III arsenide/nitride heterojunction bipolar transistors [3]. Observation using cross sectional transmission electron microscope (TEM) of as-bonded and 400-℃ annealed GaAs/GaN interfaces shows that no voids are formed at the GaN/GaAs interfaces by the 400-℃ annealing. We also fabricate n+-GaAs/n-GaN heterojunctions for comparison.We measure the capacitance-voltage characteristics and current-voltage characteristics for reverse-bias voltages of the p+-GaAs/n-GaN and n+-GaAs/n-GaN junctions and find that the characteristics of the two junctions are close to each other, which suggests that the band profiles of GaN layers in the p+-GaAs/n-GaN and n+-GaAs/n-GaN junctions are almost the same, i.e., the Fermi-level pinning occurs at the GaAs/GaN interfaces. More importantly, the reverse-bias characteristics are measured up to -60 V. The reverse-bias voltage corresponds to an electric field of as high as ~2 MV/cm, which is comparable to the breakdown field of GaN (3-4 MV/cm) [4]. We also excite minority electrons in the p+-GaAs layer using a 488-nm Ar laser and successfully observe the photo current due to the transport of minority electrons across the reverse-biased GaAs/GaN interfaces.Acknowledgement: TEM samples were fabricated under the Inter-University Cooperative Research in IMR of Tohoku University. Epi wafers used in the work were provided from Sciocs (present Sumitomo Chemical) Co., Ltd.[1] H. Takagi, et al. Appl. Phys. Lett. 68, 2222 (1996). [2] S. Yamajo, et al., Jpn. J. Appl. Phys. 57, 02BE02 (2018). [3] C. Lian, et al. Appl. Phys. Lett. 91, 063502 (2007). [4] T. Maeda, et al., IEEE Electron Device Lett., 43, 96 (2022).

Conduction mechanism of Schottky contacts fabricated on etch pits originating from single threading dislocation in a highly Si-doped HVPE GaN substrate

This paper reports the relationship between a single dislocation structure and current leakage mechanism observed in a Pt/n+-GaN Schottky contacts with the combined use of transmission electron microscopy (TEM) and conductive atomic force microscopy (C-AFM). The contacts were fabricated on a substrate grown by hydride vapor phase epitaxy. TEM analysis clarified that large- and small-size etch pits are attributed to mixed/screw and edge dislocations, respectively. Schottky contacts formed on the surfaces of large and small etch pits (L-EPC, S-EPC), a dislocation-free flat area (FLAT), and a Schottky barrier diode with a diameter of 50 μm (SBD) were prepared and their temperature-dependent current-voltage characteristics were measured using C-AFM. Analysis of the forward bias characteristics revealed that the Schottky barrier height increased and the ideality factor approached unity with increasing temperature. Such temperature dependence of the barrier height was well explained by barrier inhomogeneity at the metal/semiconductor interface. From the analysis of the reverse bias characteristics, we found that field emission (FE) currents caused by the high donor concentration of the sample were the dominant conduction mechanism for S-EPC and FLAT. On the other hand, for L-EPC, the FE currents were overlapped by Poole-Frenkel emission (PFE) currents. For SBD, the leakage currents were successfully explained by the FE and thermionic field emission (TFE) mechanisms by assuming dislocation regions with a lowered barrier height. These findings suggest that mixed dislocations are associated with PFE and lower the Schottky barrier.

Voltage-margin limiting mechanisms of AlScN-based HEMTs

In this work, the off-state characteristics of AlScN/GaN high electron mobility transistors (HEMTs) grown by metalorganic chemical vapor deposition (MOCVD) were studied and directly compared to an AlGaN- and an AlN-HEMT grown in the same MOCVD. Pinch-off instability and leaky capacitive measurements were observed for AlScN-based HEMTs, which was correlated with a higher ideality factor and lower effective potential barrier height than the AlGaN and AlN-HEMTs. However, the reverse bias characteristics exhibited a sudden drain-current increase without a significant increase in gate-leakage current. The drain-leakage current is assumed to be related to a parasitic channel across the AlScN-barrier as a result of trap-assisted carrier transport with a Poole–Frenkel characteristic. The demonstrated pinch-off instability led to significant gain expansion in load-pull measurements and early soft-breakdown, which, in turn, limits the achievable voltage-margin. The results demonstrate a key issue to reveal the full potential of AlScN-based HEMTs for mm-wave applications.

Electrical treeing and luminescence in a polymer p-n junction under extreme reverse bias

A polymer p-n junction was subjected to an extreme reverse bias, up to 1000 V, to investigate its reverse bias characteristics and possible breakdown. A full breakdown did not take place even after more than 2 h of continuous stress. Electrical treeing, a pre-breakdown phenomenon commonly encountered in insulating polymers, was instead observed. The electrical treeing was accompanied by intense light emission, and incurred significant material loss in the polymer film, the latter which was attributed to hot electron bombardment. The impact by hot electrons also caused light emission from the luminescent conjugated polymer. The polymer junction was realized via in situ redox doping reactions in a planar solid-state light-emitting electrochemical cell made from a polymer mixed conductor. The cell was activated with a forward bias and subsequently cooled to freeze the p-n junction. The reverse bias of a doped polymer junction brings interesting opportunities to study the electrical treeing phenomenon, the luminescent conjugated polymer and the still lacking reverse-bias characteristics of an organic p-n junction.

Effects of fast and thermal neutron irradiation on Ga-polar and N-polar GaN diodes

Studies of the radiation tolerance and electrical behavior of gallium nitride (GaN) based devices are important for the next generation of high-power and high-voltage electronics that may be subjected to harsh environments such as nuclear reactor and fusion facilities, particle accelerators, and post-denotation environments. In this work, we study the behavior of Ga-polar and N-polar GaN Schottky diodes before and after exposure to fast and thermal + fast neutrons. Temperature-dependent current–voltage (I–V) and circular transmission line method (CTLM) measurements were used to study the electrical characteristics. A strong reduction in reverse leakage current and an increase in differential resistance in forward bias were observed after neutron irradiation. Thermionic emission (TE), Frenkel–Poole (FP) emission, and Fowler–Nordheim (FN) tunneling models were used to explain the forward and reverse I–V characteristics pre- and post-irradiation. The study confirms that Ga-polar and N-polar GaN Schottky diodes exhibit different electrical responses to fast and thermal neutron irradiations. The reverse bias characteristics of N-polar diodes are less affected after the fast neutron irradiation compared to Ga-polar diodes, while in the forward bias region, the electrical behavior after fast and thermal neutron irradiations is similar in Ga-polar and N-polar diodes. The results indicate that the role of orientation should be considered in the design of GaN-based radiation-tolerant electronics.

Effect of temperature on forward and reverse bias characteristics of GaN based Schottky diode

One of the major implementations of wide bandgap semiconductors is in harsh environments like in radiative and extreme temperatures, which owes to their superior physical and electrical properties. The loss of energy within the device at high power levels, in high radiation fluence environments leads to rise in its temperature. Moreover, temperature is the main parameter which infer with the performance and dependability of the semiconductor gadgets. So, the main objective of the present work is to investigate the features of forward and the reverse bias of field plate edge terminated GaN-based Schottky diodes. The features of reverse and forward bias of the device have been simulated using TCAD software at a range of temperature from 100 K to 600 K. The observed characteristics have been explained in terms of current conduction mechanism in the device.

Electrical properties of Si/diamond heterojunction diodes fabricated by using surface activated bonding

We evaluated the current–voltage ( I-V ) and temperature-dependent I-V characteristics of n + -Si/p-diamond heterojunction diodes (HDs) fabricated by using surface activated bonding and Ru/p-diamond Schottky barrier diodes (SBDs). Both types of diodes were fabricated on diamond surface that was activated using a fast atom beam of Ar. Their room-temperature reverse-bias characteristics were explained using the trap-assisted tunneling model, wherein the surface activation process affects the carrier transport across the Si/diamond and Ru/diamond interfaces. In addition, we compared the electrical properties of n + -Si/p-diamond HDs and Ru/p-diamond SBDs with those of previously fabricated p + -Si/p-diamond HDs and Al/p-diamond SBDs. A correlation between the barrier heights and the work function of the contact materials (n + -Si, p + -Si, and Ru) was observed. • We fabricated n + -Si/p-diamond heterojunction diodes (HDs) and Ru/p-diamond Schottky barrier diodes (SBDs) on a diamond surface activated using a fast atom beam (FAB) of Ar. • The reverse-bias characteristics of HDs and SBDs were explained by the trap-assisted tunneling model. • The S factors of HDs and SBDs after annealing at 703 K and 873 K were close to 0.7 and 0.6, respectively, which suggested that the partially “pinning-free” interfaces were formed thank to FAB irradiation.

Stable reverse bias or integrated bypass diode in HIP‑MWT+ solar cells

The Metal Wrap Through+ (HIP-MWT+) solar cell is based on the PERC concept but features two additional electrical contacts, namely the Schottky contact between p-type Si bulk and Ag n-contact and the metal-insulator-semiconductor (MIS) contact on the rear side of the cell below the n-contact pads. To prevent thermal hotspots under reverse bias, both contacts shall either restrict current flow or allow a homogenous current flow at low voltage. In this work we present both options. First the stable reverse bias characteristics up to −15 V with a MIS contact using industrially manufactured SiON passivation and second, an integrated by-pass diode using AlOX as insulator in the passivation stack allowing current flows at approximately Vrev = –3.5 V depending on the chosen screen-print paste. The examined Schottky contacts break down at around Vrev = –2.5 V. Reverse bias testing of the cells reveals a solid performance of the cells under reverse bias and an average conversion efficiency of η = 21.2% (AlOX) and η = 20.7% (SiON), respectively.

Exploring reverse-bias characteristics of CIGS solar cells: impact of alkali-post-deposition treatment and CdS buffer layer

The characteristics of solar cells in the reverse voltage direction are essential for the resilience of a photovoltaic module against partial-shading induced damage. Therefore, it is important to establish a thorough understanding of the mechanisms that lead to reverse breakdown in solar cells. This work studies thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) absorber layers. Systematic material variations are investigated in order to learn more about the mechanisms governing reverse breakdown in these devices. To this end, devices with different thicknesses of the CdS buffer layer and with and without a RbF-post-deposition treatment (PDT) of the absorber layer were fabricated. The resulting current-voltage characteristics at negative voltage biases reveal that devices break down at much more negative voltages if they underwent a PDT, if the buffer layer thickness is increased, or if the buffer layer is not photoexcited. This implies that possibly a PDT may be disadvantageous for the shading tolerance of a module. The further analysis indicates that several mechanisms are involved in the reverse breakdown. Whereas tunneling currents in the buffer layer seem to play a major role for the actual breakdown, the strong effect of the PDT is probably caused by a reduction of shunt leakage currents along grain boundaries which lowers material heating.

Fabrication of p+-Si/p-diamond heterojunction diodes and effects of thermal annealing on their electrical properties

We fabricate p+-Si/p-diamond heterojunction diodes by using surface-activated bonding and examine the electrical properties of Si/diamond interfaces by measuring current-voltage and current-voltage-temperature characteristics. We find that the electrical properties of heterojunction diodes are improved by post-bonding annealing at temperatures up to 873 K in terms of the ideality factor and reverse-bias current. Additionally, fabricated diodes also reveal thermal stability better than that of Cu/diamond Schottky diodes in terms of the rectification factor. The barrier height at Si/diamond bonding interfaces annealed at 873 K is estimated to be 0.55 and 0.66 eV by analyzing the relationship between the saturation current density and temperature and the reverse-bias characteristics at room temperature, respectively. These values are close to that obtained by assuming that no offset is formed in the vacuum level across the Si/diamond bonding interfaces (0.36 eV).

A comparison of electrical characteristics of Au/n-Si (MS) structures with PVC and (PVC: Sm2O3) polymer interlayer

The effects of polyvinylchloride (PVC) and samarium oxide-polyvinylchloride (PVC: Sm2O3) polymer interlayers on the electrical characteristics in detail. The fabricated reference sample Au/n-Si, Au/PVC/n-Si, and Au/(PVC: Sm2O3)/n-Si were named as Metal-Semiconductor (MS), Metal-Polymer-Semiconductor (MPS1), and MPS2 structure, respectively. The procedure of providing Sm2O3 is also described in detail. XRD, FE-SEM), EDX, and UV–vis spectroscopy, have been applied to study the mean crystalline structure, morphology, elemental characterization, and optical features of the provided Sm2O3. After structural analysis, the I-V features were performed in the wide range voltage (±3.5 V), and then, the basic electronic parameters of these structures were extracted from various techniques and compared with each other. Experimental results show that (PVC: Sm2O3) leads to an increase of barrier-height (BH), rectifying-rate shunt-resistance (R sh ), and decrease of ideality-factor (n), surface-states (N ss ). The RR of the MPS2 structure was found 117 times higher than the MS structure. The energy-dependent profile of N ss was also obtained from the forward bias I–V data by considering voltage-dependent n and BH. The plots reverse-bias characteristics show that Schottky-emission (SE) type conduction mechanism is effective for MS structure, whereas Poole-Frenkel-emission (PFE) is effective for MPS structures.

Ni Schottky barrier on heavily doped phosphorous implanted 4H-SiC

The electrical behavior of Ni Schottky barrier formed onto heavily doped (N D > 1019 cm−3) n-type phosphorous implanted silicon carbide (4H-SiC) was investigated, with a focus on the current transport mechanisms in both forward and reverse bias. The forward current–voltage characterization of Schottky diodes showed that the predominant current transport is a thermionic-field emission mechanism. On the other hand, the reverse bias characteristics could not be described by a unique mechanism. In fact, under moderate reverse bias, implantation-induced damage is responsible for the temperature increase in the leakage current, while a pure field emission mechanism is approached with bias increasing. The potential application of metal/4H-SiC contacts on heavily doped layers in real devices is discussed.

Thin-Entrance Window Process for Soft X-Ray Sensors

New free electron lasers, such as SLAC’s LCLS-II, will provide unique scientific imaging opportunities. In order to fully utilize these facilities, we need to develop detectors with shallow entrance windows that will enable detection of soft x-rays from 250 eV to 1.5 KeV. Achieving adequately shallow entrance windows is challenging because the high temperature anneal needed to activate the dopant also drives the dopant profile deeper, growing the region that is insensitive to soft x-rays. A new microwave annealing technology provides an efficient way to achieve shallow entrance windows in fully depleted high-resistivity silicon sensors. The microwave anneal technique can activate dopants at low substrate temperature, with minimal dopant diffusion, and can be used to fabricate both n-type and p-type entrance windows. SRP and SIMS measurements were used to verify dopant activation with negligible dopant diffusion. We then applied the microwave anneal process to a planar sensor wafer, using the new process to create the backside diode contact. Electrical test of the resulting sensors shows good reverse bias characteristics. The sensors have been bump-bonded to a read-out ASIC and used successfully to measure an Fe-55 x-ray spectrum. Process and device simulations were performed to characterize the quantum efficiency of the entrance window for soft x-rays. This technique is useful for other sensor applications requiring a shallow entrance window, including detectors for UV photons, low energy ions and low energy electrons.

High-temperature reverse bias characteristics of highly reliable GaN MOS-HFET

We previously proposed a GaN MOS-HFET that practically reduces the maximum electric field strength in the MOS gate to one-third under high voltage reverse bias because a JFET is placed between the MOS gate and the drain electrode. We report here that this structure significantly improves high-temperature reverse bias (HTRB) characteristics in reverse bias retention compared to our conventional GaN MOS-HFET. The proposed GaN MOS-HFET with a saturation current of more than 60 A achieved excellent HTRB characteristics with a rated voltage of 650 V at 150 °C for 1000 h, in addition to a low on-resistance of 110 mΩ and good switching characteristics of rise time (Tr)/fall time (Tf) of 5.1/6.5 ns.

Analysis of Leakage Mechanisms in AlN Nucleation Layers on p-Si and p-SOI Substrates

In this paper, the carrier transport mechanisms in AlN nucleation layers grown by MOCVD on p-Si and p-silicon-on-insulator (SOI) substrates were investigated by using electrical characterizations at various electric fields and temperatures on Ni-Au/AlN/Si metal-insulator-semiconductor capacitors. For the case of the AlN on Si substrate, different contact schemes, i.e., top-bottom versus top-top, have been compared. It was found by capacitance-voltage measurements that an inversion channel related to the Shockley-Read-Hall (SRH) minority carrier generation in the silicon bulk and SOI substrates was formed. It showed that the AlN on p-SOI platform exhibited the same leakage mechanisms as for the AlN on the Si substrate. The contact scheme mainly affects the low forward and reverse bias characteristics, by adding a surface leakage contribution. At increasing the forward (positive) gate bias, the carrier transport process becomes in turn ohmic, variable-range hopping, SRH generation, and Fowler-Nordheim (FN) direct tunneling. Moreover, at the reverse bias, the carrier transportation is in turn dominated by the Schottky emission, Poole-Frenkel (PF) conduction, and FN direct tunneling with increasing bias.

Switchable Schottky Contacts: Simultaneously Enhanced Output Current and Reduced Leakage Current.

Metal-semiconductor contacts are key components of nanoelectronics and atomic-scale integrated circuits. In these components Schottky diodes provide a low forward voltage and a very fast switching rate but suffer the drawback of a high reverse leakage current. Improvement of the reverse bias characteristics without degrading performance of the diode at positive voltages is deemed physically impossible for conventional silicon-based Schottky diodes. However, in this work we propose that this design challenge can be overcome in the organic-based diodes by utilizing reversible transitions between distinct adsorption states of organic molecules on metal surfaces. Motivated by previous experimental observations of controllable adsorption conformations of anthradithiophene on Cu(111), herein we use density functional theory simulations to demonstrate the distinct Schottky barrier heights of the two adsorption states. The higher Schottky barrier of the reverse bias induced by a chemisorbed state results in low leakage current, while the lower barrier of the forward bias induced by a physisorbed state yields a larger output current. The rectifying behaviors are further supported by nonequilibrium Green's function transport calculations.

Impact of the manufacturing process on the reverse-bias characteristics of high-efficiency n-type bifacial silicon wafer solar cells

In this paper, bifacial n-type silicon wafer solar cells with a front boron-diffused emitter and a rear phosphorus-diffused back surface field are investigated. The cell structure is abbreviated as nFAB (n-type Front and Back Contact). Three different process flows are evaluated for the fabrication of nFAB cells: (1) process flow with a silicon nitride diffusion mask deposited by plasma-enhanced chemical vapour deposition (PECVD), (2) process flow with a silicon oxynitride diffusion mask also deposited by PECVD, and (3) process flow using single-sided atmospheric pressure chemical vapour deposition (APCVD), without diffusion masks. An average batch efficiency of 21.3% was measured (front side illumination only) for all three nFAB process flows. However, the reverse bias I-V characteristics are observed to be remarkably different and dependent on the process flow, especially with regards to the masking processes. For process flow 1 that uses a silicon nitride PECVD mask, the reverse bias I-V characteristics are poor with a severe breakdown occurring at − 5 V and a reverse current of 18 A at −10 V. Process flow 2 with a silicon oxynitride PECVD mask leads to significantly improved reverse characteristics with a reverse current of 1 A at −10 V. For the APCVD process flow without diffusion masks, excellent reverse bias characteristics are observed with a reverse current of only 3 mA at −10 V. Reverse-biased electroluminescence imaging is used to determine the breakdown regions of the cells for the different process flows and to analyse the impact of the diffusion masking step on the cells’ breakdown characteristics.

Breakdown-induced conductive channel for III-nitride light-emitting devices

III-nitride semiconductor-based light-emitting diodes (LEDs) have superior physical properties, such as high thermal stability and brightness, for application to solid-state lighting sources. With the commercialization of GaN-based LEDs, improving LED reliability is important because they can be affected by electrostatic discharge, reverse leakage, and breakdown. However, research on the reverse bias characteristics of GaN-based LEDs is insufficient. We studied the reverse breakdown mechanism and demonstrated that a local breakdown can form a conductive channel in GaN-based LEDs, which can be expanded to a novel planar-type LED structure without an n-contact electrode. Furthermore, we found that this approach can be applied to AC-controllable light-emitting devices without any AC–DC converter.

Investigation of the Robust Edge Termination Applied to 6.5kV SiC MOSFET

High breakdown voltage and smaller size of edge termination are required in SiC power devices. We simulated reverse bias characteristics of a variety of edge terminations targeting 6.5 kV MOSFET and the FLR showed the best trade-off between the size and the implanted Al dose. Fabricated pn diode TEGs with a FLR demonstrated over 6.5 kV breakdown voltage. We observed the avalanche breakdown visually by light emission and it corresponded to the simulated electric field. These indicate that we can fabricate the desirable FLR for 6.5 kV MOSFET.