Common-base Current Gain Research Articles

Effects of Current Filaments on IGBT Avalanche Robustness: A Simulation Study

With the increase in voltage level and current capacity of the insulated gate bipolar transistor (IGBT), the avalanche effect has become an important factor limiting the safe operating area (SOA) of the device. The hole injection into the p/n junction on the backside of the IGBT after avalanche is the main feature that distinguishes the avalanche effect from other devices. In this paper, the avalanche breakdown characteristics of IGBT and the nature of current filament are investigated using theoretical analysis and numerical simulation, and the underlying physical mechanism controlling the nature of current filaments is revealed. The results show that the hole injection on the backside of the IGBT leads to an additional negative differential resistance (NDR) branch on the avalanche breakdown curve. The device’s common-base current gain, αpnp, is a crucial factor in determining the current filament. As αpnp increases, the avalanche-induced current filament becomes stronger and slower, resulting in weaker avalanche robustness of the device.

Study on avalanche ruggedness of high voltage IGBTs

With the increase of IGBT voltage and current ratings, the avalanche effect has become an important factor limiting the safe operating area (SOA) of the device. The hole injection of the p+n junction on the back of the device when the avalanche effect occurs is the main feature that distinguishes the avalanche effect of the IGBT from other devices. In this paper, the avalanche breakdown characteristics of IGBT and the behavior of avalanche-generated current filaments are studied through theoretical analysis and numerical simulation, and the physical mechanism dominating the behavior of avalanche-generated current filaments is revealed. The results show that the hole injection on the backside of IGBT leads to an additional negative differential resistance branch on the avalanche breakdown curve, and the strength of the negative differential resistance effect depends on the common base current gain of IGBT α<sub>pnp</sub>. With the increase of α<sub>pnp</sub>, the negative differential resistance effect becomes stronger, the avalanche current at the valley point where the additional negative differential resistance branch transforms into the positive differential resistance branch also becomes higher. And the valley point at the avalanche breakdown curve of IGBT dominates the strength of the avalanche-generated filament. As a result, the strength of avalanche-generated filament depends on the α<sub>pnp</sub>. With the lattice temperature increasing, the avalanche breakdown voltage of IGBT increases, leading to the shifting of the avalanche breakdown curve towards a higher voltage. And with the increase of α<sub>pnp</sub>, the offset of the avalanche breakdown curves at high and low temperature becomes smaller, which dominates the lateral movement speed of the avalanche-generated filament. With the increase of the α<sub>pnp</sub> and the decrease of the offset of avalanche breakdown curves at high and low temperature, the avalanche-generated filament laterally moves more slowly. To sum up, with the increase of the α<sub>pnp</sub> of IGBT, the avalanche-generated filament becomes stronger and moves more slowly, which extremely causes the local overheating where the filaments exist and lowers the avalanche robustness of the device. Therefore, the α<sub>pnp</sub> of IGBT must be controlled precisely in order to have a good trade-off between the characteristics and the reliability of IGBT.

A silicon-graphene-silicon transistor with an improved current gain

In history, semiconductor-metal-semiconductor transistor (SMST) was proposed for frequency improvement. However, a general fabrication method is still missing due to the unsolved technological problem of deposition of a general crystalline semiconductor on metal, and a thinner metal base is also difficult to be fabricated with high quality. Recently, due to the atomic thickness of graphene, the concept of semiconductor-graphene-semiconductor transistor (SGST) has emerged which leads to the renaissance of SMST, however the experimental study is in its infancy. In this letter, SMST and SGST are fabricated using Si membrane transfer. It is found the common base current gain can be improved from about 0.5% in a Si-Au-Si transistor to about 1% in a Si-Gr-Ge one, and to above 10% in a Si-Gr-Si one, which is attributed to both the ultra-thin thickness and the quantum capacitance effect of graphene.

High-Frequency Graphene Base Hot-Electron Transistor.

The integration of graphene and other two-dimensional (2D) materials with existing silicon semiconductor technology is highly desirable. This is due to the diverse advantages and potential applications brought about by the consequent miniaturization of the resulting electronic devices. Nevertheless, such devices that can operate at very high frequencies for high-speed applications are eminently preferred. In this work, we demonstrate a vertical graphene base hot-electron transistor that performs in the radio frequency regime. Our device exhibits a relatively high current density (∼200 A/cm2), high common base current gain (α* ∼ 99.2%), and moderate common emitter current gain (β* ∼ 2.7) at room temperature with an intrinsic current gain cutoff frequency of around 65 GHz. Furthermore, cutoff frequency can be tuned from 54 to 65 GHz by varying the collector-base bias. We anticipate that this proposed transistor design, built by the integrated 2D material and silicon semiconductor technology, can be a potential candidate to realize extra fast radio frequency tunneling hot-carrier electronics.

Bipolar Transistor Test Structures for Extracting Minority Carrier Lifetime in IGBTs

Vertical PNP bipolar transistor test structures were fabricated and measured, attempting to electrically obtain information on carrier lifetime in the voltage-supporting base region of Insulated Gate Bipolar Transistors (IGBTs). Owing to the structural similarity, the test structures and functional IGBTs can be integrated on the same wafers, making it possible to directly correlate lifetime data and IGBT characteristics. To solve a problem of leaky backside PN junction, common base current gain of the test devices was measured without applying a reverse bias between the collector and base terminals, which suppressed the leakage to an acceptable level. A simple analytical formula to convert the current gain to hole lifetime in the N-type base region was proposed and used, that takes into account the existence of a commonly used N-buffer layer adjacent to the backside P-collector layer. The validity of the formula was confirmed using TCAD simulations. This method was applied to IGBT wafers with two different wafer thicknesses (i.e., base lengths): $120~{{\mu }}\text{m}$ and $360~{{\mu }}\text{m}$ . Consistent lifetime values extracted in spite of the largely different thicknesses supports the validity of the proposed lifetime estimation method.

Graphene Base Transistors With Bilayer Tunnel Barriers: Performance Evaluation and Design Guidelines

Graphene-based capacitors and Graphene base transistors (GBTs) featuring innovative engineered tunnel barriers are characterized in DC and the data are thoroughly analyzed by means of an electrical model and a Monte Carlo transport simulator. Following model calibration on experiments, we then propose strategies to improve the DC common-base current gain and the cutoff frequency of GBTs. The DC and RF performance of optimized GBT structures based on realistic technology data are analyzed in detail to highlight advantages and potential limits of this device concept.

Simulation of Graphene Base Transistors With Bilayer Tunnel Oxide Barrier: Model Calibration and Performance Projection

A simulation study of the graphene base transistor is presented based on the most recent experimental results involving a novel double-layer emitter-to-base insulator made of TmSiO and TiO2. The simulations are based on a 1-D quantum transport model with the effective mass approximation, and reproduce well the experiments both in terms of current levels and common-base current gain $\alpha $ . Performance projections are then investigated: with transparent graphene, the cutoff frequency is predicted to increase from 1 to 100 MHz by reducing the thickness of TmSiO from 1 to 0.5 nm. In order to go beyond this limit toward the terahertz range, a material with a lower barrier height than TmSiO should be considered. A technological breakthrough boosting the graphene interface quality is also needed in order to obtain acceptable values of $\alpha $ , which are still low.

Backscattering and common-base current gain of the Graphene Base Transistor (GBT)

In this paper, we investigate electron transport and electron scattering in the insulators of the Graphene Base Transistor (GBT) by means of a Monte Carlo transport model. We focus on electron backscattering in the base-collector insulator as the possible root cause of the large experimental base current and small measured common-base current gain (αF) of GBTs. Different GBT structures have been simulated and the impact of the scattering parameters on the base current is analyzed. Simulated backscattering-limited αF values are found to be much higher than available experimental data, suggesting that state-of-the-art technology is still far from being optimized. However, those simulated αF values can be low enough to limit the maximum achievable GBT performance.

Simulation of DC and RF Performance of the Graphene Base Transistor

We examined the DC and RF performance of the graphene base transistor (GBT) in the ideal limit of unity common base current gain. To this purpose, we developed a model to calculate the current-voltage characteristics of GBTs with semiconductor or metal emitter taking into account space charge effects in the emitter-base and base-collector dielectrics that distort the potential profile and limit the upper value of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(f_T\) </tex-math></inline-formula> . Model predictions are compared with available experiments. We show that, in spite of space charge high current effects, optimized GBT designs still hold the promise to achieve intrinsic cutoff frequency in the terahertz region, provided that an appropriate set of dielectric and emitter materials is chosen.

Growth, Electrical Characterization, and Electroluminescence of GaN/SiC Heterojunction Diodes and Bipolar Transistors Fabricated on SiC Off-Axis Substrates

The growth, electrical characterization, and electroluminescence (EL) of GaN/SiC heterojunction bipolar transistors (HBTs) are presented. GaN grown on off-axis SiC by molecular beam epitaxy showed step bunching owing to the large off-angle of SiC substrates, which contributed to the annihilation of edge dislocations. We investigated the impact of base doping concentration and SiC polytype (4H and 6H) on the characteristics of GaN/SiC heterojunction diodes. By utilizing a reduced doping concentration of 1×1018 cm-3 instead of 1×1019 cm-3, we suppressed the tunneling current via interface traps, resulting in an improved rectifying behavior in the diodes. Capacitance–voltage (C–V) and EL characteristics revealed that the band lineup of GaN/SiC is of type II, and 6H-SiC is better for electron injection. In accordance with diode characteristics, the fabricated GaN/SiC HBTs showed an improved common-base current gain of 0.03 by employing a reduced base doping concentration of 1×1018 cm-3 and 6H-SiC, whereas a current gain below 1×10-4 was obtained in the HBTs with a base doping concentration of 1×1019 cm-3.

Performance of hybrid p-type vertical transistors with poly(N-vinylcarbazole) as emitter and the transfer mechanism of charge carriers through the base

We report hybrid vertical architecture p-type transistors with poly(N-vinylcarbazole) as the emitter, p-type silicon as the collector and Al:Ca alloy layer as the base. The investigation of the common-base and common-emitter characteristics clearly demonstrates that the devices operate as permeable-base transistors (PBTs). The PBTs show common-base current gain α of 0.98 at −VBC = 1.5 V and common-emitter gain β of over 100. Atomic force microscope images of the base layer show an uneven surface, showing that the annealing does not dissolve the charge trap states but offers ‘pinholes’ for the oxidation in-depth even through the whole base layer. In this case, the charge carriers must tunnel the thin oxidized layer, and then are collected. It is clearly seen that there exists a barrier against holes injection from the base to the collector semiconductor at the interface, and the further oxidation caused by exposing the devices in air changes the operational mode of the resulting devices from the PBT to the metal-base transistor.

Vertical Graphene-Base Hot-Electron Transistor

We demonstrate vertical graphene-base hot-electron transistors (GB-HETs) with a variety of structures and material parameters. Our GB-HETs exhibit a current saturation with a high current on-off ratio (>10(5)), which results from both the vertical transport of hot electrons across the ultrathin graphene base and the filtering of hot electrons through a built-in energy barrier. The influences of the materials and their thicknesses used for the tunneling and filtering barriers on the common-base current gain α are studied. The optimization of the SiO2 thickness and using HfO2 as the filtering barrier significantly improves the common-base current gain α by more than 2 orders of magnitude. The results demonstrate that GB-HETs have a great potential for high-frequency, high-speed, and high-density integrated circuits.

Demonstration of Lateral IGBTs in 4H-SiC

Lateral insulated gate bipolar transistors (IGBTs) in 4H-SiC are demonstrated for the first time. The devices were fabricated based on three different designs to investigate the effects of buffer doping and carrier lifetime on device performance. Experimental results show that, with a lightly doped buffer, a short drift region length, and an improved carrier lifetime, the common base current gain of the parasitic bipolar junction transistor (BJT) is improved, leading to a higher current capability of the IGBT. The differential on-resistance of the lateral IGBT with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ld</i> = 20 μm is smaller than that of a lateral MOSFET counterpart, implying partial conductivity modulation in the drift region.

Fabrication and characteristics of permeable-base organic transistors based on co-evaporated pentacene:Al base

In this letter, fabrication and characteristics of a p-type permeable-base organic transistor are reported. A hole-type organic semiconductor pentacene is used as emitter and collector. The permeable pentacene:Al base is formed by co-evaporating pentacene and Al in vacuum. The device performance has shown to depend on the evaporation rate of pentacene in the pentacene:Al co-evaporation, where the evaporation rate of Al is controlled at 1 nm/s. Controlling the evaporation rate of pentacene at 0.1 nm/s in the pentacene:Al co-evaporation, the device shows the optimal common-base current gain of near 1 and common-emitter current gain of ∼137.

Improved Current Gain in GaN/SiC Heterojunction Bipolar Transistors by Insertion of Ultra-Thin AlN Layer at Emitter-Junction

GaN/SiC Heterojunction Bipolar Transistors (HBTs) with ultra-thin AlN insertion layers at the n-GaN/p-SiC emitter junction are proposed to improve carrier injection efficiency. The current-voltage characteristics of n-GaN/AlN/p-SiC heterojunctions have exhibited very small reverse leakage and good rectification. The capacitance-voltage measurement have revealed that the conduction band offset between n-GaN and p-SiC has been reduced from -0.74 eV to -0.54 eV by insertion of AlN, indicating that the GaN/AlN/SiC heterojunction may show better electron-injection efficiency. A significantly improved common-base current gain (α~0.2) is obtained for GaN/AlN/SiC HBTs with initial N* pre-irradiation, while it was very low (α~0.001) for GaN/SiC HBTs without AlN layers.

Vertical structure permeable-base hybrid transistors based on multilayered metal base for stable electrical characteristics

In this work we present a permeable-base transistor consisting of a 60 nm thick N, N′-diphenyl- N, N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine layer or a 40 nm thick 2,6-diphenyl-indenofluorene layer as the emitter, a Ca/Al/Ca multilayer as the metal base, and p-Si as collector. In the base, the Ca layers are 5 nm thick and the Al layer was varied between 10 and 40 nm, the best results obtained with a 20 nm thick layer. The devices present common-base current gain with both organic layer and silicon acting as emitter, but there is only observable common-emitter current gain when the organic semiconductor acts as emitter. The obtained common-emitter current gain, ∼2, is independent on collector-emitter voltage, base current and organic emitter in a reasonable wide interval. Air exposure or annealing of the base is necessary to achieve these characteristics, indicating that an oxide layer is beneficial to proper device operation.

Vertical structure p-type permeable metal-base organic transistors based on N,N′-diphentyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine

In this article, vertical structure p-type permeable-base organic transistors were proposed and demonstrated. A hole-type organic semiconductor N,N′-diphentyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine was used as emitter and collector. In the permeable-base transistors, the metal base was formed by firstly coevaporating Al and Ca in vacuum and then annealing at 120°C for 5min in air, followed by a thin Al deposition. These devices show a common-base current gain of near 1.0 and a common-emitter current gain of ∼270.

High gain in hybrid transistors with vanadium oxide/tris(8-hydroxyquinoline) aluminum emitter

We report the electrical characterization of hybrid permeable-base transistors with tris(8-hydroxyquinoline) aluminum as emitter layer. These transistors were constructed presenting an Al/n-Si/Au/Alq(3)/V2O5/Al structure. We investigate the influence of the V2O5 layer thickness and demonstrate that these devices present high common-base and common-emitter current gain, and can be operated at very low driving voltages, lower than 1 V, in both, common-base and common-emitter modes.

High current density tris(8-hydroxyquinoline) aluminum-based hybrid transistor in vertical architecture

We report the construction of hybrid permeable-base transistors, in vertical architecture, using tris(8-hydroxyquinoline) aluminum as emitter, a thin gold layer as base, and n-type silicon as collector. These transistors present high common-base current gain, can be operated at low driving voltages, and allow high current density.

Pseudo-metal-base transistor with high gain

We use evaporated C60 fullerene as emitter, a conducting polymer blend as base, and Si as collector in a vertical transistor structure similar to a metal-base transistor. The conducting polymer blend used as a base is poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate). The measured common-base current gain of our pseudo-metal-base transistor (p-MBT) is close to 1.0. The p-MBT is straightforward to fabricate and is compatible with conventional Si-based electronics.