4H-SiC Bipolar Transistors Research Articles

Design and Characterization of an Optical 4H-SiC Bipolar Junction Transistor

In this paper, a first demonstration of the optical triggering of a 10 kV 4H-SiC Bipolar Junction Transistor is reported. A laser emitting UV (349 nm) has been used for the generation electron-hole pairs within the device. A current density of about 20 A.cm-2 has been obtained. This low value in comparison with 100 A.cm-2 for “conventional” BJT is due to the narrow pulse width (5 ns). The current waveform shows the effect of the carrier lifetime in the base and collector regions. From these measurements, we have extracted the IC (VCE) characteristics for different laser optical power and the switch-on time which is about 1 µs.

Observation of electrically detected electron nuclear double resonance in amorphous hydrogenated silicon films

We report on the electrical detection of electron nuclear double resonance (EDENDOR) through spin-dependent tunneling transport in an amorphous hydrogenated silicon thin film. EDENDOR offers a many orders of magnitude improvement over classical ENDOR and is exclusively sensitive to paramagnetic defects involved in electronic transport. We observe hyperfine interactions with 1H nuclei very close to silicon dangling bond defects. These observations substantially extend recent EDENDOR observations involving silicon vacancy defects and 14N hyperfine interactions with fairly distant nitrogen atoms in 4H-SiC bipolar junction transistors. We have improved the detection scheme utilized in the earlier study by combining magnetic field modulation with RF amplitude modulation; this combination significantly improves the operation of the automatic power leveling scheme and the overall sensitivity.

Relating Gain Degradation to Defects Production in Neutron-Irradiated 4H-SiC Transistors

Wide-gap semiconductor SiC is generally considered as a superior candidate for high-radiation applications, due to its higher displacement energy for both Si and C lattice atoms. In this work, we find that the current gain ( $\beta $ ) of 4H-SiC bipolar junction transistors (BJTs) can severely deteriorate after neutron irradiation. Deep-level transient spectroscopy (DLTS) reveals that two major carrier-killer centers, Z1/2 and EH6/7, were produced in neutron-irradiated devices. Surprisingly, we find that the shallow Z1/2 center can play a dominant role over the deep EH6/7 defects in carrier recombination under high-injection conditions, which contributes to the unusual smaller hole capture cross section than that of the electron one, as indicated by the first-principles calculations. Finally, technology computer aided design (TCAD) simulation confirms that the enhanced carrier recombination from the Z1/2 centers is responsible for the degraded performance of SiC BJTs after irradiation. Our findings not only provide a deep insight into the underlying physics for displacement damage in SiC BJTs, but also are of interest for technological applications field related to high-energy particle irradiation or implantation.

A New Technique for Analyzing Defects in Silicon Carbide Devices: Electrically Detected Electron Nuclear Double Resonance

We show that electrically detected electron nuclear double resonance (EDENDOR) can be detected with relatively high signal-to-noise ratios in fully processed 4H-SiC bipolar junction transistors (BJTs). We observe EDENDOR of nitrogen interacting with recombination center defects in the depletion region of forward-biased emitter-base junctions of these devices at room temperature. Our results indicate that EDENDOR has great potential in the investigation of SiC-based devices specifically, as well as in the investigation of solid-state devices based upon other material systems.

A 4H-SiC UV Phototransistor With Excellent Optical Gain Based on Controlled Potential Barrier

In this article, we report the experimental results of a visible-light-blind 4H-polytype silicon carbide phototransistor able to detect ultraviolet (UV) radiations for wavelengths lower than 380 nm with a significative improvement in the optical gain compared with the state-of-the-art of 4H-SiC UV phototransistors. From the electro-optical measurements, the device shows a dark current of 0.62 pA, an ON-/OFF-current ratio of seven orders of magnitude up to bias voltage of −0.5 V, and an excellent optical gain of $1.14\cdot {10}^{{{5}}}$ at 300 nm, whereas it is only $2.6\cdot {10}^{-{{3}}}$ at 400 nm demonstrating a good rejection of visible radiations. Besides having high optical gain, the phototransistor is also more sensitive than the conventional 4H-SiC UV detectors for wavelengths with low penetration depths, because its structure is designed to have the electric field up to the radiated surface where the photogenerated electron–hole pairs are efficiently swept up before recombination occurs. The operating principle of the detector is also investigated, and we experimentally proved that differently from the conventional 4H-SiC bipolar junction transistor phototransistor, it is based on the change in the potential barrier height, which controls the current flow, due to the variation in the Fermi levels when the electron–hole pairs are photogenerated. A comparison with the state-of-the-art of 4H-SiC UV phototransistors is reported.

Electrically detected electron nuclear double resonance in 4H-SiC bipolar junction transistors

We demonstrate high signal-to-noise electrically detected electron-nuclear double resonance measurements on fully processed bipolar junction transistors at room temperature. This work indicates that the unparalleled analytical power of electron-nuclear double resonance in the identification of paramagnetic point defects can be exploited in the study of defects within fully functional solid-state electronic devices.

TCAD Model Calibration of High Voltage 4H-SiC Bipolar Junction Transistors

In this project, a Technology CAD (TCAD) model has been calibrated and verified against experimental data of a 15 kV silicon carbide (SiC) bipolar junction transistor (BJT). The device structure of the high voltage BJT has been implemented in the Synopsys Sentaurus TCAD simulation platform and design of experiment simulations have been performed to extract and fine-tune device parameters and 4H-SiC material parameters to accurately reflect the 15 kV SiC BJT experimental results. The set of calibrated TCAD parameters may serve as a base for further investigations of various SiC device design and device operation in electrical circuits.

The Impact of Temperature and Switching Rate on Dynamic Transients of High-Voltage Silicon and 4H-SiC NPN BJTs: A Technology Evaluation

This paper reports the application of silicon bipolar junction transistor (BJT) modeling techniques to the modeling of dynamic behavior of high-voltage 4H-SiC BJTs, and the experimental validation thereof. High-voltage silicon BJTs are impractical due to their low current gain that requires a bulky base driver. Emergence of high-voltage 4H-SiC vertical NPN BJTs with a tenfold higher gain enables the application of efficient drivers, with ratings close to those of IGBTs. This paper demonstrates the advantages offered by 4H-SiC BJTs by means of wide-scale measurements at 800 V and 10 A in a range of temperatures up to 175 °C and adjusted base driver switching rates. This paper shows that the turn- off storage delay in the SiC BJT is two orders of magnitude lower than that of the silicon device. It also shows that the turn- on switching transients of SiC device are by an order of magnitude and the turn- off transients are by two orders of magnitude faster than that of its silicon counterpart, resulting in a tenfold reduction of the switching energy. It also demonstrates the temperature dependence of switching transients of the silicon BJT, and the relative temperature-invariance of the SiC device's performance. This paper concludes with validation of the transient models for the 4H-SiC NPN BJT, showing that the model is sufficiently accurate for transient switching and loss calculations.

15 kV-Class Implantation-Free 4H-SiC BJTs With Record High Current Gain

Implantation-free mesa-etched ultra-high-voltage (0.08 mm2) 4H-SiC bipolar junction transistors (BJTs) with record current gain of 139 are fabricated, measured, and analyzed by device simulation. High current gain is achieved by optimized surface passivation and optimal cell geometries. The area-optimized junction termination extension is utilized to obtain a high and stable breakdown voltage without ion implantation. The open-base blocking voltage of 15.8 kV at a leakage current density of 0.1 mA/cm2 is achieved. Different cell geometries (single finger, square, and hexagon cell geometries) are also compared.

Geometrical effects in JTE rings termination for 4H-SiC medium-voltage devices

Compared with a conventional junction termination extension (JTE) structure, JTE rings termination used in power devices could achieve a higher blocking performance without any extra processing. In this paper, the geometrical effects in JTE rings are analyzed and demonstrated for medium voltage applications of 4H-SiC power devices. According to the simulation results, it is found that the first space (D1) of JTE rings has a strong effect on both the maximum breakdown voltage and the optimal implantation dose window. To achieve excellent blocking characteristics, D1 should kept at around 2 μm which could be implemented conveniently by contact lithography. Furthermore, the optimized design of JTE rings is verified by experimental results of 4H-SiC bipolar junction transistors (BJTs) with drifting layer of 20 μm-thickness and doping concentration of 3 × 1015 cm−3. The measured breakdown voltages are consistent with the observations from simulation results.

Transient switch-off of a 4H-SiC bipolar transistor from the deep-saturation mode

The transient switch-off of a bipolar 4H-SiC transistor from the deep-saturation mode is studied by performing 1D numerical simulation. Switch-off in the zero base current mode and in the mode of switching-off with a negative base current is examined. It is shown that at quite real values of the switching-off base current, the switch-off time can be made ~40 times shorter than the switch-off time at zero base current. The delay time can also be made substantially (several times) shorter. It is noted that, in the deep saturation mode in which the conductivity of the collector layer is highly modulated by minority carriers, the bipolar transistor can operate in the continuous mode at a rather high current density.

Total Dose Effects on 4H-SiC Bipolar Junction Transistors

In this work, total dose effects on 4H-SiC bipolar junction transistors (BJT) are investigated. Three 4H-SiC NPN BJT chips are irradiated with 3MeV protons with a dose of 1×1011, 1×1012 and 1×1013 cm-2, respectively. From the measured reciprocal current gain it is observed that 4H-SiC NPN BJT exposed to protons suffer both displacement damage and ionization, whereas, a traditional Si BJT suffers mainly from displacement damage. Furthermore, bulk damage introduction rates for SiC BJT were extracted to be 3.3×10-15 cm2, which is an order of magnitude lower compared to reported Si values. Finally, from detailed analysis of the base current at low injection levels, it is possible to distinguish when surface recombination leakage is dominant over bulk recombination.

SiC BJT Compact DC Model With Continuous- Temperature Scalability From 300 to 773 K

The first vertical bipolar intercompany (VBIC)-based compact dc model has been developed and verified for a low-voltage 4H-SiC bipolar junction transistor to continuously map a wide temperature range from 300 to 773 K. Temperature and doping dependent physical models for bandgap, incomplete ionization, carrier mobility, and lifetime have been taken into account to give physically meaningful fitting parameters for the compact model. Isothermal simulations using the default VBIC model are performed to extract key parameter sets from measured data at seven different temperature points. Then new temperature dependent equations for the key parameters are proposed and embedded in the default VBIC model. Consequently, a single set of model parameters at 300 K is used to achieve fitting over a wide temperature range from 300 to 773 K. This new model can be used for simulating circuits that require continuous description of device dc performance over a wide temperature range.

Influence of base carrier lifetime on the characteristics of 4H-SiC BJTs

The optimum range of the base electron lifetime (τnB0) in silicon carbide (SiC) bipolar junction transistors (BJTs) is qualitatively investigated in this paper, considering both the static and dynamic performance. The TCAD simulation results show that there is tradeoff between the current gain (β) and the turn-off time (tF). With τnB0 increasing, both β and tF increase. However, with a fixed base width, the base transport factor tends toward saturation when τnB0 exceeds certain value. Thus, the eclectic values of base electron lifetime with different base thickness are obtained. The influence of the base drive current is also discussed. According to the charge control theory, the excessive storage charge caused by overdrive has a great influence on turn-off delay time (tD). To solve this, a suitable base drive current is necessary. For the common 4H-SiC BJT structure, appropriate base drive under variable base and collector doping concentration is suggested.

Modification of Etched Junction Termination Extension for the High Voltage 4H-SiC Power Devices

High voltage 4H-SiC bipolar junction transistors (BJTs) with modified etched junction termination extension (JTE) were fabricated and optimized in terms of the length (LJTE) and remaining dose (DJTE) of JTEs. It is found that for a given total termination length (Σ LJTEi), a decremental JTE length from the innermost edge to the outermost mesa edge of the device will result in better modification of the electric field. A breakdown voltage (BV) of 4.95 kV is measured for the modified device which shows ~20% improvement of the termination efficiency for no extra cost or extra process step. Equal-size BJTs by interdigitated-emitter with different number of fingers and cell pitches were fabricated. The maximum current gain of 40 is achieved for a single finger device with the emitter width of 40 µm at IC = 0.25 A (JC = 310 A/cm2) which corresponds to RON = 33 mΩ.cm2. It is presented that the current gain decreases by having more fingers while the maximum current gain is achieved at higher current density.

Transient collector modulation of 4H–SiC BJTs during switch-on process

Main physical features of the collector resistance modulation processes have been studied via a one-dimensional simulation for n+–p–n0–n+ 4H–SiC bipolar junction transistor. The motion dynamics of minority carriers (holes) across the n0 collector layer during the switch-on process is traced. It is demonstrated that the effective modulation of the collector resistance is only possible in the case of a rather fast transistor switch-on. A necessary condition for the fast switch-on is the large amplitude and short leading edge of the base current pulse.

High current gain 4H-SiC bipolar junction transistor**Project supported by the National Natural Science Foundation of China (Nos. 61306093, 61401075).

A novel 4H-SiC BJT of high current gain with a suppressing surface traps effect has been proposed. It is effective to improve the current gain due to the lower electrons density in the surface region by extending the emitter metal to overlap the passivation layer on the extrinsic base surface. The electrons trapped in the extrinsic base surface induce the degeneration of SiC BJTs device performance. By modulating the electron recombination rate, the novel structure can increase the current gain to 63.2% compared with conventional ones with the compatible process technology. Optimized sizes are an overlapped metal length of 4 μm, as well as an oxide layer thickness of 50 nm.

Tailoring the Interface between Dielectric and 4H-SiC by Ion Implantation

In this paper effects of carbon (C), silicon (Si) and nitrogen (N) implantation on the interface properties of 4H-SiC/SiO2 and the implications for 4H-SiC bipolar junction transistors (BJT) passivation are discussed. 4H-SiC epi-layer have been implanted with 12C, 14N and 28Si ion at three different doses with energies of 3, 3.5 and 6 keV, respectively, resulting in a projected range of 8 nm for the three ions. Then metal oxide semiconductor (MOS) structures with SiO2 as dielectric have been fabricated. Capacitance voltage measurements show an increase in the negative fixed charges for all the implanted samples as a function of implantation induced damage. Similarly, in the case of C and Si, the surface roughness increases as a function of dose and the mass of the ions. No reduction of Dits due to the implantations is seen for any of the ions. Furthermore, TCAD device simulations of npn bipolar junction transistors (BJT), using the interface and fixed charges extracted from CV measurements, show a way to further optimize current gain and breakdown properties for the BJT.

Improvement of Current Gain and Breakdown Voltage in 4H-SiC BJTs Employing High-k Dielectric as an Interfacial Layer

This paper proposes a new structure of 4H-SiC bipolar junction transistor, which can both achieve high current gain and high open base breakdown voltage. By introducing a groove type of metal-high k dielectric-silicon carbide (MIS) structure into the active region along the base-emitter sidewall which is formed with the process of isolation etching, a large electric field appears at the interface between high-k dielectric and bulk material by analyzing the potential distribution in forward mode, thus accelerating the electron transport. Based on a doping concentration of 4×1017cm-3 and thickness of 0.6um base region, current gain of as high as 191 is obtained using TCAD simulation, and that is almost double of the conventional structure in the same simulation setup. Furthermore, a field plate structure is composed combined with the base contact metal simultaneously, and the open base breakdown voltage is obviously increased from 634V to 948V with a 6μm-thick n-SiC collector (Nd=3×1015cm-3).

Investigation of the Breakdown Voltage in High Voltage 4H-SiC BJT with Respect to Oxide and Interface Charges

Ion implantation in silicon carbide (SiC) induces defects during the process. Implantation free processing can eliminate these problems. The junction termination extension (JTE) can also be formed without ion implantation in SiC bipolar junction transistor (BJT) using a well-controlled etching into the epitaxial base layer. The fixed charges at the SiC/SiO2 interface modify the effective dose of the JTEs, leakage current, and breakdown voltage. In this paper the influence of fixed charges (positive and negative) and also interface trap density at the SiC/SiO2 interface on the breakdown voltage in 4.5 kV 4H-SiC non-ion implanted BJT have been simulated. SiO2 as a surface passivation layer including interface traps and fixed charges has been considered in the analysis. Simulation result shows that the fixed charges influence the breakdown voltage significantly more than the interface traps. It also shows that the positive fixed charges reduce the breakdown voltage more than the negative fixed charges. The combination of interface traps and fixed charges must be considered when optimizing the breakdown voltage.