SiC Bipolar Junction Transistors Research Articles

Experimental determination of intrinsic carrier density in 4H-SiC based on electron diffusion current in an npn bipolar junction transistor

The intrinsic carrier density of 4H-SiC at temperatures ranging from 294 to 595 K was derived by analyzing a collector current in an npn-type SiC bipolar junction transistor, the structure of which was designed based on a device simulation. The obtained intrinsic carrier density was in good agreement with the value calculated from the bandgap and effective densities of states taking multiple and non-parabolic SiC bands into account. The coincidence of the intrinsic carrier density obtained by these two different approaches indicates the usefulness of the proposed method and the validity of the evaluated value of intrinsic carrier density. The temperature dependence of the bandgap was also estimated from the deduced intrinsic carrier density and compared with an empirical formula. The derived bandgap agreed well with the empirical formula showing bandgap shrinkage at high temperatures. The errors in evaluating the intrinsic carrier density and the bandgap caused by the estimation of the hole density and electron mobility in the base layer are also discussed for the proposed method.

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.

Electrothermal Model of SiC Power BJT

This paper refers to the issue of modelling characteristics of SiC power bipolar junction transistor (BJT), including the self-heating phenomenon. The electrothermal model of the tested device is demonstrated and experimentally verified. The electrical model is based on the isothermal Gummel–Poon model, but several modifications were made including the improved current gain factor (β) model and the modified model of the quasi-saturation region. The accuracy of the presented model was assessed by comparison of measurement and simulation results of selected characteristics of the BT1206-AC SiC BJT manufactured by TranSiC. In this paper, a single device characterization has only been performed. The demonstrated results of research show the evident temperature impact on the transistor d.c. characteristics. A good compliance between the measured and calculated characteristics of the considered transistor is observed even in quasi-saturation mode.

Evaluation of the off-State Base-Emitter Voltage Requirement of the SiC BJT With a Regenerative Proportional Base Driver Circuit and Their Application in an Inverter

A strong candidate device for use in high-efficiency and high-density power converters is the SiC bipolar junction transistor, which requires a continuous gate (base) current to maintain its on -state. A base driver circuit with regenerative collector current feedback using a current transformer, and a negative off -state base-emitter voltage is presented in this article. The off -state base-emitter voltage required to prevent simultaneous conduction of a commercially available device when subjected to d v /d t’ s is assessed. The device is then utilized in a three-phase dc-to-ac power converter where the efficacy of using the proposed base driver is evaluated. The off -state base-emitter voltage used is informed by the d v /d t tests. The converter is supplied from a 600-V dc rail, switches at 50 kHz and supplies a 4.1-kW load at a modulation index of 0.9. An efficiency of 97.4% was measured.

Temperature Dependence of Conductivity Modulation in SiC Bipolar Junction Transistors

Temperature dependence of conductivity modulation in SiC bipolar junction transistors (BJTs) from 300 to 573 K was experimentally investigated by evaluating current–voltage characteristics in the SiC BJTs, which exhibit the clear conductivity modulation at room temperature. The ON-resistance increased with elevating the temperature because the current gain was degraded and the resistance in the collector layer (unipolar resistance) increased due to reduced mobility. However, since the carrier lifetime in the collector layer was lengthened and the base spreading resistance decreased with an increase in the temperature due to the enhanced ionization of aluminum acceptors, stronger conductivity modulation was caused than that evoked at 300 K. Thus, the ratio of the ON-resistance to the unipolar resistance was reduced with the increase in the temperature and reached only 40% at 573 K. In addition, the expansion of the conductivity-modulated region was experimentally demonstrated, which was predicted in our previous simulation work.

Demonstration of Conductivity Modulation in SiC Bipolar Junction Transistors With Reduced Base Spreading Resistance

SiC bipolar junction transistors (BJTs) were fabricated based on the design criterion proposed in our previous study, which quantitatively proved the importance of decreasing a base spreading resistance. To reduce the base spreading resistance, Al+-implantation was performed in the parasitic base region. No negative influences due to the implantation damage on the current gain were confirmed when the implantation is performed sufficiently apart from the emitter mesa sidewall, the distance of which is longer than ${3}~\mu \text{m}$ . Since the fabricated BJTs satisfied the design criterion, clear conductivity modulation was achieved, resulting in a reduced collector-resistance, that is, 50% of the unipolar resistance. In addition, we experimentally demonstrated that the conductivity modulation in SiC BJTs could be enhanced by decreasing the base spreading resistance.

Junction Temperature Measurement Method for SiC Bipolar Junction Transistor Using Base–Collector Voltage Drop at Low Current

This paper proposes an electrical method for estimation of the vertical junction temperature of silicon carbide bipolar junction transistors (SiC BJTs). This measurement method is based on measurement of the base-collector voltage (VBC) drop at a low current (VBC(low)) during the turn-off process. This voltage shows both good sensitivity and linearity with respect to temperature. The traditional temperature-sensitive electrical parameter VBE(low) (i.e., the base-emitter voltage at a low current) and an infrared camera are used to compare the characteristics of VBC(low). The results show that use of VBC(low) provides more accurate junction temperature and thermal resistance measurement results, which can then be used to extract the vertical junction temperature of the SiC BJT under test.

An Improved Proportional Base Driver for Minimizing Driver Power Consumption of SiC BJT Over Wide Current and Temperature Range

SiC bipolar junction transistor (BJT) still offers an attractive alternative to the more popular SiC MOSFET because of its several advantages, such as low fabrication cost, fast switching speed, and high temperature durability. However, it has not been widely accepted in the market partially because the large constant base current induces large power consumption on its base driver during on-state. In this paper, a new proportional base driver is proposed to provide adaptive base current for SiC BJT and thus minimize its driver power consumption when operating within a wide range of load current and operation temperature in SiC BJT-based power converters. It simply uses a silicon BJT serially connected with a current sensor as a current-/temperature-controlled current source in the base driver circuit, promptly supplying adaptive base current for SiC BJT whenever the SiC BJT’s junction temperature and/or operation current varies. The operation principle of this proportional base driver is theoretically analyzed and then experimentally verified in a 600-V/20-A SiC BJT-based dc/dc boost converter. Experimental results show that the proposed proportional base driver is capable of driving the SiC BJT much more efficiently than the GeneSiC’s constant base driver, greatly reducing the power consumption of base driver by more than 50% over wide ranges of operation currents and temperatures.

An Improved SPICE Model of SiC BJT Incorporating Surface Recombination Effect

The SiC bipolar junction transistor (BJT) offers an attractive alternative to the more popular SiC mosfet . It is important to develop an accurate SPICE model for the SiC BJT to enable its use in power electronic applications. The current gain of an SiC BJT may degrade considerably at high current levels and/or at high temperatures largely due to the surface recombination effect. In this paper, an improved SPICE behavioral model that accurately accounts for the current gain depending on the collector current and the junction temperature is proposed for the SiC BJT. In this paper, the conventional Gummel–Poon model is extended to include this important physical effect by adding a diode between the external base and emitter terminals of the BJT. A two-step model parameter extraction method is developed. First, the basic Gummel–Poon model parameters are extracted from low-current measurement data, and then, the new surface recombination model parameters are extracted by observing the difference between the measured high-level base current and the standard Gummel–Poon model prediction. The simulated static and switching characteristics of the new SiC BJT model match the measured data very well. Finally, the new SPICE model is used in the performance assessment of a proportional base driver technique embedded in a 3.6-kW boost converter, demonstrating its validity in helping with the optimum design of power electronic applications based on SiC BJTs.

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.

Monolithic Integration of SiC Power BJT and Small-Signal BJTs for Power ICs

Monolithic integration of power transistor and specific functional integrated circuits (power ICs) offers unique advantages for wide bandgap power devices. However, few publications have reported the power ICs based on SiC because of several challenging problems such as fabrication process and cost. This paper explores the monolithic integration possibility of 4H-SiC lateral small-signal bipolar junction transistors (BJTs) and vertical power BJT without extra mask or processing step other than Vanadium isolation implantation. A new monolithic power integration architecture is proposed and optimized using TCAD simulations first, and then both the vertical and lateral SiC BJT types are fabricated on the same wafer using the standard power BJT process to experimentally verify the feasibility of the monolithic integration method. The SiC power BJT demonstrates a breakdown voltage of 1320 V and a peak common-emitter current gain of 13.1 while the lateral SiC BJT demonstrated a breakdown voltage of 25 V and a peak common-emitter current gain of 3.8 at room temperature. While the device performance can be further improved, the fact that they can be successfully fabricated on the same wafer without much process modification shows a great promise of realizing monolithic SiC power ICs, which will greatly benefit the next-generation power electronics systems.

Comparison of conventional and cascode drive of SiC BJTs

This study compares simple conventional and cascode driver circuits for the SiC bipolar junction transistor (BJT). A low-voltage silicon metal-oxide-semiconductor field-effect transistor is used in the emitter of the BJT to realise the cascode variant. The circuits are experimentally evaluated when switching a current of 2.5 A and a voltage of 600 V in a buck converter.

Impacts of Finger Numbers on ON-State Characteristics in Multifinger SiC BJTs With Low Base Spreading Resistance

Impacts of finger numbers on ON-state characteristics in multifinger 10-kV-class SiC bipolar junction transistors (BJTs), whose base spreading resistance is sufficiently reduced by using aluminum ion implantation, were investigated by performing TCAD simulations. Common-emitter current–voltage characteristics of the BJTs with different base current densities and carrier lifetimes in the collector layer were analyzed. The simulation results exhibited that the BJTs with fewer finger numbers could achieve superior characteristics owing to an expansion of a conductivity-modulated region and to a higher current density per finger. In addition, we showed that BJTs with a punchthrough structure have a potential to achieve superior characteristics suitable for power device applications under a certain condition, where strong conductivity modulation occurs. The presented results indicate that the appropriate finger numbers should be designed for a better performance of the multifinger SiC BJTs.

Effects of Parasitic Region in SiC Bipolar Junction Transistors on Forced Current Gain

Effects of a parasitic region in SiC BJTs on conductivity modulation and a forced current gain (βF) were investigated by using TCAD simulation with various device structures. By introducing an Al+-implanted region below the base parasitic region, βF can be improved because the implanted region can reduce the base spreading resistance, leading to alleviation of debiasing effect. βF in devices with various parasitic areas, whose base spreading resistances were reduced by the Al+-implantation, were compared. We found that βF can be enhanced by expanding the parasitic area if the base spreading resistance is sufficiently reduced. The higher βF is attributed to an expanded conductivity-modulated region. The collector current spreading in the collector layer and the hole injection from the parasitic region as well as from the intrinsic region can play a role to evoke the conductivity modulation. Thus, the larger parasitic region can expand the conductivity-modulated region, resulting in expansion of an active area and the enhancement of βF.

A comparative study on electrical characteristics of 1-kV pnp and npn SiC bipolar junction transistors

We investigate the electrical characteristics of 1-kV pnp SiC bipolar junction transistors (BJTs) and compare them with those of npn SiC BJTs. The base resistance, current gain, and blocking capability are characterized. It is found that the base resistance of pnp SiC BJTs is two orders of magnitude lower than that of npn SiC BJTs. However, the obtained current gains are low below unity in pnp SiC BJTs, whereas npn SiC BJTs exhibit a current gain of 14 without surface passivation. The reason for the poor current gain of pnp SiC BJTs is discussed.

Design Considerations for 4H-SiC Lateral BJTs for High Temperature Logic Applications

4H-silicon carbide (SiC)-based bipolar integrated circuits (ICs) are suitable alternatives to silicon (Si)-based ICs in high temperature applications, owing to superior properties of 4H-SiC and the robust performance of SiC bipolar junction transistors (BJTs). However, cost, size, and manufacturability of 4H-SiC ICs remains inferior to the prevalent Si-based ICs, due to large footprint and high number of epilayers in conventional SiC BJTs. An alternative to overcome these limitations is to use lateral BJTs (LBJTs). Though Si LBJTs have been demonstrated, this is the first time they are explored in 4H-SiC. This paper proposes a symmetric, self-aligned 4H-SiC LBJT design, which is relatively easier and cheaper to manufacture, has fewer epilayers, and is >90% smaller than existing structures. Extensive device simulations and optimization is performed to achieve optimal current gains, at a range of temperatures (27 °C–500 °C). The results suggest current gains of over 100 in devices with base width of 1 $\mu \text{m}$ , at room temperature. The applicability of the structure is validated by designing a 4H-SiC LBJT-based emitter-coupled logic inverter, which shows stable operation and good speeds (~3 ns) up to 500 °C, while having high integration density and lower cost.

Diffusion of minority carriers against electric field (high injection level)

A one-dimensional analytic model describing the motion of minority carriers against the electric field direction under the conditions of high injection level is developed. The results of the model can also be used to estimate the motion of carriers against the field in the case of an arbitrary injection level. The model makes it possible to describe, in good agreement with the results of computer simulation, the modulation of the collector layer resistance in a high voltage SiC bipolar transistor.

A New Proportional Base Drive Technique for SiC Bipolar Junction Transistor

As one of the most attractive postsilicon power semiconductor devices, SiC bipolar junction transistor (BJT) has been studied extensively and commercialized in the past few years. However, SiC BJT has not been widely accepted in the market partially because of high driver consumption in the on-state, which is induced by a relatively large constant base current in order to ensure it is fully turned on . In this paper, a new proportional base driver with a simple circuitry is proposed in order to reduce the steady-state base driver consumption of SiC BJT. It utilizes a silicon small-signal mosfet as a variable resistor placed between the driver and the base of the SiC BJT. The collector current of the SiC BJT is measured via a Hall current sensor whose output is connected to the gate of the mosfet . When the collector current decreases, the gate voltage of the mosfet decreases and its on-state resistance increases, resulting in the decrease of the actual base driver current of the SiC BJT. The operation principle of the proposed proportional base drive technique has been verified by theoretical analysis and experimental demonstration of a 1200 V 10 A-SiC BJT-based dc–dc boost converter. Theoretical calculation, PSpice simulation, and experimental results have shown that the steady-state power consumption of the SiC BJT's proportional base driver is reduced by approximately 70% under the light load condition compared to conventional base driver with a constant base current.

Design Criterion for SiC BJTs to Avoid ON-Characteristics Degradation Due to Base Spreading Resistance

It is empirically known that the ON-resistance (voltage) of the SiC bipolar junction transistors (BJTs) with a thin-base layer is prone to be larger than the resistance of a voltage-blocking collector layer. In this paper, we explain the mechanism of this degradation of ON-characteristics by focusing on a high base spreading resistance and a parasitic diode existing below the base contact. An equivalent circuit model of the SiC BJT was proposed, and SPICE simulation was performed.In addition, TCAD simulation confirmed the validity of the model well. Based on the model, a design criterion to avoid the unwanted increase of the ON-resistance is proposed.

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.