4H-SiC Bipolar Transistors Research Articles

4H-SiC Bipolar Junction Transistors with a Current Gain of 108

4H-SiC BJTs with a common emitter current gain (b) of 108 at 25°C have been demonstrated. The high current gain was accomplished by using a base as thin as 0.25 μm. The current gain decreases at high temperatures but is still greater than 40 at 300°C. The device demonstrates an open emitter breakdown voltage (BVCBO) of 1150 V, and an open base breakdown voltage (BVCEO) of 250 V. A low specific on-resistance of 3.6 mW-cm2 at 25°C was achieved. The BJTs have shown blocking capabilities over a wide range of operating temperatures up to 300°C.

1200 V 4H-SiC BJTs with a Common Emitter Current Gain of 60 and Low On-Resistance

This paper reports a 4H-SiC bipolar junction transistor (BJT) with a breakdown voltage (BVCEO) of 1200 V, a maximum current gain (β) of 60 and the low on-resistance (Rsp_on)of 5.2 mΩcm2. The high gain is attributed to an improved surface passivation SiO2 layer which was grown in N2O ambient in a diffusion furnace. The SiC BJTs with passivation oxide grown in N2O ambient show less emitter size dependence than reference SiC BJTs, with conventional SiO2 passivation, due to a reduced surface recombination current. SiC BJT devices with an active area of 1.8 mm × 1.8 mm showed a current gain of 53 in pulsed mode and a forward voltage drop of VCE=2V at IC=15 A (JC=460 A/cm2).

1600 V, 5.1 mΩ●cm<sup>2</sup> 4H-SiC BJT with a High Current Gain of β=70

This paper reports a newly achieved best result on the common emitter current gain of 4H-SiC high power bipolar junction transistors (BJTs). A fabricated 1600 V – 15 A 4H-SiC power BJT with an active area of 1.7 mm2 shows a high DC current gain (b) of 70, when it conducts 9.8 A collector current at a base current of only 140 mA. The maximum AC current gain (DIC/DIB) is up to 78. This high performance BJT has an open base collector-to-emitter blocking voltage (VCEO) of over 1674 V with a leakage current of 1.6 μA, and a specific on-resistance (RSP-ON) of 5.1 mW.cm2 when it conducts 7.0 A (412 A/cm2) at a forward voltage drop of VCE = 2.1 V. A large area 4H-SiC BJT with a footprint of 4.1 mm x 4.1 mm has also shown a DC current gain over 50. These high-gain, high-voltage and high-current 4H-SiC BJTs further support a promising future for 4H-SiC BJT applications.

Reliability Testing of 4H-SiC Bipolar Junction Transistors in Continuous Switching Applications

Continued improvement in silicon carbide (SiC) material processing has allowed development of efficient high temperature devices which are uniquely suited to power electronics circuit designs. The 4H-SiC structure has several intrinsic characteristics that facilitate optimal speed and power handling during high temperature device operation. These characteristics include wide bandgap (3.2 eV), high dielectric breakdown (3.5 MV/cm), and high thermal conductivity (4.9 W/cm-K)[1,2]. By combining these properties, SiC bipolar junction transistors (BJTs) can achieve fast, low impedance switching at high voltages (1.2 kV). New generation devices are being developed with increased current handling capability, as well as improved forward voltage characteristics. The device considered here, along with its on-state DC characteristic, is shown in figure 1. The BJTs are approximately 5mm by 5mm, and are nominally rated for a maximum Ice of 50A. Measurements on the Tektronix 371B curve-tracer indicate current gains over 60 at 25 oC and roughly 40 at 150 oC. These results were obtained at collector currents up to 20A. The base current for BJTs is typically 300 to 800 mA, depending on device temperature and the maximum device current required. In order to meet current handling requirements of up to 80A, as required for power conversion in modern military systems such as the hybrid-electric vehicle (HEV), it is necessary to configure these devices in parallel with minimal external cooling. The resulting switching circuits must therefore be validated for operation at high temperatures (package temperatures of 90 oC, and junction temperatures to 150 oC). Validation includes characterization of the devices in clamped inductive circuits with devices configured both alone and in parallel over time. Figure 4 shows measurement waveforms obtained during continuous clamped inductive switching. The primary focus of this work is to establish the overall performance and reliability of these newer generation SiC BJTs in power conversion circuits. Failure analysis and critical performance issues, such as current sharing, energy loss, and total reverse recovery charge are addressed. The initial results of the experiments indicate that these SiC switches have the potential to perform reliably in high temperature power conversion.

Fabrication and Characterization of High-Current-Gain 4H-SiC Bipolar Junction Transistors

This paper reports on newly developed high-performance 4H-SiC bipolar junction transistors (BJT) with improved current gain and power handling capabilities based on an intentionally designed continuously grown 4H-SiC BJT wafer. The measured dc common-emitter current gain is as high as 70, the specific ON-state resistance (RSP-ON) is as low as 3.0 mOmegamiddotcm2, and the open-base breakdown voltage (VCEO) reaches 1750 V. Large-area 4H-SiC BJTs with a footprint of 4.1 times 4.1 mm have been successfully packaged into a high-gain (beta = 50.8) high-power (80 A times 700 V) all-SiC copack and evaluated at high temperature up to 250degC. Small 4H-SiC BJTs have been stress tested under a continuous collector current density of 100 A/cm2 for 24 h and, for the first time, have shown no obvious forward voltage drift and no current gain degradation. Numerical simulations and experimental results have confirmed that simultaneous high current gain and high open-base breakdown voltage could be achieved in 4H-SiC BJTs.

High-Current-Gain SiC BJTs With Regrown Extrinsic Base and Etched JTE

This paper describes successful fabrication of 4H-SiC bipolar junction transistors (BJTs) with a regrown extrinsic base layer and an etched junction termination extension (JTE). Large-area 4H-SiC BJTs measuring 1.8 times 1.8 mm (with an active area of 3.24 ) showed a common emitter current gain of 42, specific on-resistance of 9 , and open-base breakdown voltage of 1.75 kV at room temperature. The key to successful fabrication of high-current-gain SiC BJTs with a regrown extrinsic base is efficient removal of the regrown layer from the surface of the emitter-base junction. The BJT with regrown layer has the advantage of lower base contact resistivity and current gain that is less sensitive to the distance between the emitter edge and the base contact, compared to a BJT with ion-implanted base. Fabrication of BJTs without ion implantation means less lifetime-reducing defects, and in addition, the surface morphology is improved since high-temperature annealing becomes unnecessary. BJTs with flat-surface junction termination that combine etched regrown layers show about 250 V higher breakdown voltage than BJTs with only etched flat-surface JTE.

Theoretical and Experimental Analyses of Safe Operating Area (SOA) of 1200-V 4H-SiC BJT

The safe operating area (SOA) of 1200-V SiC bipolar junction transistor (BJT) is investigated by experiments and simulations. The SiC BJT is free of the second breakdown even under the turn-off power density of 3.7 MW/cm2. The theoretical boundary of reverse-biased SOA caused by the false turn-on is obtained by simulations. The short-circuit capability of the 1200-V SiC BJT is also investigated theoretically and experimentally. Self-heating is considered by the nonisothermal simulation, and 1800-K maximum local temperature is the simulated critical temperature of device failure. The surface condition is very critical for short-circuit capability. From simulations, when the interface trap density increases, the critical temperature decreases. This is believed to be the reason why the experimental results show much shorter short-circuit withstand time than the simulation showed.

Implantation-Free 4H-SiC Bipolar Junction Transistors With Double Base Epilayers

This letter reports the first 4H-SiC power bipolar junction transistor (BJT) with double base epilayers which is completely free of ion implantation and hence of implantation-induced crystal damages and high-temperature activation annealing-induced surface roughness. Based on this novel design and implantation-free process, a 4H-SiC BJT was fabricated to reach an open base collector-to-emitter blocking voltage of over 1300 V, with a common-emitter current gain up to 31. Improvements on reliability have also been observed, including less forward voltage drift (< 2%) and no significant degradation on current gain in the active region.

Surface passivation oxide effects on the current gain of 4H-SiC bipolar junction transistors

Effects of surface recombination on the common emitter current gain have been studied in 4H-silicon carbide (SiC) bipolar junction transistors (BJTs) with passivation formed by conventional dry oxidation and with passivation formed by dry oxidation in nitrous oxide (N2O) ambient. A gradual reduction of the current gain was found after removal of the passivation oxide followed by air exposure. Comparison of the measurement results for two different passivated BJTs indicates that the BJTs with passivation by dry oxidation in nitrous oxide (N2O) ambient show a half order of magnitude reduction of base current, resulting in a half order of magnitude increase of current gain at low currents. This improvement of current gain is attributed to reduced surface recombination caused by reduced interface trap densities at the base-emitter junction sidewall.

Comparison of Static and Switching Characteristics of 1200 V 4H-SiC BJT and 1200 V Si-IGBT

In this paper, static and switching characteristics of a 1200 V 4H-silicon carbide (SiC) bipolar junction transistor (BJT) at a bus voltage of 600 V are reported for the first time. Comparison was made between the SiC BJT and a 1200 V Si insulated gate bipolar transistor (IGBT). The experimental data show that the SiC BJT has much smaller conduction and switching losses than the Si IGBT. The SiC BJT also shows an extremely large reverse bias safe operation area, and no second breakdown was observed. This removes one of the most unattractive aspects of the BJT. The results prove that, unlike Si BJTs, BJTs in 4H-SiC are good competitors for Si IGBTs.

Effect of Base Impurity Concentration on DC Characteristics of Double Ion Implanted 4H-SiC BJTs

ABSTRACTDouble ion implanted 4H-SiC bipolar junction transistors (BJTs) are fabricated by Al and N ion implantation to the base and emitter. The current gain of 3 is obtained at the base Al concentration of 1 × ∼ 1017 /cm3. The collector current as a function of the base Gummel number suggests that double ion implanted 4H-SiC BJT operates in the intrinsic region below the emitter in the low injection level. The high base resistance restricts the base current at VBE as low as 3 V.

1200 V 4H-SiC Bipolar Junction Transistors with A Record β of 70

A common current gain of 70 has been achieved in 4H-SiC bipolar junction transistors (BJTs) at room temperature, which is the highest among those reported. BJTs having an active area of 4 mm × 4 mm exhibit a specific on-resistance of 6.3 mΩ cm2 at 25°C, which increases to 17.4 mΩ cm2 at 250°C. BVCEO (the breakdown voltage from collector to emitter with open base) and BVCBO (the breakdown voltage from collector to base with open emitter) of 1200 V were observed at <5 μA leakage currents at all temperatures up to 250°C. Dynamic characteristics were measured using the IXYS RF/Directed Energy IXDD415 gate driver evaluation board to drive the BJT. A collector current (I C) rise time at turn-on of 32 ns was measured with a 1.6 A gate current provided to support the collector current of 63 A. An I C fall time at turn-off of 16 ns was achieved.

Deep level defects which limit current gain in 4H SiC bipolar junction transistors

The authors employ a very sensitive electrically detected electron spin resonance technique called spin dependent recombination to observe recombination centers in fully processed 4H SiC n-p-n bipolar junction transistors. Their measurements indicate that the observed dominating recombination defect in these transistors is an intrinsic center of high symmetry, most likely a vacancy. This defect likely plays a dominating role in limiting the current gain in these 4H SiC devices.

Performance Assessment of 4H-SiC Bipolar Junction Transistors and Insulated Gate Bipolar Transistors

The performance prospects for 4H-SiC Bipolar Junction Transistors (BJTs) and Insulated Gate Bipolar Transistors (IGBTs) are theoretically evaluated. The total power dissipated (Ptotal) for both devices is calculated as a function of lifetime in the drift region and blocking voltage and used as a figure of merit to compare and contrast the effectiveness of different semiconductor materials for bipolar device applications. Assuming a maximum of 300W/cm2 for the total permissible power dissipation due to heat sink constraints we estimate an upper limit of 5kV for SiC BJT operation.

Current Gain Dependence on Emitter Width in 4H-SiC BJTs

This paper reports the fabrication of epitaxial 4H-SiC bipolar junction transistors (BJTs) with a maximum current gain β=64 and a breakdown voltage of 1100 V. The high β value is attributed to high material quality obtained after a continuous epitaxial growth of the base-emitter junction. The current gain of the BJTs increases with increasing emitter width indicating a significant influence of surface recombination. This “emitter-size” effect is in good agreement with device simulations including recombination in interface states at the etched termination of the baseemitter junction.

4H-SiC Bipolar Transistors with UHF and L-Band Operation

We report for the first time on RF SiC BJTs fabricated on semi-insulating (SI) substrates with L-band performance. Small-periphery (4x150μm) devices were tested using on-wafer load-pull measurements up to 1.5GHz. Under pulsed conditions, the devices exhibited 10dB of power gain at 1GHz and a peak power density of 2.3W/mm (1.4W) with a 100μs pulse width and a 1% duty cycle. The power gain decreased to 8dB at 1GHz under CW conditions at a power density of 1.6W/mm (1W). The load-pull measurements were performed up to 125oC, which resulted in a 1 dB reduction of power gain compared to the room temperature performance. Results at 0.5 and 1.5 GHz are presented as well.

Identification of Deep Level Defects in SiC Bipolar Junction Transistors

In this study, we report on the observation of recombination center defects in the base of 4H-SiC bipolar junction transistors. The defects are observed through a very sensitive electrically detected electron spin resonance technique called spin dependent recombination. To the best of our knowledge, these results represent the first electron spin resonance results of any kind reported in a fully processed SiC bipolar junction transistor and provide the first direct observations of the chemical and physical nature of recombination centers in SiC bipolar junction transistors. Our results clearly demonstrate the power of SDR techniques in the detection of recombination centers in SiC bipolar junction transistors.

High Temperature Characterization of 4H-SiC Bipolar Junction Transistors

This paper summarizes the recent demonstration of 3200 V, 10 A BJT devices with a high common emitter current gain of 44 in the linear region, and a specific on-resistance of 8.1 mΩ- cm2 (10 A at 0.90 V with a base current of 350 mA and an active area of 0.09 cm2). The onresistance increases to 40 mΩ-cm2 at 350°C, while the DC current gain decreases to 30. A sharp avalanche behavior was observed with a leakage current of 10 μA at a collector voltage of 3.2 kV.

4H-SiC power bipolar junction transistor with a very low specific ON-resistance of 2.9 m/spl Omega//spl middot/cm/sup 2/

This letter reports a newly achieved best result on the specific ON-resistance (R/sub SP/spl I.bar/ON/) of power 4H-SiC bipolar junction transistors (BJTs). A 4H-SiC BJT based on a 12-/spl mu/m drift layer shows a record-low specific-ON resistance of only 2.9 m/spl Omega//spl middot/cm/sup 2/, with an open-base collector-to-emitter blocking voltage (V/sub ceo/) of 757 V, and a current gain of 18.8. The active area of this 4H-SiC BJT is 0.61 mm/sup 2/, and it has a fully interdigitated design. This high-performance 4H-SiC BJT conducts up to 5.24 A at a forward voltage drop of V/sub CE/=2.5 V, corresponding to a low R/sub SP-ON/ of 2.9 m/spl Omega//spl middot/cm/sup 2/ up to J/sub c/=859 A/cm/sup 2/. This is the lowest specific ON-resistance ever reported for high-power 4H-SiC BJTs.

Analysis of Transit Times and Minority Carrier Mobility in n-p-n 4H-SiC Bipolar Junction Transistors

An analysis of the transit times and minority carrier mobility in n-p-n 4H-SiC RF bipolar junction transistors is presented. These parameters were extracted from small signal RF measurements on 4H-SiC RF transistors with three different base thicknesses: 100, 140, and 200 nm. The study shows that the room temperature minority carrier electron mobility is 215 cm/sup 2//V/spl middot/s for a base Al doping of N/sub B/=4/spl times/10/sup 18/ cm/sup -3/. The analysis reveals that the collector charging time /spl tau//sub C/ and the parasitic charging time /spl tau//sub P/ from the capacitance between metal pads and the underlying collector region have a significant effect on the transistors RF performance. The calculated RF gain is in good agreement with the measured results.