Jian Hui Zhang

High Power(500V-70A) and High Gain(44-47) 4H-SiC Bipolar Junction Transistors

This paper reports high power 4H-SiC BJTs with record high current handling capabilities. Wet-oxygen low-temperature re-oxidation and aluminum-free Ohmic contacts were applied in the fabrication processing. Up to 7A(Jc=583A/cm 2 ) with a single BJT cell has been achieved at V CE =5.5V. The peak DC common emitter current gain reached 47 at Ic=4.7A (Jc=392A/cm 2 ) and Vce=6.5V. The open base blocking voltage (Vceo) reached up to 858V with 1mA leakage current. The specific on-resistance was measured to be 8.7m Ω·cm 2 up to Ic=5.5A (Jc=458A/cm 2 ) at Vce=4.0V. At 150 o C, the peak current gain was still as high as 36 at Ic=3.6A at Vce=10V. The specific on-resistance at 150 o C was 13.2m Ω·cm 2 up to Ic=4.0A (Jc=333A/cm 2 ) at Vce=4.4V. At 100 o C, the open base blocking voltage was measured up to 875V with 1mA leakage current. A 4H-SiC BJT package containing nine similar BJT cells was also fabricated and tested. The highest collector current was measured up to 70A. The peak DC current gain was 44.3 at Ic=44.3A(Jc=410A/cm 2 ) and Vce=5.5V. The open base blocking voltage for the package at 500V showed a leakage current of 8mA. Its inductively-loaded half-bridge switching results are also reported.

1600 V, 5.1 mΩ●cm2 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.

1836 V, 4.7 mΩ•cm2 High Power 4H-SiC Bipolar Junction Transistor

This paper reports recent progress in the development of high power 4H-SiC BJTs based on an improved device design and fabrication scheme. Near theoretical limit high blocking voltage of VCEO=1,836 V has been achieved for 4H-SiC BJTs based on a drift layer of only 12 μm, doped to 6.7x1015 cm-3. The collector current measured for a single cell BJT with an active area of 0.61 mm2 is up to IC=9.87 A (JC=1618 A/cm2). The collector current is 7.64 A (JC=1252 A/cm2) at VCE=5.9 V in the saturation region, corresponding to an absolute specific on-resistance (RSP_ON) of 4.7 m9·cm2. From VCE=2.4 V to VCE= 5.8 V, the BJT has a differential RSP_ON of only 3.9 m9·cm2. The current gain is about 8.8 at Ic=5.3 A (869 A/cm2). This 4H-SiC BJT shows a V2/RSP_ON of 717 MW/cm2, which is the highest value reported to date for high-voltage and high-current 4H-SiC BJTs. A verylarge area 4H-SiC BJT with an active area of 11.3 mm2 is also demonstrated.

A High Voltage (1,750V) and High Current Gain (β=24.8) 4H-SiC Bipolar Junction Transistor using a Thin (12 μm) Drift layer

This paper reports the first, near theoretical limit breakdown voltage (1,750V) 4H-SiC BJTs with both high DC current gain(24.8) and low specific on-resistance (8.4m Ω·cm 2 at 50A/cm 2 and 12m Ω·cm 2 at 356A/cm 2 ) based on a drift layer of only 12 μm, doped to 8.5x10 15 cm -3 . The high performance is achieved through the use of an optimum single-step junction termination extension (JTE), Al-free base Ohmic contact and wet-oxygen low-temperature re-oxidation annealing. Detailed processing conditions are reported. Room temperature and high temperature current- voltage characteristics are also reported.

4H-SiC Bipolar Junction Transistors with Graded Base Doping Profile

This work reports 4H-SiC bipolar junction transistor (BJT) results based upon our first intentionally graded base BJT wafer with both base and emitter epi-layers continuously grown in the same reactor. The 4H-SiC BJTs were designed to improve the common emitter current gain through the built-in electrical fields originating from the grading of the base doping. Continuously-grown epi-layers are also believed to be the key to increasing carrier lifetime and high current gains. The 4H-SiC BJT wafer was grown in an Aixtron/Epigress VP508, a horizontal hot-wall chemical vapor deposition reactor using standard silane/propane chemistry and nitrogen and aluminum dopants. High performance 4H-SiC BJTs based on this initial non-optimized graded base doping have been demonstrated, including a 4H-SiC BJT with a DC current gain of ~33, specific on-resistance of 2.9 mcm2, and blocking voltage VCEO of over 1000 V.

10 kV, 87 mΩcm2 Normally-Off 4H-SiC Vertical Junction Field-Effect Transistors

SiC JFET, compared with SiC MOSFET, is attractive for high power, high temperature applications because it is free of gate oxide reliability issues. Trenched-and-Implanted VJFET (TIVJFET) does not require epi-regrowth and is capable of high current density. In this work we demonstrate two trenched-and-implanted normally-off 4H-SiC vertical junction field-effect transistors (TI-VJFET), based on 120μm, 4.9×1014cm-3 and 100μm, 6×1014cm-3 drift layers. The corresponding devices showed blocking voltage (VB) of 11.1kV and specific on-resistance (RSP_ON) of 124m7cm2, and VB of 10kV and RSP_ON of 87m7cm2. A record-high value for VB 2/RSP_ON of 1149MW/cm2 was achieved for normally-off SiC FETs.

A 500V, Very High Current Gain (β=1517) 4H-SiC Bipolar Darlington Transistor

This paper reports the demonstration of a record high current gain high voltage 4H-SiC hybrid bipolar Darlington transistor. The DC current gain of the bipolar Darlington was measured up to 1517 at Ic=27.3A (Jc=284A/cm 2 ) with Vce=20V at room temperature, which substantially surpasses the past record of a 500V Darlington with a DC current gain of 430. The differential specific on-resistance (RSP_ON) is 12.9mΩ⋅cm 2 up to Ic=25.6A (Jc=267A/cm 2 ) at Vce=7.0V. This high current gain SiC Darlington has a 7.8mA leakage current at a blocking voltage of 500V. At 150 o C, the Darlington still maintains a high current gain of 1015 at Ic=20.3A (Jc=211A/cm 2 ), Vce=20V, and a low leakage current of 6.5mA at 500V. The Darlington’s RSP_ON is increased to 19.0mΩ⋅cm at Ic up to 16.4A (Jc=171A/cm) at Vce=6.5V at 150C. An inductively-loaded halfbridge switching measurement (320V-50A) at RT and 150 o C is also reported. Introduction 4H-SiC has been exploited for high temperature and high power applications since late 1980’s due to its high critical field and large bandgap. 4H-SiC power bipolar junction transistors (BJTs) are gaining more and more interest in the recent years[1,2,3], partly because BJTs are free of gate oxide problems and have the potential to achieve a low on-state voltage at high current density. The main disadvantage of SiC BJTs, however, is the low current gain or the high base driving current requirement. Bipolar Darlington transistor can drastically reduce the base current requirement but at the expense of increased forward voltage drop (VF), making Darlington transistor attractive only at a relatively high voltage region. Among the best results of earlier efforts are: (i) a hybrid Darlington of 500V, >200A/cm 2 (>23A) at VCE=6.4V with a DC current gain of 430 at JC~200A/cm 2 [4], (ii) a monolithic 4H-SiC Darlington of 0.3A at VCE=7.5V with a corresponding DC current gain β~40 at JC ≥ 50A/cm 2 [5], and (iii) a hybrid Darlington of 1800V, 3.9A(278A/cm2) at VCE=6.3V with a maximum AC current gain of 500 (Estimated DC current gain is 367)[6]. This paper reports a Darlington transistor with a drastically increased current gain of β > 1517 (measurement set-up limited) at Ic=27.3A (Jc=284A/cm 2 ). Device Design and Fabrication The 4H-SiC BJT driving transistor (active area=1.2mm 2 ) and output transistors (active area=9.6mm 2 ) were fabricated on the same chip. The 4H-SiC wafer was purchased from Cree Inc. The emitter was formed by heavily-doped n-type epi-layer, with a thickness of 0.7μm. The base was a 0.8μm p-type epi-layer with a concentration of 3.0×10cm. The collector was formed by a 12μm drift layer with n-type doping of 6.0×10cm and the n-type 4H-SiC substrate. The emitter mesa was formed by inductively coupled plasma etching, and the mesa depth was 0.82μm. The base implanted region has a 5μm spacing to the emitter mesa edge. The base implantation was done by aluminum and carbon co-implantation at room temperature, and the implanted sample was annealed at 1550 o C for 30 minutes in Ar. The devices were isolated by a mesa etching of ~1.4μm into the drift layer. Device passivation included 2 hours wet thermal oxidation at 1100 o C, 1 hour Ar Materials Science Forum Vols. 457-460 (2004) pp. 1165-1168 online at http://www.scientific.net

A High Voltage (1570V) 4H-SiC Bipolar Darlington with Current Gain β>640 and Tested in a Half-Bridge Inverter up to 20A at VBus=900V

This paper reports the design, fabrication and characterization of a 4H-SiC bipolar Darlington transistor with both high common emitter current gain and high blocking voltage. The driving and output transistors were and fabricated on the same chip with a 12μm, 8.5x10cm doped drift layer. The Darlington’s driving transistor was capable of 1,600V and 5.3A with a maximum DC current gain β1=26 at a collector current IC1=3.12A (JC1=260A/cm 2 ) and VCE1=4.2V, and a specific on-resistance (RSP_ON) of 12.2mΩ⋅cm 2 for currents up to IC1=3.65A (JC1=304A/cm 2 ) and VCE1=3.7V. The output transistor can handle over 23A and a blocking voltage higher than 1600V with a peak DC current gain β2=22.3 at IC2=15.7A (JC2=262A/cm 2 ) and VCE2=4.54V, and an RSP_ON of 16.7mΩ⋅cm 2 for currents up to IC2=18A (JC2=300A/cm 2 ) and VCE2=4.1V. The maximum AC current gain of the hybrid BJT Darlington at room temperature was >640. The DC current gain at room temperature was found to increase with the collector current, up to 462 at IC=13.9A (JC2~232A/cm 2 ) and VCE2=10.6V, limited only by the measurement instrument. The Darlington can block voltages up to 1571V, conduct an IC of 14A at VF= 7.7V and has a differential RSP_ON of 16.7mΩ⋅cm at JC2 up to over 240A/cm 2 (Ic=14.4A). Inductively-loaded half-bridge inverter switching is also reported at 900V-20A. Introduction Due to its excellent material properties of high critical field and wide bandgap, 4H-SiC has been widely investigated for high temperature and high power applications. 4H-SiC power bipolar junction transistors (BJTs) are gaining increased attention in recent years partly because BJTs are free of gate oxide problems and have the potential to achieve low on-state voltage at high current density[1-5]. The main disadvantage of SiC BJTs, however, is the low current gain or the high base driving current requirement. Darlington transistor can drastically reduce the base current requirement but at the expense of increased forward voltage drop (VF), making Darlington transistors attractive only at a relatively high voltage region. Among the best results of earlier efforts are (i) a monolithic 4H-SiC Darlington of Ic=0.3A at VCE=7.5V with a corresponding DC current gain β~40 at JC ≥ 50A/cm 2 [6], (ii) a hybrid Darlington of 500V, >200A/cm 2 (>23A) at VCE=6.4V with a DC current gain of 430 at JC~200A/cm 2 [7], and (iii) a hybrid Darlington of 3.9A(278A/cm 2 ) at VCE=6.3V with a maximum AC current gain of 500 and an estimated DC current gain of 367[8]. This paper reports a 4H-SiC high power(1,570V-14A) hybrid Darlington with an AC current gain of 640 and a DC current gain >462 (limited by measurement instrument). Design and Fabrication The 4H-SiC BJT driving transistor (active area=1.2mm 2 ) and output transistors (active area=6.0 mm 2 ) were fabricated on the same chip. The 4H-SiC wafer was purchased from Cree Inc. The emitter n-type epi-layer is 0.8μm, doped to 2×10cm. The base is a 1.0μm p-type epi-layer with a Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 1169-1172 doi:10.4028/www.scientific.net/MSF.457-460.1169

High Voltage 4H-SiC BJTs with Deep Mesa Edge Termination

This paper reports our recent study on 4H-SiC power bipolar junction transistors (BJTs) with deep mesa edge termination. 1200 V – 10 A 4H-SiC power BJTs with an active area of 4.64 mm2 have been demonstrated using deep mesa for direct edge termination and device isolation. The BJT’s DC current gain () is about 37, and the specific on-resistance (RSP-ON) is ~ 3.0 m-cm2. The BJT fabrication is substantially simplified and an overall 10% reduction in the device area is achieved compared to the multi-step JTE-based SiC-BJTs.

Work Hardening and Flow Softening of γ-TiAl Containing Ni

This paper presents the true stress - strain curves and data analyses of a Ni-containing TiAl and its reference alloy based on the isothermal compression tests at 1000°C and 0.01 - 1.0s-1 strain rates. The results show that the minor Ni addition makes the flow softening coming sooner and therefore significantly lowers the peak stress. Those effects, in addition with a better balance between the work hardening and flow softening during hot deformation, improve the steady state flow behavior of TiAl. The Ni-influence mechanisms are also suggested based on the TEM observation of dislocation configurations and lamellar breakdown during the deformation.