4H-SiC Power Bipolar Junction Transistors Research Articles

Effects of base doping and carrier lifetime on differential current gain and temperature coefficient of 4H-SiC power bipolar junction transistors

4H-SiC NPN bipolar junction transistor (BJT) is studied systematically by performing two-dimensional numerical simulations. Several design issues are discussed. Depending on the doping concentration of the base and the carrier lifetimes, both positive and negative temperature coefficients in the common emitter current gain could exist in 4H-SiC NPN BJTs with aluminium-doped base. The temperature coefficients of the current gain at different base doping concentrations and different carrier lifetimes have been determined. A high base doping concentration can reduce the requirement for the carrier lifetime in order to obtain negative temperature coefficient in current gain. Device simulations are performed to evaluate the carrier lifetimes by fitting the measured output IC –VCE curves. An excellent fitting is obtained and the base electron lifetime and the emitter hole lifetime are extracted to be about 22 and 5.7 ns, respectively.

Impact of Ionizing Radiation on the $\hbox{SiO}_{2}/ \hbox{SiC}$ Interface in 4H-SiC BJTs

Degradation of SiO2 surface passivation for 4H-SiC power bipolar junction transistors (BJTs) as a result of ion irradiation has been studied to assess the radiation hardness of these devices. Fully functional BJTs with 2700 V breakdown voltage are implanted with 600 keV helium ions at fluences ranging from 1 ×1012 to 1 ×1016 cm-2 at room temperature. These ions are estimated to reach the SiO2/ SiC interface. The current-voltage characteristics before and after irradiation show that the current gain of the devices starts degrading after a helium fluence of 1×1014 cm-2 and decreases up to 20% for the highest fluence of ions. Simulations show that the helium ions induce ionization inside the SiO2, which increases the interface charge and leads to a degradation of the BJT performance. Thermal annealing of the irradiated devices at 300 °C, 420 °C, and 500 °C further increases the amount of charge at the interface, resulting in increased base current in the low-voltage range.

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.

Effect of 3.0 MeV helium implantation on electrical characteristics of 4H-SiC BJTs

Degradation of 4H-SiC power bipolar junction transistors (BJTs) under the influence of a high-energy helium ion beam was studied. Epitaxially grown npn BJTs were implanted with 3.0 MeV helium in the fluence range of 1010–1011 cm−2. The devices were characterized by their current–voltage (I–V) behaviour before and after the implantation, and the results showed a clear degradation of the output characteristics of the devices. Annealing these implanted devices increased the interface traps between passivation oxide and the semiconductor, resulting in an increase of base current in the low-voltage operation range.

Modeling and Characterization of Current Gain Versus Temperature in 4H-SiC Power BJTs

Accurate physical modeling has been developed to describe the current gain of silicon carbide (SiC) power bipolar junction transistors (BJTs), and the results have been compared with measurements. Interface traps between SiC and SiO2 have been used to model the surface recombination by changing the trap profile, capture cross section, and concentration. The best agreement with measurement is obtained using one single energy level at 1 eV above the valence band, a capture cross section of 1 × 10-5 cm2, and a trap concentration of 2 × 1012 cm-2. Simulations have been performed at different temperatures to validate the model and characterize the temperature behavior of SiC BJTs. An analysis of the carrier concentration at different collector currents has been performed in order to describe the mechanisms of the current gain fall-off at a high collector current both at room temperature and high temperatures. At room temperature, high injection in the base (which has a doping concentration of 3 × 1017 cm-3) and forward biasing of the base-collector junction occur simultaneously, causing an abrupt drop of the current gain. At higher temperatures, high injection in the base is alleviated by the higher ionization degree of the aluminum dopants, and then forward biasing of the base-collector junction is the acting mechanism for the current gain fall-off. Forward biasing of the base-collector junction can also explain the reduction of the knee current with increasing temperature by means of the negative temperature dependence of the mobility.

A Simple and Reliable Electrical Method for Measuring the Junction Temperature and Thermal Resistance of 4H-SiC Power Bipolar Junction Transistors

To determine the maximum allowed power dissipation in a power transistor, it is important to determine the relationship between junction temperature and power dissipation. This work presents a new method for measuring the junction temperature in a SiC bipolar junction transistor (BJT) that is self-heated during DC forward conduction. The method also enables extraction of the thermal resistance between junction and ambient by measurements of the junction temperature as function of DC power dissipation. The basic principle of the method is to determine the temperature dependent I-V characteristics of the transistor under pulsed conditions with negligible self-heating, and compare these results with DC measurements with self-heating. Consistent results were obtained from two independent temperature measurements using the temperature dependence of the current gain, and the temperature dependence of the base-emitter I-V characteristics, respectively.

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.

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.

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

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

Demonstration of 4H-SiC power bipolar junction transistors

The first demonstration of 4H-SiC power bipolar junction transistors is reported. The detailed device structure along with the multi-step junction termination extension design dimensions is described. The DC I–V characteristics at room temperature and 250°C of the fabricated bipolar junction transistors are measured and reported.