Benedetto Buono

Surface-Passivation Effects on the Performance of 4H-SiC BJTs

In this brief, the electrical performance in terms of maximum current gain and breakdown voltage is compared experimentally and by device simulation for 4H-SiC BJTs passivated with different surface-passivation layers. Variation in bipolar junction transistor (BJT) performance has been correlated to densities of interface traps and fixed oxide charge, as evaluated through MOS capacitors. Six different methods were used to fabricate SiO2 surface passivation on BJT samples from the same wafer. The highest current gain was obtained for plasma-deposited SiO2 which was annealed in N2O ambient at 1100°C for 3 h. Variations in breakdown voltage for different surface passivations were also found, and this was attributed to differences in fixed oxide charge that can affect the optimum dose of the high-voltage junction-termination extension (JTE). The dependence of breakdown voltage on the dose was also evaluated through nonimplanted BJTs with etched JTE.

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.

High-Voltage 4H-SiC PiN Diodes With Etched Junction Termination Extension

Implantation-free mesa-etched 4H-SiC PiN diodes with a near-ideal breakdown voltage of 4.3 kV (about 80% of the theoretical value) were fabricated, measured, and analyzed by device simulation and optical imaging measurements at breakdown. The key step in achieving a high breakdown voltage is a controlled etching into the epitaxially grown p-doped anode layer to reach an optimum dopant dose of ~ 1.2 times 1013 cm-2 in the junction termination extension (JTE). Electroluminescence revealed a localized avalanche breakdown that is in good agreement with device simulation. A comparison of diodes with single- and double-zone etched JTEs shows a higher breakdown voltage and a less sensitivity to varying processing conditions for diodes with a two-zone JTE.

Fabrication of 2700-V 12-

High-voltage blocking (2.7-kV) implantation-free SiC bipolar junction transistors with low ON-state resistance (12 mOmegaldrcm2) and high common-emitter current gain of 50 have been fabricated. A graded-base doping was implemented to provide a low-resistive ohmic contact to the epitaxial base. This design features a fully depleted base layer close to the breakdown voltage providing an efficient epitaxial JTE without ion implantation. Eliminating all ion implantation steps in this approach is beneficial for avoiding high-temperature dopant activation annealing and for avoiding generation of lifetime-killing defects that reduce the current gain.

\hbox{m}\Omega \cdot \hbox{cm}^{2}

In this paper, implantation-free 4H-SiC bipolar junction transistors (BJTs) with a high breakdown voltage of 2800 V have been fabricated by utilizing a controlled two-step etched junction-termination extension in the epitaxial base layer. The small-area device shows a maximum direct-current (dc) gain of 55 at J_C = 0.33 A (J_C = 825 A/cm²) and V_CESAT = 1.05 V at I_c = 0.107 A that corresponds to a low specific ON-state resistance of 4 mΩ · cm². The large-area device has a maximum dc gain of 52 at J_C = 9.36 A (J_C = 289 A/cm²) and V_CESAT = 1-14 V at I_c = 5 A that corresponds to a specific ON-state resistance of 6.8 mΩ · cm². In addition, these devices demonstrate a negative temperature coefficient of the current gain (β = 26 at 200 °C) and a positive temperature coefficient of the specific ON-state resistance (R_ON = 10.2 mΩ · cm² at 200 °C). The small-area BJT shows no bipolar degradation and a low-current-gain degradation after a 150-h stress of the base-emitter diode with a current level of 0.2 A (J_E = 500 A/cm²). Furthermore, the large-area BJT shows a V_CE fall time of 18 ns during turn-on and a V_CE rise time of 10 ns during turn-off for 400-V switching characteristics.

Non Ion-Implanted 4H- SiC BJTs With Common-Emitter Current Gain of 50

The influence of the emitter-base geometry on the current gain has been investigated by means of measurements and simulations. Particular attention has been placed on the emitter width and on the distance between the emitter edge and the base contact. When the emitter width is decreased from 40 to 8 m, the current gain is reduced by 20%, whereas when the distance between the base contact and the emitter edge is decreased from 5 to 2 m, the current gain is reduced by 10%. Simulations have been used to investigate the reasons for the current gain reduction. The reduction of the emitter width induces two mechanisms of current gain reduction: earlier forward biasing of the base-collector junction and higher recombination in the emitter region. Both mechanisms result from the higher current density flowing under the emitter region. Placing the base contact very close to the emitter edge increases the base current by increasing the gradient of the electron concentration toward the base contact. The effect of increasing the base doping in the extrinsic region has been simulated, and the results demonstrate that the current gain can be improved if a high doping concentration in the range of 5 × 1018 cm-3 is used.

High-Voltage (2.8 kV) Implantation-Free 4H-SiC BJTs With Long-Term Stability of the Current Gain

The on-resistance of silicon carbide bipolar transistors is characterized and simulated. Output characteristics are compared at different base currents and different temperatures in order to validate the physical model parameters. A good agreement is obtained, and the key factors, which limit the improvement of RON, are identified. Surface recombination and material quality play an important role in improving device performances, but the device design is also crucial. Based on simulation results, a design that can enhance the conductivity modulation in the lowly doped drift region is proposed. By increasing the base doping in the extrinsic region, it is possible to meet the requirements of having low voltage drop, high current density, and satisfactory forced current gain. According to simulation results, if the doping is 5 ×1018 cm-3, it is possible to conduct 200 A/cm2 at VCE = 1 V by having a forced current gain of about 8, which represents a large improvement, compared with the simulated value of only one in the standard design.

Influence of Emitter Width and Emitter–Base Distance on the Current Gain in 4H-SiC Power BJTs

SiC BJTs are very attractive for high power application, but long term stability is still problematic and it could prohibit commercial production of these devices. The aim of this paper is to investigate the current gain degradation in BJTs with no significant degradation of the on-resistance. Electrical measurements and simulations have been used to characterize the behavior of the BJT during the stress test. Current gain degradation occurs, the gain drops from 58 before stress to 43 after 40 hours, and, moreover, the knee current shows fluctuations in its value during the first 20 hours. Current gain degradation has been attributed to increased interface traps or reduced lifetime in the base-emitter region or small stacking faults in the base-emitter region, while fluctuations of the knee current might be due to stacking faults in the collector region.

Modeling and Characterization of the on-Resistance in 4H-SiC Power BJTs

Non ion-implantation mesa etched 4H-SiC BJT with three-zone JTE of optimized lengths and doses (descending sequences) has been simulated. This design presents an efficient electric field distribution along the device. The device area has been optimized and considerably reduced. As a result of this comprehensive optimization, a high breakdown voltage and high current gain have been achieved; meanwhile the device area with a constant emitter and base contact area has been reduced by about 30%.

Current Gain Degradation in 4H-SiC Power BJTs

In this work, implantation-free 4H-SiC bipolar transistors with two-zone etched-JTE and improved surface passivation are fabricated. This design provides a stable open-base breakdown voltage of 2.8 kV which is about 75% of the parallel plane breakdown voltage. The small area devices shows a maximum dc current gain of 55 at Ic=0.33 A (JC=825 A/cm2) and VCESAT = 1.05 V at Ic = 0.107 A that corresponds to a low ON-resistance of 4 mΩ•cm2. The large area device shows a maximum dc current gain of 52 at Ic = 9.36 A (JC=312 A/cm2) and VCESAT = 1.14 V at Ic = 5 A that corresponds to an ON-resistance of 6.8 mΩ•cm2. Also these devices demonstrate a negative temperature coefficient of the current gain (β=26 at 200°C) and positive temperature coefficient of the ON-resistance (RON = 10.2 mΩ•cm2).