Materials Science Forum Vols. 556-557

Abstract: A novel high temperature wire bondless packaging technique is numerically investigated in this paper. Extraction of device effective resistivity with temperature from numerical characteristics of 1.2kV 4H-SiC MOSFETs at a current density of 400A/cm2 have demonstrated a T−2 temperature dependence. Electro-thermal finite element analysis (FEA) of 1.2kV 4H-SiC MOSFETs sandwiched between two etched direct-bonded-copper substrate tiles has been performed. The thermal resistance of the ceramic sandwich package shows a 75% reduction in thermal resistance compared to conventional wire bonded assemblies. Mechanical analysis of the assembly has been used to investigate the residual stresses in the SiC dies at room temperature, which are then alleviated at higher temperatures during device operation. Mismatch of the expansion coefficients of the auxiliary materials in the assembly result in elevated stresses at full load operation, however these are well below the tensile strength of the respective materials and hence do not compromise the mechanical integrity of the package.

Abstract: Both unipolar and injection SiC devices can be used for high voltage switching applications; it is not determined, however, for which applications one approach is preferred over the other. In this paper, simulation studies are used to compare the suitability of unipolar devices, in this case a JFET (Junction Field Effect Transistor) against an equivalent FCD (Field Controlled Diode) configuration up to very high voltages. The calculations are performed in a finite element approach, with commercial drift-diffusion software. Numerous drift layers have been simulated in a Monte-Carlo approach to ensure that the optimal design of the drift layers for different breakdown is used. In a static case, purely conductive losses in the drift layer in both unipolar and injection configuration are compared. Additionally the total losses are studied and compared in switched applications for different switching frequencies and current levels.

Abstract: This work utilizes silicon carbide (SiC) vertical JFETs in a cascode configuration to exploit the inherent advantages of SiC and demonstrate the device under application conditions. The all-SiC cascode circuit is made up of a low-voltage normally-off vertical JFET, and high-voltage normally on vertical JFET to form a normally-off cascode switch. In this work, a half-bridge inverter was developed with SiC cascode switches for DC to AC power conversion. The inverter uses high-side and a low-side cascode switches that are Pulse Width Modulated (PWM) from a 500 V bus to produce a 60 Hz sinusoid at the output. An inductor and a capacitor were used to filter the output, while a load resistor was used to model the steady-state current of a motor.

Abstract: We fabricated a multi-chip module of 4H-SiC reduced surface field (RESURF)-type lateral JFETs. A single chip consists of 4 unit devices of 2.0 mm × 0.5 mm in size, which were isolated electrically from each other. The multi-chip module consists of 8 chips mounted on an AMC substrate. The drain current and the breakdown voltage of the module are over 3 A and 771 V, respectively. The turn-on time and the turn-off time are 36ns and 166ns, respectively. The module resistance is proportional to the absolute temperature to the 1.05th power.

Abstract: Silicon Carbide (SiC) power devices are increasingly in demand for operations which require ambient temperature over 300°C. This paper presents circuit applications of normally-on SiC VFETs at temperatures exceeding 300°C. A DC-DC boost converter using a 4H-SiC VJFET and a SiC Schottky Diode was fabricated and operated up to 327°C. A power amplifier achieved a voltage gain of 3.88 at 27°C dropping to 3.16 at 327°C. This 20 % reduction is consistent with the fall in transconductance of the device.

Abstract: The performance and characterization of SiC JFETs and BJTs, used as inverter switching devices, in a 2 kW, high temperature, 33 kHz, 270-28 V DC-DC converter has been accomplished. SiC and Si power devices were characterized in a phase shifted H-bridge converter topology utilizing novel high temperature powdered ferrite transformer material, high temperature ceramic filter capacitors, SiC rectifiers, and 10 oz. 220oC polyimide printed circuit boards. The SiC devices were observed to provide excellent static and dynamic characteristics at temperatures up to 300oC. SiC JFETs were seen to exhibit on-resistance trends consistent with temperature-mobility kinetics and temperature invariant dynamic loss characteristics. SiC BJTs exhibited positive temperature coefficients (TCE) of VCE and negative β TCEs, with only a 2-fold increase in on-resistance at 300oC. Both SiC power devices possessed fast inductive switching characteristics with τon and τoff ~100-150 ns when driving the transformer load. The SiC converter characteristics were compared to Si-MOSFET H-bridge operation, over its functional temperature range (30-230oC), and highlights the superiority of SiC device technology for extreme environment power applications.

Abstract: The purpose of this paper is to present an all-SiC switched AC-DC converter using active power factor correction. The typical boost-converter approach is employed using continuous conduction mode. A SiC Schottky barrier diode performs the free-wheeling diode function, and a 600 V, 0.12 % SiC vertical junction field effect transistor performs the switching function under the control of a Fairchild ML4821 integrated circuit. The converter is operable off-line over the full universal voltage range (85-260 VAC), but it was optimized for a 400-600 W application operating at 208 VAC. Results are presented that demonstrate extremely high efficiency at a switching frequency of 500 kHz, the highest operating frequency of the ML4821.

Abstract: 4H-SiC p-i-n diodes were designed, fabricated and characterized for use in microwave applications. The diodes exhibited a blocking voltage of 1100 V, a 100 mA differential resistance of 1-3 &, a capacitance below 0.5 pF at a punchthrough voltage of 100 V and a carrier effective lifetime between 15-27 ns. Single 4H-SiC p-i-n diode switches, operating in X-band, exhibited insertion loss 0.7 dB, isolation up to 25 dB and were able to handle microwave power up to 2.2 kW in pulsed mode of operation. The switching speed of the switches has not exceeded 20 ns.

Abstract: In this paper, we propose new designs of Schottky, JBS and PiN diodes, which process technology is compatible with that of vertical power SiC JFETs. Three novel diode designs are proposed and we report their electrical characteristics. The P+ buried layer implant of the JFET is used for the PiN anode formation and for the P+ islands of the JBS. The Schottky diode differs from a standard Schottky diode since buried rings below the Schottky contact region have been included and the anode metal layer also contacts the buried P+ region at the diode periphery. With this last approach, the resulting Schottky diodes show low leakage currents and surge current capability, with a lower on-state voltage than the JBS.

Abstract: High voltage SiC semiconductor devices have been successfully fabricated and some of them are commercially available [1]. To achieve experimental breakdown voltage values as close as possible to the theoretical value, i.e. value of the theoretical semi-infinite diode, it is necessary to protect the periphery of the devices against premature breakdown due to locally high electric fields. Mesa structures and junction termination extension (JTE) as well as guard rings, and combinations of these techniques, have been successfully employed. Each of them has particular drawbacks. Especially, JTE are difficult to optimize in terms of impurity dose to implant, as well as in terms of geometric dimensions. This paper is a study of the spreading of the electric field at the edge of bipolar diodes protected by JTE and field rings, by optical beam induced current.