Reinhold Schörner

Detailed investigation of n-channel enhancement 6H-SiC MOSFETs

Basic MOSFET parameters like inversion layer mobility, threshold voltage, intrinsic mobility reduction factor and interface state density extracted from the subthreshold slope were examined in detail for 6H-SiC enhancement-mode n-channel MOSFETs. The inversion layer mobility and the threshold voltage were determined as a function of substrate doping concentration as well as device temperature. The interface state density was studied for different substrate doping concentrations. The inversion layer mobility was found to decrease strongly with increasing substrate doping. In contrast to earlier reports the inversion layer mobility decreases also with temperature. Furthermore, the threshold voltage depends more pronounced on substrate doping and temperature than theoretically expected. The interface state density extracted from the subthreshold slope increases significantly with substrate doping concentration. All these phenomena are consistently interpreted by the classical MOSFET behavior which is extended by acceptor like interface states. These states are located close to the conduction band and exhibit a density increasing drastically toward the band edge.

Interface properties of metal‐oxide‐semiconductor structures on n‐type 6H and 4H‐SiC

This work reports on the interface properties of metal‐oxide‐semiconductor (MOS) structures formed by thermal oxidation of n‐type silicon carbide (SiC). The SiC layers, grown homoepitaxially on the silicon‐face of 6H‐SiC and 4H‐SiC substrates, were oxidized at 1100 °C in dry or wet atmosphere. The interface properties of the MOS structures were investigated using both, the Terman and the high‐low frequency method. The validity of these methods for wide band‐gap semiconductors is clarified in a short theoretical analysis. The experimental results reveal moderate densities of interface states for MOS devices on 6H‐SiC as well as on 4H‐SiC. Only minor differences were observed between both polytypes. Current‐voltage measurements prove earlier results and show good quality independent of the polytype used as substrate material.

Planar aluminum-implanted 1400 V 4H silicon carbide p-n diodes with low on resistance

Planar p-n diodes with edge termination were fabricated by aluminum implantation on n-type 4H silicon carbide. These diodes exhibited an excellent blocking behavior up to 1400 V reverse voltage with stable avalanche breakdown at an electric field strength of 2.8 MV/cm. In addition, a nearly classical forward characteristic was observed with both recombination and diffusion current mechanism represented by ideality factors of 1.05 and 1.93, respectively. The turn-on voltage was 2.8 V. At a forward voltage drop of 6.2 V a current density of 4000 A/cm2 and a differential on resistance below 1 mΩ cm2 were achieved.

Enhanced channel mobility of 4H–SiC metal–oxide–semiconductor transistors fabricated with standard polycrystalline silicon technology and gate-oxide nitridation

This work presents improved channel mobility of n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) on 4H–SiC, achieved by gate-oxide nitridation in nitric oxide. Lateral enhancement mode MOSFETs were fabricated using standard polycrystalline silicon gate process and 900 °C annealing for the source and drain contacts. The low field mobility of these MOSFETs was as high as 48 cm2/Vs together with a threshold voltage of 0.6 V, while the interface state density—determined from the subthreshold slope—was about 3×1011 eV−1 cm−2. The 43-nm-thick gate oxide of coprocessed metal–oxide–semiconductor structures exhibited a breakdown field strength of 9 MV/cm.

An 1800 V triple implanted vertical 6H-SiC MOSFET

6H silicon carbide vertical power MOSFETs with a blocking voltage of 1800 V have been fabricated. Applying a novel processing scheme, n/sup +/ source regions, p-base regions and p-wells have been fabricated by three different ion implantation steps. Our SiC triple ion implanted MOSFETs have a lateral channel and a planar polysilicon gate electrode. The 1800 V blocking voltage of the devices is due to the avalanche breakdown of the reverse diode. The reverse current density is well below 200 /spl mu/A/cm/sup 2/ for drain source voltages up to 90% of the breakdown voltage. The MOSFETs are normally off showing a threshold voltage of 2.7 V. The active area of 0.48 mm/sup 2/ delivers a forward drain current of 0.3 A at Y/sub GS/=10 V and V/sub DS/=8 V. The specific on resistance was determined to 82 m/spl Omega/dcm/sup 2/ at 50 mV drain source voltage and at V/sub GS/=10 V which corresponds to an uppermost acceptable oxide field strength of about 2.7 MV/cm. This specific on resistance is an order of magnitude lower than silicon DMOSFETs of the same blocking capability could offer.

SiC power devices with low on-resistance for fast switching applications

Silicon carbide switching devices exhibit superior properties compared to silicon devices. Low specific on-resistance for high breakdown voltages is believed to be the most outstanding feature of SiC power switching devices. In this paper, MOSFETs and JFETs capable to block 1800 V with a specific on-resistance of 47 m/spl Omega/ cm/sup 2/ and 14.5 m/spl Omega/ cm/sup 2/, resp., are discussed. However, there are additional advantages making SiC devices attractive for the system designer. The authors present fast recovery of the 6H-SiC MOSFET reverse diode (Q/sub rr/ 30 nC, t/sub rr/ 20 ns) and fast switching as well as short circuit capability (1 ms) of vertical VJFETs. Finally, a short outlook to future SiC switching devices is given.

Dielectric strength of thermal oxides on 6H‐SiC and 4H‐SiC

This work reports on the dielectric strength of oxide layers formed by thermal oxidation of silicon carbide (SiC). SiC epilayers grown homoepitaxially on the silicon face of 6H‐SiC and 4H‐SiC substrates were oxidized in dry or wet ambient at 1100 °C. The dielectric strength was investigated using metal–oxide‐semiconductor capacitors and was found to be tightly bound to 10 MV/cm for oxide thicknesses around 65 nm and independent of the SiC polytype and substrate doping. Considering the current‐voltage characteristics in the prebreakdown region, dry oxides exhibit superior quality. Fowler–Nordheim tunneling was identified as the limiting current mechanism in the dry oxides. The corresponding barrier heights between the two SiC polytypes and thermal silicon dioxide were determined.

Stacked high voltage switch based on SiC VJFETs

Based on the serial connection of high voltage SiC VJFETs a stacked solution able to block very high voltages is presented. By connecting VJFETs in series, a unipolar high voltage switch with 8kV blocking voltage and an on-resistance of 2/spl Omega/ was fabricated. The basic functions of this stacked switch are analyzed by discussing the electrical behavior. The static and dynamic behavior indicates an interesting perspective for high voltage and high power applications. Especially the dynamics are carefully analyzed using a low voltage version of the stacked solution. Additionally, the potential of SiC VJFETs as a 4kV single switch is demonstrated.

Properties and Suitability of 4H-SiC Epitaxial Layers Grown at Different CVD Systems for High Voltage Applications

This work reports on properties of epitaxial grown SiC layers and their suitability for high voltage devices using cold wall and hot wall CVD systems. Differences of fundamental machine parameters like temperature gradients and flow conditions were investigated. Based on these parameters structural and electrical layer properties were analyzed and compared. The layer suitability for high voltage devices was proven using pin-diodes with Al-implanted emitters.

Switching behaviour of fast high voltage SiC pn-diodes

4H-SiC p-n diodes with an active area of 1 mm/sup 2/ and up to 3 kV blocking voltage have been fabricated, characterized and compared to simulations. The static forward characteristics demonstrate the expected forward power loss with a negative temperature coefficient. The diodes exhibit a stable avalanche breakdown, showing a small positive temperature coefficient (0.3 V/K). The turn-on switching behaviour shows a relatively small voltage overshoot as compared to silicon diodes. The turn-off resembles that of a Schottky diode. In both cases, the dynamics can be attributed to a rapid recombination of the storage charge, even under high forward injection conditions. Numerical simulations may point to a local lifetime reduction at the p-n junction.