Papers by Author: Lothar Frey

Abstract: In this work, a lateral 4H-SiC n-LDMOS transistor, based on the principle of a reduced surface field due to charge compensation, is investigated by numerical simulations, in order to find adequate fabrication parameters for a lightly doped p-type epitaxy in combination with a higher doped channel region. The purpose of this work is the integration into an existing technology for a 10 V 4H-SiC-CMOS process. The simulations predict in a blocking voltage of 1.3 kV in combination with an On-resistance of 17 mΩcm² for a device with a RESURF structure with a total implanted Al concentration of 6∙10¹⁶ cm^-3 and a depth of 1 μm, a field plate of 5 μm and a drift region of 20 μm. The threshold voltage varies from 5 V to 10 V, depending on the thickness of the gate oxide (50 nm to 100 nm).

Abstract: Aluminum implanted 4H-SiC often shows an unexpected increase of the free hole density at elevated temperatures in Hall Effect measurements. Here we show that this phenomenon cannot solely be traced down to the Hall scattering factor and the presence of excited acceptor states. It is necessary to assume an additional defect center in the lower half of the band gap with ionization energies higher than that of aluminum to explain this behavior. Therefore, we investigated ion-implanted square van-der-Pauw samples with Hall Effect and complementary SIMS measurements. An analysis of the data using the neutrality equation reveals compensation ratios of 20 % to 90 %, depending on the aluminum concentration and the concentration of the deep defect center of up to 50 % of the doping.

Abstract: A comparison between self-aligned process (using lift-off) and Ni-SALICIDE used in fabrication of ohmic contacts for SiC Power MOSFET is done. Both processes are demonstrated for 3.3 kV SiC VDMOS transistors fabricated on 100 mm substrates. It is shown that the Ni-SALICIDE process with first silicidation at 500 °C does not degrade the electrical properties of silicon dioxide; particularly, a degradation of the interlayer dielectric between source and gate is not evident. Additionally, this first silicidation is found to have a positive impact on the specific resistance of contacts formed on p-type SiC using NiAl2.6% as an ohmic metal.

Abstract: In this paper we report on the performance of 4H-SiC bipolar diodes as temperature sensors far beyond 273K. The sensor is measured from 150K to 445K covering a temperature range of 295K. In this operating temperature range, the sensor characteristic V_D-T is highly linear and it is dominated by the typical dependence of the p-i-n diode voltage on the temperature. The sensor sensitivity is -4.48mV/K for a diode current of 2nA with a maximum error of 4.3K across the full temperature range. Although 4H-SiC p-i-n are mainly focused on very high temperature applications, our analysis on the performance of bipolar diodes at low temperatures highlights its feasibility as temperature sensor for aerospace and high altitude applications where cryogenic temperatures are achieved.

Abstract: In this work, the impact of a shallow aluminum channel implantation on the channel properties of SiC p-MOSFETs and digital SiC CMOS devices is investigated. For this purpose, p-MOSFETs, CMOS inverters and ring oscillators with different channel implantation doses were fabricated and electrically characterized. The threshold voltage of the resulting p-MOSFETs was shifted from-5 V to-3.6 V whereas the effective channel mobility was slightly decreased from 11.8 cm2/Vs to 10.2 cm2/Vs for a p-MOSFET channel implantation dose of 2∙10¹³ cm^-2 compared to the non-implanted channel. The resulting p-MOSFETs enable SiC CMOS logic circuits to operate with a 5 V power supply and to satisfy 5 V TTL input level specification over the whole temperature range of 25°C to 400°C. Furthermore the propagation delay time of inverters was reduced by 80% at 25°C and 40% at 400°C compared to inverters without p-MOSFET channel implantation.

Abstract: In this work, the feasibility of the Bipolar-Injection Field Effect-Transistor (BIFET) [5] in two different Dual Thyristor type circuits [4] for an application as solid-state circuit breaker (SSCB) is experimentally verified. The Dual Thyristor type circuits are assembled from discrete silicon JFETs and a silicon carbide BIFET and are electrically characterized at various temperatures. The current-voltage characteristic shows the expected regenerative self-triggered turn-off capability under over-currents and the option to control the turn-off current by a passive resistor network. The issue with the adverse positive temperature coefficient of the trigger-current can be solved by putting the SiC BIFET in a cascode arrangement with a silicon Dual Thyristor. In this configuration the SiC BIFET provides the high voltage blocking capability and the silicon Dual Thyristor with its negative temperature coefficient controls the trigger-current. Transient analyses of both circuits indicate fast switching times of less than 50 μs seconds. It is demonstrated for the first time, that the SiC BIFET, due to its normally-on behaviour, used in a Dual Thyristor type circuit is a promising concept for self-triggered fuses in high current and high voltage applications.

Abstract: The usability of 4H-SiC pin-diodes as nearly linear temperature sensors up to 800 K is demonstrated. Two sensor concepts were evaluated including the constant current forward bias (CCFB) concept and the integrated proportional to absolute temperature (PTAT) concept. The maximum sensitivity was 4.5 mV/K for the CCFB and an applied current density of 118 nA/cm². Additionally, this device can be used for UV detection, too, demonstrating the feasibility of 4H-SiC multi-sensor integration.

Abstract: In this study, the basic device features of a novel monolithically integrated solid-state-circuit-breaker (MI-SSCB) are demonstrated and analyzed using numerical simulations. Thereby, the MI-SSCB is built according to the concept of the dual thyristor. But, in comparison to similar technical solutions reported in literature, due to the advanced device structure proposed in this study a monolithically integration could be achieved for the first time.

Abstract: In this study, the potential of forward conduction loss reduction of Bipolar-Injection Field-Effect-Transistors (SiC-p-BIFET) with an intended blocking voltage of 10kV by adjusting the doping concentration in the channel-region is analyzed. For the optimization of the SiC-p-BIFET, numerical simulations were carried out. Regarding a desired turn-off voltage of approximately 25V, the optimum doping concentration in the channel-region was found to be 1.4x10¹⁷cm^-3. Based on these results, SiC-p-BIFETs were fabricated and electrically characterized in the temperature range from 25°C up to 175°C. In this study, the differential on-resistance was found to be 110mΩcm² for a temperature of 25°C and 55mΩcm² for a temperature of 175°C. In comparison to our former results, a reduction of the differential on-resistance of about 310mΩcm² at room temperature is demonstrated.

Abstract: This paper describes the fabrication, characterization, and simulation of 4H-SiC pin-photodiodes for solar UV radiation detection. The devices were produced with an aluminum implanted emitter unlike most previously published detectors which use epitaxy for all applied doping regions (see e.g. [1-3]). They were electrically characterized at different temperatures with and without UV-illumination and afterwards a spectral analysis of the photocurrent was performed. A quantum efficiency up to 55% at 260 nm will be shown. Furthermore, the capability of the diodes for visible blind sun UV monitoring e.g. within a building, is demonstrated.