The radiation damage of 4H-SiC Metal Semiconductor Field Effect Transistors (MESFETs) due to gamma rays was studied. The threshold voltage and Schottky property of gate contact varied only slightly after absorbed dose of 10.4 MGy(SiC). In addition, the transient response of charge collection was studied by using transient ion beam induced current (TIBIC) system. It was found that the collected charge was several orders of magnitudes higher than the charge induced in SiC by direct ionization. The most likely explanation of the enhanced charge collection was the bipolar and the channel modulation effects.

Transient Response of Charge Collection by Single Ion Strike in 4H-SiC MESFETs

Characterization of SiC MESFETs with narrow channel layer

The drain-induced barrier lowering (DIBL) effect in 4H-SiC MESFETs has been studied using the physical drift and diffusion model. Our simulation results showed that the high drain voltage typically applied in short-channel 4H-SiC MESFETs could substantially reduce the channel barrier and result in large threshold voltage shift. It is also found that the DIBL effect is more dependent on the ratio of the gate length to channel thickness (Lg/a), rather than the channel thickness itself. In order to minimize the DIBL effect, the ratio of Lg/a should be kept greater than 3 for practical 4H-SiC MESFETs.

Physical Simulation of Drain-Induced Barrier Lowering Effect in SiC MESFETs

We have fabricated planar 4H-SiC, metal-semiconductor field-effect transistors (MESFETs) with high-quality metal/SiC contacts. To eliminate potential damage to the gate region caused by etching and simplify the device fabrication process, gate Schottky contacts were formed without any recess gate etching, and an ideality factor of 1.03 was obtained for these gate contacts. The interface state density between the contact metal and SiC was 5.7×1012 cm−2eV−1, which was found from the relationship between the barrier height and the metal work function. These results indicate that the interface was well controlled. Thus, a transconductance of 30 mS/mm was achieved with a 3-µm gate length as the performance figure of these MESFETs with high-quality metal/SiC contacts. Also, a low ohmic contact resistance of 1.2×10−6 Θcm2 was obtained for the source and drain ohmic contacts by using ion implantation.

High-quality schottky and ohmic contacts in planar 4H-SiC metal semiconductor field-effect transistors and device performance

The impact of radiation damage on the device performance of 4H-SiC metal Schottky field effect transistors, which are irradiated at room temperature with 2-MeV electrons, is studied. No performance degradation is observed by 1 × 10 15 e / cm 2 , while a slight increase of the linear drain current together with a decrease of the threshold voltage are noticed for 1 × 10 16 e / cm 2 . The degradation for low electron fluence is mainly attributed to the radiation-induced decrease of the Schottky barrier height at the gate contact. For exposures over 1 × 10 16 e / cm 2 , the drain current and transconductance decrease. The maximum transconductance for 1 × 10 17 e / cm 2 is only 18% of the value before irradiation. Although no electron capture levels are observed before irradiation, three electron capture levels ( E 1 – E 3 ) are induced after irradiation. The observed increase of the channel resistance is due to the induced lattice defects creating electron traps.

Radiation-induced defects in SiC-MESFETs after 2-MeV electron irradiation

Silicon carbide (SiC) MESFETs–with gate lengths from 0.3 μm to 1.5 μm–were fabricated on a semi-insulating substrate, and their RF characteristics were measured. These measurements showed that when the gate length is less than 1.0 μm, the cutoff frequency f t and maximum oscillation frequency f max of the SiC-MESFETs is less dependent on gate length. It was also found that the degradation of these RF characte risti s is apparently caused by the short-channel effect. To uncover the mechanism underlying this finding, static electrical characteristics, such as the transconductance and thr eshold voltage of the fabricated short-channel SiC-MESFETs, were investigated; conse quently, it was found that the short-channel effect occurred when the aspect ratio of gate l ength to channel thickness was five or less. Introduction Silicon carbide (SiC) is a promising material for high-power high-frequency device s, because of its combination of high breakdown electrical field, high saturation velocity, and high thermal conductivity. A cutoff frequency f t of 22 GHz and a maximum oscillation frequency f max of 50 GHz have been reported for a SiC-MESFET with a 0.4-μm gate length [1]. In addition, CW output power of 80 W and pulsed output power of 120 W under 3.1-GHz operation has been reported [2],[3]. We previously reported that the RF characteristics, f t and fmax, of a SiC-MESFET depend on the unit gate width [4]. In that study, a 0.5-μm-gate FET with unit gate widths of 1000 μm and 100 μm produced f t of 10 GHz and f max of 18 GHz. The output-power density was 2.5 W/mm under 1-GHz operation. However, these results were inferior to those reported in the references mentioned above. Accordingly, we tried to increase f t and fmax by optimizing the gate length. Although Nilsson et al. earlier reported the gatelength dependence of SiC-MESFETs, their MESFETs were fabricated on conducting substr ate [5]. The current study therefore focuses on the gate-length dependence of RF chara cteristics of SiC-MESFETs fabricated on a semi-insulating substrate. It al so reveals the relationship between this gate-length dependence and the short-channel effect in SiC-M E FETs. Experiments MESFETs were fabricated on epitaxial layers grown on a semiinsulating 4H-SiC substrate purchased from CREE Inc. A cross section of the fabric ated SiC-MESFET is shown in Figure 1. The vertical structure consists (from bottom to top) of a 1.0-μm-thick p-buffer layer doped to 4.0×10 15 /cm, a 0.29-μm-thick channel layer doped to 3.1×10 17 /cm and a 0.3-μm-thick n+ layer doped to 1.0×10 19 /cm. A conventional GaAs-MESFET fabrication process was applied. Plasma dry etching was used to is olate adjoining MESFETs and was followed by oxidization to remove the damaged layer from the etched surface. Materials Science Forum Online: 2003-09-15 ISSN: 1662-9752, Vols. 433-436, pp 745-748 doi:10.4028/www.scientific.net/MSF.433-436.745 © 2003 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-12/03/20,10:57:01) Nickel metal was deposited by electron-beam evaporation and formed an ohmic contact after rapid thermal annealing at 950 °C for 2 min in an argon atmosphere. Specific contact resistivity of 2.0×10 -6 Ω·cm and contact sheet resistance of 380 Ω/ were obtained from a transmission line model (TLM) fabricated on the same wafer. Titanium (100 nm)/platinum (50 nm)/gold (400 nm) gate metal was deposited by electron-beam evaporation and formed by a lift-off process. The gate length was varied from 0.3 to 1.5 μm. All MESFETs were prepared on one wafer in order to minimize technological fluctuations in characteristics. Experimental Results DC measurements The fabricated devices were analyzed using a HP4142B semiconductor parameter analyzer. Figure 2 shows the typical I-V characteristics for a 1.5-μm-gate MESFET. The saturation drain current I dss is 270 mA/mm and the transconductance g m is 23.7 mS/mm. Figure 3 shows the typical I-V characteristics for a short-channel 0.3-μm-gate MESFET. It is clear that the drain curr ent shows parallel-triode operating characteristics when the gate bias voltage exceeds –4 V. The saturation drain current Idss is 410 mA/mm and g m is 23.4 mS/mm. Small-signal measurements Small-signal S-parameters were measured on-wafer using a HP8510 network analyzer and Cascade Microteck probes. Figure 4 shows the typical small-signal RF characteristics for a 0.3-μm-gate MESFET with a 0.4-mm gate width. The cutoff frequency f t of this MESFET is 10.9 GHz and its maximum oscillation frequency fmax is 30 GHz. Figure 5 shows the dependence of f t and fmax on gate length (L g). It is clear that f t increased in inverse proportion to gate length until L g reached 1.0 μm, and then saturated; fmax decreased when gate length became less than 0.5 μm. The best v alu of ft is 11.3 GHz for a 0.3-μm×0.2-mm gate device and that of f max is 33 GHz for a 0.5-μm×0.2-mm gate device. These frequency characteristics are superior to those r eported in [4], because we Fig. 1. Schematic crosssection of a SiC-MESFET Semi-insulating substrate p-type buffer layer Source Drain Gate Back metal n-type channel layer n+ layer n+ layer Fig. 3. I-V characteristics o f a 0.3-μm-gate SiCMESFET with 0.4-mm gate width Fig. 2. I-V characteristics of a 1.5-μm-gate SiCMESFET with 0.4-mm gate width 0 100 200 300 400 500

RF Characteristics of Short-Channel SiC MESFETs

With rapid growth of mobile communications represented by portable telephones, further increases in the output of high-frequency power transistors for the base stations are required. Also, in the fields of satellite communications and radar, realization of high-power transistors replacing electron tubes such as TWTs and magnetrons is desirable for size reduction, weight reduction, and extension of transmitter life. As a transistor meeting such needs, the metal semiconductor field effect transistor (MESFET) based on silicon carbide (SiC) has been attracting attention. SiC is superior in material characteristics such as the electric breakdown field and thermal conductivity and is expected theoretically to provide high power density as a high-frequency transistor. To date, higher output power density than MESFETs based on Si and GaAs of the same size has been achieved, including an output power density of 4.2 W/mm at 1 GHz. In this paper, the high-frequency characteristics of the tested SiC high-frequency MESFET are presented and trends in research on improvement of the high-frequency characteristics are described. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 86(11): 1–10, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.10160

Development of High-Frequency SiC-MESFETs

Source Resistance Analysis of SiC-MESFET

Influence of Gate Finger Width on RF Characteristics of 4H-SiC MESFET

We have developed high power SiC-MESFETs for high-freqeuncy applications. We obtained a cut-off freqeuncy of 9.3 GHz and a maximum oscillation frequency of 34.2 GHz from a 0.5μm gate MESFET. We measured pulsed output power characteristics of 54.1 W at 1.0 GHz and 11.2 W at 9.4 GHz from a 39.2mm-gate-MESFET and a 9.6mm-gate-width MESFET, respectivly.

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RF Characteristics of Short-Channel SiC MESFETs

Development of High-Frequency SiC-MESFETs

Source Resistance Analysis of SiC-MESFET

Influence of Gate Finger Width on RF Characteristics of 4H-SiC MESFET

Development of High-Frequency SiC-MESFETs