High-power device technology is a key technological factor for wireless communication, which is one of the information network infrastructures in the 21st century, as well as power electronics innovation, which contributes considerably to solving the energy saving problem in the future energy network. Widegap semiconductors, such as SiC and GaN, are strongly expected as high-power high-frequency devices and high-power switching devices owing to their material properties. In this paper, the present status and future prospect of these widegap semiconductor high-power devices are reviewed, in the context of applications in wireless communication and power electronics.

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Present Status and Future Prospect of Widegap Semiconductor High-Power Devices

In the present brief review modern views on the reasons of time instability of gas sensors parameters, as well as approaches, which could be used for the improvement of this important sensor's parameter, are summarized. In particular, the influence of factors such as structure transformation, phase transformation, poisoning, degradation of contacts and heaters, bulk diffusion, errors in design, change of humidity, fluctuations of temperature in the surrounding atmosphere, and interference effect was analyzed. It was shown that while designing devices such as solid-state gas sensors, there are no secondary issues or tasks—all are important. Sensors work in extreme temperatures in the presence of active gases, and therefore every element of the sensor could be responsible for its long-term stability. The conclusions, regarding the efficiency of approaches such as optimization of technological processes and optimization of exploitation processes used for improvement of stability of conductometric metal oxide gas sensors, were made as well.

Instability of metal oxide-based conductometric gas sensors and approaches to stability improvement (short survey)

The electrodes of semiconductor gas sensors are important in characterizing sensors based on their sensitivity, selectivity, reversibility, response time, and long-term stability. The types and materials of electrodes used for semiconductor gas sensors are analyzed. In addition, the effect of interfacial zones and surface states of electrode-semiconductor interfaces on their characteristics is studied. This study describes that the gas interaction mechanism of the electrode-semiconductor interfaces should take into account the interfacial zone, surface states, image force, and tunneling effect.

Electrodes for Semiconductor Gas Sensors

This paper presents the thermal sensitivity variation trend of Ni/4H-nSiC (0001) Schottky diode based temperature sensor, equipped with floating metal guard ring and oxide field plate as edge terminations in low current regime, i.e., ranging from 1 nA to 5 pA. Various measurements were carried out at temperatures ranging from 233 K to 473 K in steps of 20 K. An imperative outcome of the present study, which is in contrast with the theory, is that there exists an anomaly in the device thermal sensitivity behaviour after a range of current. The thermal sensitivity of the fabricated device, calculated from the slope of forward voltage versus temperature plot, was found to be varied from 3.11 mV/K at 1 nA to 3.32 mV/K at 5 pA with standard error of ±0.03 mV/K. A detailed analysis of I-V-T characteristics by taking into account all the possibilities for variation in the barrier height and the ideality factor with temperature emphasizes that there exist barrier height inhomogeneities at the metal–semiconductor interface in the fabricated device. These observations indicate that anomaly in the device thermal sensitivity was due to the barrier height inhomogeneities present in the device.

Barrier height inhomogeneities induced anomaly in thermal sensitivity of Ni/4H-SiC Schottky diode temperature sensor

This report is on the diameter dependent thermal sensitivity variation trend of Ni/4H-nSiC Schottky barrier diode (SBD) temperature sensors. Scaled SBDs of 2, 1.6, and 1.2 mm in diameter were fabricated using standard photolithography process comprising a field plate and a guard ring as edge terminations on the same epitaxial wafer. Taking into consideration the heat loss and power consumption, the thermal sensitivities of the fabricated SBDs were measured in the current range from 1 μA to 50 pA. The temperature was varied from 273 to 473 K in step of 25 K. An authoritative consequence of the present study is the observed increase in thermal sensitivity with the diameter of the fabricated SBDs. An exhaustive investigation confirms that in all diodes, there exist nanosized patches, which assumed to have different barrier heights and hence ascertained to be the main cause of anomalies in thermal sensitivity variation with diode size. Taking into account the effective area of these patches, theoretically fit...

Diameter dependent thermal sensitivity variation trend in Ni/4H-SiC Schottky diode temperature sensors

Long-term reliability of Ti–Pt–Au metallization system for Schottky contact and first-level metallization on SiC MESFET

We report a comparative investigation of the activation kinetics of nitrogen and phosphorus ions implanted at room temperature in 4H-SiC semi-insulating substrates. We characterised the electrical activation through a fast non-contact method (eddy current) which gives the sheet resistance after every annealing step. The surface roughness was monitored by AFM. The crystal recovery was characterised by Raman spectroscopy. For a given annealing temperature, we show that, the N + -implanted sample reaches always a maximum activation rate at a shorter time ( ~ 3 times less) than the P + -implanted one. We find also that the best activation results are not obtained by using simply N + or P + but rather a co-implantation of N + and P + to the same dose.

Room Temperature Implantation and Activation Kinetics of Nitrogen and Phosphorus in 4H-SiC Crystals

Several passivation schemes on 4H-SiC MESFETs have been studied. Two main configurations are compared. MESFET structures with thin passivation layer or no passivation layer (Configuration 1) exhibit high breakdown voltage but also current instability after high voltage Vds stress. Thick SiO2 passivation covering the gate (Configuration 2) improves the current stability but yields lower breakdown voltage and higher gate leakage current. Surface trapping effects are considered as the main cause of the observed phenomena. Introduction Transistors based on wide band gap semiconductors have been studied for about 10 years for RF and microwave power amplifiers. Today, the most promising prototype structures, such as III-N HEMT [1], SiC SITs and SiC MESFETs [2], provide good RF power performance, paving the way to industrial applications such as base station, or radar transmitters [3]. Still, the reliability of these new devices remains a weak point on the way towards industrialization. One key concern resides in the fact that, today, the same insulator materials optimized for the previous silicon and GaAs technologies are used for passivating the new wide band gap devices, although they are submitted to electric field five to ten-fold higher. Another key issue is related to the wide band gap semiconductor surface or interface with insulators. This surface exhibits large density of surface states, inducing charge trapping effects related to carrier injection [4,5]. Surface charges can have electrostatic influence on the device channel, and may induce DC and / or RF-dispersion, together with limitation of RF power performance. Recently, surface states instabilities as well as the effects of passivation on III-N HEMT devices characteristics, have received an increasing attention [6, 7]. On the other hand, only limited literature is available so far on surface effects in SiC MESFET [8,9]. In this paper, we report on the effect of the passivation on the stability of SiC MESFET characteristics. Two main variants have been studied and compared. First, MESFET with no passivation or a thin insulator ( 200nm) covering the device (Configuration. 2). Devices in Config.1 will generally exhibit high breakdown voltage but unstable DC characteristics. In contrast, a device in Config.2 will generally exhibit high stability but lower breakdown voltage. Device structure and processing HTCVD Semi Insulating substrates were mainly supplied by OKMETIC except one HPSI substrate from CREE Research. The epitaxy, supplied by OKMETIC on all wafers, includes a P-type buffer layer (Na ≈1-3 x10 cm), a N-type active layer (Nd =1-3x10 cm) and a N+ contact layer (Nd >1x10 cm). Standard process consists in MESA isolation by RIE etch and a 250nm deep recess, also RIE etched, to define the channel in the active layer. In order to protect the active layer, and prior to ohmic contact metal deposition, 50nm thick insulator is either grown by 6 hours wet oxidation (resulting in SiO2), or deposited using PECVD (SiO2 or Si3N4). We call this thin insulating layer “prepassivation”. Then, ohmic contacts are made, consisting of 100nm Ni based metallisation e-beam evaporated after self-aligned RIE etch of the prepassivation thin layer, and metal lift-off. After annealing (950°; 5min), good morphology and electrical characteristics of the ohmic contact are reproducibly achieved (Ra∼2nm, ρc=2x10 Ω.cm). The same kind of lift-off process is used to deposit the multilayer gate Ti(50nm) Pt(50nm) Au(500nm) with 2.5μm Lgd spacing. At this stage, the transistor is completed and can be measured (Config.1). Then, thick passivation is deposited, either mineral SiO2 or Si3N4 layer using PECVD, or BCB polymer from Dow Chemicals deposited by spin-on technique and annealed [10]. At this stage, the device has reached its final state (Config. 2, see figure 1). Table 1 summarises the different combinations studied for “prepassivation” and “passivation” layers Fig 1: Main passivation schemes: config.1 (thin mineral protection or free SiC surface), and config.2 (thick passivation covering the gate). Table 1: Prepassivation and passivation combinations explored in this study. Also indicated, the breakdown voltage measured before and after thick passivation. “WO “ indicates the presence of a “walk out” phenomenon: Sample Substrate Prepassivation (50nm) BVgd1 Passivation BVgd2 X_59 HTCVD (OKM) Thermal Oxide >250V PECVD SiO2/ Si3N4 (400/50nm) 80-180 X_84 HTCVD (OKM) Thermal Oxide >250V PECVD SiO2/ Si3N4 (400/50nm) 110 X_85 HTCVD (OKM) Thermal Oxide >250V PECVD SiO2/ Si3N4 (400/50nm) 140 X_60 HTCVD (OKM) PECVD Oxide >250V PECVD SiO2/ Si3N4 (400/50nm) 200 X_62 HTCVD (OKM) PECVD Oxide >250V No X_82 HTCVD (OKM) PECVD Si3N4 >250V PECVD Si3N4 (200nm) 120 (WO) X_83 HTCVD (OKM) PECVD Si3N4 >250V PECVD SiO2/ Si3N4 (400/50nm) 190 (WO) X_86 HPSI (CREE) PECVD Si3N4 200V PECVD SiO2/ Si3N4 (400/50nm) 150 (WO) X_61 HTCVD (OKM) No >250V BCB 180 (WO) Measurements and experimental results Current stability for Vds 10s). This relaxation cannot be described with one time-constant. PSiC Buffer Semi-insulating SiC Substrate N+ Epi (200nm) N+ Epi (200nm) Ti/Pt/Au Schottky Gate Ni Ohmic Contact

Effect of Passivation on Device Stability and Gate Reverse Characteristics on 4H-SiC MESFETs

SiC MESFET and III-N HEMT based on SiC substrates are the most promising structures for the next generation of RF power devices. One of the most useful material properties of SiC substrate is its high thermal conductivity, 3 ti mes that of Silicon. But still, due to extremely high power density dissipated in this devices, an effective thermal management will be the key for future development. The main other re quirement put on SiC substrate for SiC MESFET applications is purity, for minimizing carrier trapping, but also for thermal issues. For III-N HEMT, the impact of SiC substrate pur ity on the device has hardly been studied yet. Improvement in bulk growth techniques has led to large r diameter and purer substrates though not cheaper yet but many crystal defects , still present in the SiC substrates, are detrimental to large devices for both SiC MESFET and IIIN HEMT. WIDE BANDGAP DEVICE DEVELOPPED FOR MICROWAVE POWER Mainly two families of wide bandgap semiconductor transistors are being developed for applications in high-frequency amplifiers ranging from RF up to mi llimeter-waves. Although there exist some attempts to use “vertical” structures, such as SIT, BJT, or HBT, the two main tracks today are « horizontal » field effect structures : SiC MESFET and III-N « piezomorphic » HEMT. While there is not any III-N microwave pow er device product on the market yet, there has already been some kind of commercial s amp ing for SiC MESFET. As “horizontal” structures, the two kind of devices are preferably f abricated on insulating support. The main physical support in use today for the active laye r is single crystal semiinsulating SiC substrate which allows deposition of the active layer by epitaxy on top of it. THE IMPORTANCE OF SUBSTRATE THERMAL CONDUCTIVITY The power density achieved in operating wide bandgap devices is ranging from 2 to more than 10 W / mm, much higher than in previous similar structures based on silicon or on heavier III-V compounds. Since the efficiency is still limited to 50-60 % at best on large devices, the dissipated power density is extremely large. As a consequence, thermal management is a clear bottleneck in this device development, esp ecially for large devices. As an example, figure 1a shows the output power density versus the device p riphery on the same SiC MESFET wafer. A thermal crowding effect is obser ved as the local temperature of the device increases with the total periphery. The decrease in out put power is considered to be largely due to electron mobility drop with temperature (μ ∼ T) as shown on figure 1b. Materials Science Forum Vols. 433-436 (2003) pp 731-736 online at http://www.scientific.net © (2003) Trans Tech Publications, Switzerland Online available since 2003/Sep/15 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 the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.33-14/04/08,13:03:37)

Influence of Material Properties on Wide-Bandgap Microwave Power Device Characteristics

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Papers

Long-term reliability of Ti–Pt–Au metallization system for Schottky contact and first-level metallization on SiC MESFET

Room Temperature Implantation and Activation Kinetics of Nitrogen and Phosphorus in 4H-SiC Crystals

Effect of Passivation on Device Stability and Gate Reverse Characteristics on 4H-SiC MESFETs

Influence of Material Properties on Wide-Bandgap Microwave Power Device Characteristics