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

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)