Abstract
The present work considers electrothermal simulation of LDMOS devices and associated nonequilibrium effects.Simulations have been performed on three kinds of LDMOS: bulk Si, partial SOI and full SOI. Differences betweenequilibrium and nonequilibrium modeling approaches are examined. The extent and significance of thermal nonequilibriumis determined from phonon temperature distributions obtained using a common electronic solution andthree different heating models (Joule heating, electron/lattice scattering, phonon scattering). The results indicate that,under similar operating conditions, nonequilibrium behavior is more significant in the case of full SOI devices, wherethe extent of nonequilibrium is estimated to be twice that of the partial SOI device and four times that of the bulkdevice. Time development of acoustic phonon and lattice temperatures in the electrically active region indicates thatnonequilibrium effects are significant for times less than 10 ns._ 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
Metal oxide semiconductor field effect transistors(MOSFETs) are widely used in a variety of powerelectronic systems. For example, the devices are particularlyuseful as high speed switches because of their lowon-state resistance [1]. In high-frequency RF applications,they exhibit low capacitance and high gain (inlateral double-diffused MOS) or high stability (in verticaldouble-diffused MOS) [2]. MOSFETs can also beintegrated with signal processing circuits to form ‘‘smartchips’’ and in-system programmable large-scale integrationdevices (IPLSI) [3]. The present work focuses onlateral double-diffused MOS (LDMOS), which is wellsuited for high frequency applications such as telecommunicationcircuits. These lateral surface effect devicesare, however, susceptible to hot-carrier currents thatinduce several breakdown mechanisms. Various effortshave been directed towards characterizing hot-carrier effects in LDMOS, and suggestions have been made tomitigate such effects [4–6]. These attempts representmodest improvements in thermal management, but thedevices are still likely to exhibit thermal problems.Therefore, thermal simulations of LDMOS are crucialfor accurate estimates of device performance.Nonequilibrium in power devices can be defined froman electrical and a thermal perspective. Electrical nonequilibriumincludes all conditions that eventually causea flow of charge (i.e. a current) [7]. However, the energygained by charge carriers due to an externally appliedelectrical field, can be transferred to the lattice. Inequilibrium, the electrons and lattice exhibit similarenergy levels. Yet, power device operation requires theapplication of large fields and high currents. Therefore,these devices normally operate in electrical nonequilibrium.Thermal nonequilibrium refers to the condition whencharge carriers are not able to transfer their excess energyto the lattice efficiently. A temperature difference iscreated between electrons and the lattice resulting inlocalized heating in the active area of the device.


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