Trapping of hot electron behavior by trap centers located in buffer layer of
a wurtzite phase GaN MESFET has been simulated using an ensemble Monte Carlo
simulation. The results of the simulation show that the trap centers are
responsible for current collapse in GaN MESFET at low temperatures. These
electrical traps degrade the performance of the device at low temperature. On
the opposite, a light-induced increase in the trap-limited drain current,
results from the photoionization of trapped carriers and their return to the
channel under the influence of the built in electric field associated with the
trapped charge distribution. The simulated device geometries and doping are
matched to the nominal parameters described for the experimental structures as
closely as possible, and the predicted drain current and other electrical
characteristics for the simulated device including trapping center effects show
close agreement with the available experimental data.Comment: 5 pages, 2 figure

A. Binesh

Arabshahi

H. Arabshahi

Papers in Physics

English

arXiv

Trapping of hot electron behavior by trap centers located in buffer layer of a wurtzite phase GaN MESFET has been simulated using an ensemble Monte Carlo simulation. The results of the simulation show that the trap centers are responsible for current collapse in GaN MESFET at low temperatures. These electrical traps degrade the performance of the device at low temperature. On the opposite, a light-induced increase in the trap-limited drain current, results from the photoionization of trapped carriers and their return to the channel under the influence of the built in electric field associated with the trapped charge distribution. The simulated device geometries and doping are matched to the nominal parameters described for the experimental structures as closely as possible, and the predicted drain current and other electrical characteristics for the simulated device including trapping center effects show close agreement with the available experimental data

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Papers in Physics, vol. 1, art. 010006 (2009)Received: 10 November 2009, Accepted: 26 November 2009Edited by: A. GoñiLicence: Creative Commons Attribution 3.0DOI: 10.4279/PIP.010006www.papersinphysics.orgISSN 1852-4249Light effect in photoionization of traps in GaN MESFETsH. Arabshahi,1∗A. Binesh2Trapping of hot electron behavior by trap centers located in buffer layer of a wurtzitephase GaN MESFET has been simulated using an ensemble Monte Carlo simulation. Theresults of the simulation show that the trap centers are responsible for current collapsein GaN MESFET at low temperatures. These electrical traps degrade the performanceof the device at low temperature. On the opposite, a light-induced increase in the trap-limited drain current, results from the photoionization of trapped carriers and their returnto the channel under the influence of the built in electric field associated with the trappedcharge distribution. The simulated device geometries and doping are matched to thenominal parameters described for the experimental structures as closely as possible, andthe predicted drain current and other electrical characteristics for the simulated deviceincluding trapping center effects show close agreement with the available experimentaldata.I. IntroductionGaN has become an attractive material for powertransistors [1-3] due to its wide band gap, highbreakdown electric field strength, and high ther-mal conductivity. It also has a relatively highelectron saturation drift velocity and low relativepermitivity, implying potential for high frequencyperformance. However, set against the virtues ofthe material are the disadvantages associated withmaterial quality. GaN substrates are not readilyavailable and the lattice mismatch of GaN tothe different substrate materials commonly usedmeans that layers typically contain between 108and 1010 threading dislocations per cm2. Further,several types of electron traps occur in the device∗E-mail: arabshahi@um.ac.ir1 Department of Physics, Ferdowsi University of Mashhad,P.O. Box 91775-1436, Mashhad, Iran.2 Department of Physics, Payam-e-Nour University, Fari-man, Iran.layers and have a significant effect on GaN devices.In the search for greater power and speed per-formance, the consideration of different aspectsthat severely limit the output power of GaN FETsmust be accounted for. It is found that presenceof trapping centers in the GaN material is themost important phenomenon which can effect oncurrent collapse in output drain current of GaNMESFET. This effect was recently experimentallyinvestigated in GaN MESFET and was observedthat the excess charge associated with the trappedelectrons produces a depletion region in the con-ducting channel which results in a severe reductionin drain current [4]. The effect can be reversedby librating trapped electrons either thermallyby emission at elevated temperatures or opticallyby photoionization. There have been severalexperimental studies of the effect of trapping levelson current collapse in GaN MESFET. For exam-ple, Klein et al. [5-6] measured photoionizationspectroscopy of traps in GaN MESFET transistorsand calculated that the current collapse resultedfrom charge trapping in the buffer layer. Binari et010006-1Papers in Physics, vol. 1, art. 010006 (2009) / H. Arabshahi et al.al. [7] observed decreases in the drain current of aGaN FET corresponding to the deep trap centerslocated at 1.8 and 2.85 eV.In this work, we report a Monte Carlo simulationwhich is used to model electron transport inwurtzite GaN MESFET including a trappingcenters effect. This model is based upon the factthat since optical effect can emit the trappedelectrons that are responsible for current collapse,the incident light wavelength dependence of thiseffect should reflect the influence of trap centerson hot electron transport properties in this device.This article is organized as follows. Details ofthe device fabrication and trapping model whichis used in the simulated device are presented insection 2, and the results from the simulationcarried out on the device are interpreted in section3.II. Model, device and simulationsAn ensemble Monte Carlo simulation has been car-ried out to simulate the electron transport proper-ties in GaN MESFET. The method simulates themotion of charge carriers through the device by fol-lowing the progress of 104 superparticles. Theseparticles are propagated classically between colli-sions according to their velocity, effective mass andthe prevailing field. The selection of the propaga-tion time, scattering mechanism and other relatedquantities, is achieved by generating random num-bers and using these numbers to select, for example,a scattering mechanism. Our self-consistent MonteCarlo simulation was performed using an analyt-ical band structure model consisting of five non-parabolic ellipsoidal valleys. The scattering mech-anisms considered for the model are acoustic andpolar optical phonon, ionized impurity, piezoelec-tric and nonequivalent intervalley scattering. Thenonequivalent intervalley scattering is between theΓ1, Γ3, U, M and K points.The parameters used for the present Monte Carlosimulations for wurtzite GaN are the same as thoseused by Arabshahi for MESFET transistors [8-9].The device structure illustrated in figure 1.a is usedin all simulations. The overall device length is 3.3µm in the x -direction and the device has a 0.3 µmgate length and 0.5 µm source and drain length.Figure 1: (a) Cross section of wurtzite GaN MES-FET structure which we have chosen in our simu-lation. Source and drain contacts have low resis-tance ohmic contacts, while the gate contact formsa Schottky barrier between the metal and the semi-conductor epilayer, (b) The instantaneous distribu-tion of 104 particles at steady forward bias (drainvoltage 50 V, gate voltage −1 V), superimposed onthe mesh. Note that in the simulation there aretwo types of superparticles. The mobile particleswhich describe unbound electron flow through thedevice and trapping center particles which are fixedat the center of each electric field cell (in this casein the buffer layer only). The ellipse represents atrap center which is fixed at the center of an electricfield cell and occupied by some mobile charges.The source and drain have ohmic contacts and thegate is in Shottky contact in 1 eV to reperesent thecontact potential at the Au/Pt. The source anddrain regions are doped to 5 × 1023 m−3 and the010006-2Papers in Physics, vol. 1, art. 010006 (2009) / H. Arabshahi et al.top and down buffer layers are doped to 2 × 1023m−3 and 1× 1022 m−3, respectively. The effectivesource to gate and gate to drain separation are 0.8µm and 1.2 µm, respectively. The large dimensionsof the device need a long simulation time to ensureconvergence of the simulator. The device is simu-lated at room temperature and 420 K.In the interests of simplicity it is assumed that thereis just a single trap with associated energy level ETin all or just part of the device. Further, it is as-sumed that only electrons may be captured fromthe conduction band by the trap centers, whichhave a capture cross-section σn and are neutralwhen unoccupied, and may only be emitted froman occupied center to the conduction band. We usethe standard model of carrier trapping and emission[9-10].For including trapping center effects, the followingassumption has been considered. The superparti-cles in the ensemble Monte Carlo simulation areassumed to be of two types. There are mobile par-ticles that represent unbound electrons throughoutthe device. However, the particles may also un-dergo spontaneous capture by the trap centers dis-tributed in the device. The other type of super-particles are trapping centers that are fixed at thecenter of each mesh cell. As illustrated in figure1.b, each trap center has the capacity to trap afinite amount of mobile electronic charge from par-ticles that are in its vicinity and reside in the lowestconduction band valley. The vicinity is defined asexactly the area covered by the electric field meshcell. The finite capacity of the trapping center ineach cell of a specific region in the device is set bya density parameter in the simulation programme.The simulation itself is carried out by the followingsequence of events. First, the device is initializedwith a specific trap which is characterized by itsdensity as a function of position, a trap energy leveland a capture cross-section. Then at a specific gatebias the source-drain voltage is applied.Some of the mobile charges passing from the sourceto the drain in each timestep can be trapped bythe centers with a probability which is dependenton the trap cross-section and particle velocity in thecell occupied at the relevant time t. The quantity ofcharge that is captured from a passing mobile parti-cle is the product of this probability and the chargeon it. This charge is deducted from the charge ofthe mobile particle and added to the fixed chargeof the trap center. The emission of charge is sim-ulated using the emission probability. Any chargeemitted from a trap center is evenly distributed toall mobile particles in the same field cell. Such cap-ture and emission simulations are performed for theentire mesh in the device and information on theensemble of particles is recorded in the usual way.III. ResultsThe application of a high drain-source voltagecauses hot electrons to be injected into the bufferlayer where they are trapped by trap centers. Thetrapped electrons produce a depletion region in thechannel of the GaN MESFET which tends to pinchoff the device and reduce the drain current. Thiseffect can be reversed by any factor which substan-tially increases the electron emission rate from thetrapped centers, such as the elevated temperaturesconsidered previously. Here we consider the effectof exposure to light [11-13].There have been several experimental investiga-tions of the influence of light on the device char-acteristics. Binari et al. [6] were the first to ex-perimentally study the current collapse in GaNMESFETs as a function of temperature and illu-mination. They showed that the photoionization oftrapped electrons in the high-resistivity GaN lay-ers and the subsequent return of these electrons tothe conduction band could reverse the drain currentcollapse. Their measurements were carried out asa function of incident light wavelength with valuesin the range of 380 nm to 720 nm, corresponding tophoton energies up to 3.25 eV which is close to theGaN band gap. Their results show that when thephoton energy exceeds the trap energy, the elec-trons are quickly emitted and a normal set of draincharacteristics is observed.To examine the photoionization effect in our simu-lations, the thermal emission rate etn was changedto etn + eon, where eon ∼ σonΦ is the optical emissionrate, with σon, the optical capture cross-section andΦ the photon flux density given byΦ =Ihν=Iλhc(1)where I is the light intensity, ν is the radiation fre-quency and λ is the incident light wavelength.Our modeling of photoionization effects in GaN010006-3Papers in Physics, vol. 1, art. 010006 (2009) / H. Arabshahi et al.Figure 2: I-V characteristics of a GaN MESFETunder optical and thermal emission of trapped elec-trons (solid curve) and thermal emission of trappedelectrons (dashed curve) at two different tempera-tures. (a) At T = 300 K with trap centers at 1.8eV and illuminated with a photon energy of 2.07eV. (b) At T = 420 K with trap centers at 2.85 eVand illuminated with a photon energy of 3.1 eV.MESFETs is based on parameters used by Binariand Klein [5-7]. The simulations were all carriedout for two different deep trap centers, both with aconcentration of 1022 m−3, and with photoioniza-tion threshold energies at 1.8 and 2.85 eV and cap-ture cross-sections of 6× 10−21 m2 and 2.8× 10−19m2, respectively. A fixed incident light intensity of5 Wm−2 at photon energies of 2.07 eV and 3.1 eVis used. The simulations have been performed ata sufficiently high temperature (420 K), for boththermal and optical emission, to be significant aswell as at room temperature.Figure 2a illustrates the effect on the drain currentcharacteristics of exposure of the device to light atroom temperature. The GaN MESFET has a deeptrap center at 1.8 eV and is illuminated at a pho-ton energy of 2.07 eV. It can be seen that in thelight the I-V curves generally exhibit a larger draincurrent, especially at higher drain voltages, reflect-ing the fact that the density of trapped electrons ismuch lower.Simulations have also been performed at 420 K for adevice with deep level traps at 2.85 eV. The simula-tion results in figure 2b for illumination of a photonenergy of 3.1 eV are compared with the collapsedI-V curves in the absence of light. Comparison offigures 2a and 2b shows that the currents are gen-erally higher at 420 K and that the light has lesseffect at the highest temperature.IV. ConclusionsThe dependence upon light intensity (exposure) ofthe reversal of current collapse was simulated in aGaN MESFET for a single tapping center. Trapsin the simulated device produce a serious reductionin the drain current and consequently the outputpower of GaN MESFET. The drain current behav-ior as a function of illumination with photon en-ergy was also studied. Our results show that asthe temperature and photon energy are increased,the collapsed drain current curve moves up towardthe non-collapsed curve due to more emission oftrapped electrons.Acknowledgements - The authors wish to thankM. G. Paeezi for the helpful comments and criticalreading of the manuscript.[1] B Gil, Group-III Nitride Semiconductor Com-pounds, Oxford Science Pub. (1998).[2] M A Khan, M S Shur, AlGaN/GaN Metal Ox-ide Semiconductor Heterostructure Field Ef-fect Transistor, Mater. Sci. Eng. B 42, 69(1997).[3] P B Klein, S C Binari, J A Freitas, A E Wick-enden, Photoionization spectroscopy of trapsin GaN metal-semiconductor field-effect tran-sistors, J. Appl. Phys. 88, 2843 (2000).010006-4Papers in Physics, vol. 1, art. 010006 (2009) / H. Arabshahi et al.[4] M A Khan, M S Shur, Q C Chen, J NKuznia, Low frequency noise in GaN metalsemiconductor and metal oxide semiconductorfield effect transistors, Electron. Lett. 30, 2175(1994).[5] P B Klein, S C Binari, J A Freitas, AE Wickenden, Observation of deep traps re-sponsible for current collapse in GaN metal-semiconductor field-effect transistors, J. Appl.Phys. 88, 2843 (2000).[6] P B Klein, J A Freitas, S C Binari, A EWickenden, AlGaN/GaN heterostructure field-effect transistor model including thermal ef-fects, Appl. Phys. Lett. 75, 4016 (1999).[7] S C Binari, W Kruppa, H B Dietrich, G Kel-ner, A E Wickenden, J A Freitas, Trapping ef-fects and microwave power performance in Al-GaN/GaN HEMTs, Solid State Electron. 41,1549 (1997).[8] H Arabshahi, Monte Carlo simulations of elec-tron transport in Wurtzite phase GaN MES-FET including trapping effect, Modern Phys.Lett. B 20, 787 (2006).[9] H Arabshahi, The frequency response and ef-fect of trap parameters on the characteristicof GaN MESFETs, The Journal of DamghanUniversity of Basic Sciences 1, 45 (2007).[10] S Trassaert, B Boudart, C Gaquiere, Investiga-tion of traps induced current collapse in GaNdevices, a1404 ORSAY France, 127 (1999).[11] A Kastalsky, S Luryi, A C Gossard, W KChan, Switching in NERFET circuits, IEEEElectron Device Lett. 6, 347 (1985).[12] J C Inkson, Deep impurities in semiconduc-tors. II. The optical cross section, J. Phys. C:Solid State Phys. 14, 1093 (1981).[13] D V Lang, R A Logan, M Jaros, Monte Carloevaluations of degeneracy and interface rough-ness effects, Phys. Rev. B 19, 1015 (1979).010006-5

Light effect in photoionization of traps in GaN MESFETs

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