ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Comparison Study of Uniformly-Doped and Delta-Doped Al0.22Ga0.78As/In0.22Ga0.78As Pseudomorphic HEMTs

K. N. Mohd. Kharuddin, Student Member IEEE and B. Yeop Majlis, Senior Member IEEE

Institute of Microengineering and Nanoelectronics (IMEN) Universiti Kebangsaan Malaysia (UKM)

43600 Bangi, Selangor, Malaysia E-mails: nisha78@hotmail.com, burhan@vlsi.eng.ukm.my

Abstract - Device performance of uniformly-doped and delta-doped AlGaAs/InGaAs pseudomorphic high electron mobility transistors are investigated with two-dimensional numerical simulator, ATLAS/Silvaco. Simulation results demonstrates superior performance for the delta-doped structure. The advantages shown by delta-doped structure include better electron confinement and reduced parasitic conduction which are manifested as higher transconductance and improved drain current.

I. INTRODUCTION The AlGaAs/InGaAs pseudomorphic displays superior device performance compared to the conventional AlGaAs/GaAs HEMT due to enhanced electron confinement within the two-dimensional quantum-well system [1], [2], [3]. Many attempts have been made to increase the sheet carrier concentration in the InGaAs channel to improve the current driving capability. Among other approach to achieve this is by using multiple quantum well structures and incorporating delta-doped layer in the PHEMT structure [4], [5]. Obtaining high transconductance and drain current requires a trade-off between the doping level in AlGaAs layer and gate-to-channel separation. Delta-doping will prevent the total doping in the AlGaAs layer from being reduced by a reduction in the gate-to-channel distance. A delta-doping in an AlGaAs layer can be realized by growing pure silicon for a short period of time within the growth of an undoped AlGaAs layer [6]. In this study, we examine the DC device characteristics of uniformly-doped and delta-doped structure to compare the device performance.

II. DEVICE STRUCTURES AND SIMULATION The PHEMT structures used in the simulation are shown in Fig. 1 and Fig. 2. For D-PHEMT, delta-doped layer are incorporated 3 nm above and 3 nm below the channel with doping 8×1018 cm3 and 2×1018 cm3 respectively.

Fig. 1 : Cross-section of uniformly doped PHEMT.

Fig. 2 : Cross-section of double-delta doped PHEMT.

Source Drain Gate

undoped AlGaAs 30 nm

InGaAs channel 14 nm AlGaAs spacer 3 nm

AlGaAs spacer 3 nm

p-type AlGaAs buffer 38 nm

p-type GaAs buffer

GaAs n+ capping 50 nm

GaAs n+ capping 50 nm

delta doped

n+ GaAs 50 nm

Source Drain Gate

n+ AlGaAs 50 nm

AlGaAs spacer 2 nm InGaAs channel 10 nm

GaAs p-type buffer

n+ GaAs 50 nm

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Both PHEMT has non-rectangular recessed passivation layer with 250 nm unpassivated region on both sides of the 80 nm gate. Physical models applied to AlGaAs and GaAs regions are concentration dependent mobility, parallel electric field dependence mobility and concentration dependent recombination models. The same models except concentration dependent mobility and concentration dependent recombination models are applied to InGaAs region where Schockley-Read-Hall recombination with fixed carrier lifetimes model is used instead. The Gummel method is used to start a few solutions before switching to the Newton method to complete the solution. Throughout simulations reported, the VD is fixed at 2.0 V and the lattice temperature is 300K.

III. RESULTS AND DISCUSSION

Simulation were performed to observe the performance of both uniformly-doped and delta-doped structures. Advantages of D-PHEMT over U-PHEMT are demonstrated in aspect of carrier confinement and reducing parasitic conduction.

Fig. 3 : Cross-sectional of conduction band of U-PHEMT and D-PHEMT at VG = 0 V PHEMT.

Fig. 3 shows a cross-sectional view of conduction band profile for both U-PHEMT and D-PHEMT with gate length of 80 nm at VG = 0. This cross-section is done perpendicular to the middle of the gate. An Al and In mole fraction of 22 % and a Schottky barrier height of 0.72 eV are assumed for both structures. At zero gate bias, it is expected that maximum charge carriers i.e. electrons are

confined in the two-dimensional electron gas (2DEG) layer since charge carriers are fully depleted from AlGaAs donor or barrier layer. As shown, in this depleted region, the conduction band of D-PHEMT has a linear shape near the gate due to low doping compared to the conduction band for U-PHEMT which is nearly parabolic. Both profiles show potential well in the channel due to discontinuity of band edges and their gradient approaches zero in the buffer layer. The D-PHEMT structure shows a deeper and larger channel thus 2DEG layer. There is a large separation between the channel and the bottom of the conduction band edge in AlGaAs buffer compared to the U-PHEMT structure. Control of two-dimensional electrons in the 2DEG layer is by application of gate voltage since the sheet charge density is related to the electric field applied across the heterojunction [4]. Application of a bias voltage to the drain terminal with the source grounded causes current flow in the X-axis direction. At VD = 2.0 V and VG ramped from -2 V to 2 V, the maximum drain saturation current is 439 mA/mm for D-PHEMT and 425 mA/mm for U-PHEMT as shown in Fig. 4. The improved carrier confinement also contributes to higher transconductance in D-PHEMT as shown in Fig. 5 where the gmax for D-PHEMT is 425 mS/mm compared to gmax of 416 mS/mm for U-PHEMT.

Fig. 4 : Drain current, ID against gate voltage, VG curves for both PHEMT structures. Drain current is in A/µm.

420 0-7803-8658-2/04/$20.00(c)2004 IEEE

ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

2,uo ~~~~~~~~~ J~~-PHIMMT

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Fig 5 Maximum transconductance, g,nw, againstgate voltage, VG for both PHEMT strucrures.

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Fig. 6: Logl0 of electron concentration withinInGaAs channel of both U-PHEMT and D-PHEMTat VG=OVand VG= -0.4V.

Fig. 4 shows further advantage of D-PHEMT. The cross-sectional view of electronconcentration is taken in the InGaAs channelat V(;= 0 V and VcG -0.4 V. As shown,electron concentration is higher for D-PHEMTat both gate voltages compared to U-PHEMT.As lower VG is applied, electron concentrationis lower near the channel resulting smallerdrain current. In the shape of the curves, itappears that as electrons approach the drainside of the gate, the layer occupancy ofelectrons become minimum to reflect theeffects of real space transfer [4]. When anegative bias is applied, the AlGaAs layer isnearly depleted in both U-PHEMT and D-PHEMT. The detailed difference in the actualshape in the conduction band edge in thatregion does not introduce any considerable

change. As a result, at negative gate bias, bothstructures exhibit almost the same devicecharacteristics including nearly identicalelectron concentration [4]. However, as thegate bias is increased and becomes positive,the differences between them begin to appear.Both structures exhibit short-channel effects

in their output characteristics where poorsaturation characteristics are obtained asshown in Fig. 7 and Fig. 8. Stenzel etal.reported that short channel effects dominatethe characteristics of short-channel HEMTsespecially single-gated structure is used [7]which is the case in this simulation study.

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>,-I

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Fig. 7: ID-VD characteristics of U-PHEMT withgate length of 80 nm. Drain current is in A/pm.

22Q ' P~ .42#t'.* ~ ,1' *'sIf- lsvX?:ASZsCkfSZ-v?i>sXtx>9eo??;^ ,..........,j+,Rii:7?.2'- '' 422- - '.)

SI, 2 isSP,tl"9zzi:.X):~~~Vvt;

Fig. 8: ID-VD characteristics of D-PHEMT withgate length of 80 nm. Drain current is in A/wm.

The poor saturation characteristic isobserved as increasing of Ih relative to drainvoltage, VD. This may be caused by velocityovershoot as electric field increases [81 andalso by significant current conduction occuringin the buffer layer [9]. The current conductionin buffer layer, also known as parasiticconduction path is displayed in Fig. 9 for bothPHEMT structures. Electron concentration in

0-7803-8658-2/04/$20.00(c)2004 IEEE

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

channel and buffer layers of U-PHEMT andD-PHEMT are shown. From Fig. 9, it isobvious that concentration in the buffer ishigher in U-PHEMT than in D-PHEMT. Thisimprovement over U-PHEMT comes fromhigher conduction band edge over most of theAlGaAs layer in the D-PHEMT whichprovides less charge accummulation in theparallel channel [4]. The application ofAlGaAs as the buffer layer also helps tosuppress short channel effects caused byparasitic conduction[ 10].

A D -FtllEtr c1on.^Tel,

-C 1,-i. 1__ 1

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Fig. 9 Log1o of electron concentration in thechannel and buffer layer of both U-PHEMT and D-PHEMT structures at VG = -0.4 V.

IV. CONCLUSION

The device performance of uniformly-dopedand delta-doped PHEMT structure are reportedwhere the performance of the D-PHEMT isfound to be superior. This attributed to itsdeeper conducting channel and reducedparasitic conduction. These propertiescontribute to higher transconductance andimproved drain current.

REFERENCES

[1 ] F. Diette, D. Langrez, J.L Codron, E.Delos, D. Theron and G. Salmer, "1510mS/mm 0.1 jim gate lengthpseudomorphic HEMTs with intrinsiccurrent gain cutoff frequency of 220GHz", Electronic Letters, Vol. 32, No. 9,pp. 848-849, 1996.

[2] J. Dickmann, Heinrich Daembkes, H.Nickel, W. Schlapp and R. Ldsch,"Double-Side Planar DopedAlGaAs/lnGaAs/AlGaAs MODFET with

Current Density of 1 A/mm", IEEEElectron Device Letters, vol.12, no. 6,pp.327-328, 1991.

[3] Nick Moll, Mark H. Hueschen and AliceFischer-Colbrie,"Pulse-DopedAlGaAs/lnGaAs PseudomorphicMODFETs", IEEE Transactions onElectron Devices, vol.35, no. 7, pp. 879-886, 1991.

[4] Ki Wook Kim, Hong Tian and Michael A.Littlejohn, "Analysis of Delta-Doped andUniformly-Doped AlGaAs/GaAs HEMTsby Ensemble Monte Carlo Simulations",IEEE Transactions on Electron Devices,vol.38, no. 8, pp. 1737-1742, 1991.

[5] Loi D. Nguyen, David Radolescu, Paul J.Tasker, William J. Schaff and Lester F.Eastman, "0.2 sum Gate-Length Atomic-Planar Doped PseudomorphicAl0.3Ga0.7As/In0.25Ga0 75As MODFET'swith overfT 120 GHz", IEEE ElectronDevice Letters, vol. 9, no. 8, pp. 374-376,1988.

[6] Helmut Brech, PhD. dissertation, ViennaUniversity of Technology, Austria, 1998.

[7] R. Stenzel, J. Hontschel and W. Klix,"Simulation of ultra-short channel HEMTswith different gate concepts by 2D/3D-hydrodynamics models", Proc. of 14thWorkshop on Modelling and Simulation ofElectron Devices, Barcelona, Spain,October 16-17 2003.

[8] Yuji Awano, Makoto Kosugi, KinjiroKosemura, Takashi Mimura and MasayukiAbe, "Short-Channel Effects inSubquarter-Micrometer-Gate HEMT's:Simulation and Experiment", IEEETransactions on Electron Devices, vol.36,no. 10, pp. 2260-2266, 1989.

[9] S. M. Sze, 'High Speed SemiconductorDevices", John Wiley & Sons, Inc.,Canada, 1990.

[10] James A. Adams, I.G. Thayne, C.D.W.Wilkinson, S.P. Beaumont, N.P. Johnson,A.H. Kean and C.R. Stanley, "Short-Channel Effects and Drain-InducedBarrier Lowering in Nanometer-ScaleGaAs MESFETs", IEEE Transactions onElectron Devices, vol.40, no. 6, pp. 1047-1052, 1993.

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