IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 45,NO.10,OCTOBER 1998 2089 Numerical Analysis of Device Performance of Metamorphic In Al As/In Ga As HEMT’s on GaAs Substrate HenriHappy,Sylvain Bollaert, Herv´ e Four ´ e, and Alain Cappy, Senior Member, IEEE Abstract—A numerical model describing the influence of InAs mole fraction on metamorphic HEMT structures (MM-HEMT) is proposed. The material properties are calculated using the Monte Carlo method, while the charge control law is calculated using a self-consistent solution of Poisson’s and Schr ¨ odinger’s equations. The modeling of the dc, ac, noise and high frequency performance of a device with 0.25- m gate length is performed using the quasi- two-dimensional (Q2D)approach. This analysis shows that an InAs mole fraction of about 0.40 is an optimum composition for manufacturing high gain, low noise amplifiers. In this range of composition, the performance of MM-HEMT structures is similar to that obtained for lattice-matched HEMT’s on InP substrates. I. I NTRODUCTION T HE operating frequency of monolithic integrated circuits has recently been extended into the millimeter-wave range [1],[2].HEMT’sgrown on GaAsor InP substrates are used to reach this frequency range, because these are the only active devices showing high cutoff frequencies, low noise figure, and high power-handling capabilities. In addition, the development of commercial millimeter-wave integrated cir- cuits requires a high performance device with low production cost.In thisrespect, the GaAssubstrate offers three main advantages over the InP substrate: it is less fragile and less expensive than the InP substrate, and wafers with diameters as large as six inches are now available. However, lattice matched HEMT’s on InP (LM-HEMT) exhibit higher cutoff frequencies and lower noise figure than GaAs HEMT’s. Without using an InP substrate while maintaining performance similar to that of LM-HEMT,a new structure called metamorphic HEMT (MM-HEMT) has been proposed [3]. The MM-HEMT structures typically usean unstrained In Al As/In Ga As material system grown on GaAs (Fig.1). The InAsmole fractions “ ” and “ ” are chosen to match the lattices of the InAlAs and InGaAs layers. The metamorphic buffer (gradual layer composition of In Ga As or In Al As with grown at low temperature, is used to accommodate the largelatticemismatch between the GaAs substrateand the activelayer.By optimizing the metamorphic buffergrowthconditions, the threading dislocations are suppressed, and high-quality active layers Manuscript received March 26, 1998;revised May 15, 1998.The review of this paper was arranged by Editor W. Weber. The authors are with the Institut d’Electronique et de Micro ´ electronique du Nord,UMR CNRS 9929, Universit ´ e des Sciences et Technologies de Lille, 59652 Villeneuve d’Ascq-France (e-mail: happy@iemn.univ-lille1.fr). Publisher Item Identifier S 0018-9383(98)07456-5. Fig.1. Cross section of the metamorphic HEMT (MM HEMT) structure. are achieved. High performance devices have already been manufactured in the field of low noise and power amplifiers [4]–[6]. In the MM-HEMT structure, an unstrained channel can be obtainedfor a largeInAs mole fractionrange.In In Al As/In Ga As heterojunctions, the InAs mole fraction has a significant influence on the material properties and consequently on the device performance. In fact, determining an optimum composition for a specific applicatio is important because the InAs mole fraction influences all the material parameters. For instance, as the InAs mole fraction increases, the low field mobility increases while the conducti band discontinuity decreases. So it is still an open question a to the optimum composition for high device performance. The aim of this work is to analyze numerically the effect of material composition on metamorphic HEMT devices. The application considered is low noise amplification in the millimeter-wave range (V and W bands). This work is based on the Monte Carlo method, the self-consistent resolution of Poisson’sand Schr ¨ odinger’sequations, and the quasi-two- dimensional (Q2D) approach [7]–[9]. The advantages and weaknesses of these modeling techniques have been discusse in a numberof papers [10]–[12]. The aim ofthis paper is notto explore these works again, butrather to use them in determining an optimum InAs mole fraction for the given application. The method used here comprises three phases. The material properties are firstinvestigated as a function of InAs mole fraction; nextthe electron transport properties of the structurearestudied using Monte Carlo analysis; finally the charge control analysis of the layers is calculated 0018–9383/98$10.00  1998 IEEE