1554 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 51, NO. 10, OCTOBER 2004 Surface-Related Drain Current Dispersion Effects in AlGaN–GaN HEMTs G. Meneghesso, Member, IEEE, Giovanni Verzellesi, Member, IEEE, Roberto Pierobon, Fabiana Rampazzo, Alessandro Chini, Umesh K. Mishra, Fellow, IEEE, Claudio Canali, Associate Member, IEEE, and E. Zanoni, Senior Member, IEEE Abstract—Drain current dispersion effects are investigated in AlGaN–GaN HEMTs by means of pulsed, transient, and small-signal measurements. Gate- and drain-lag effects charac- terized by time constants in the order of – s cause dispersion between dc and pulsed output characteristics when the gate or the drain voltage are pulsed. An activation energy of 0.3 eV is extracted from temperature-dependent gate-lag measurements. We show that two-dimensional numerical device simulations accounting only for polarization charges and donor-like traps at the ungated AlGaN surface can quantitatively reproduce all dispersion effects observed experimentally in the different pulsing modes, provided that the measured activation energy is adopted as the energetic distance of surface traps from the valence-band edge. Within this hypothesis, simulations show that surface traps behave as hole traps during transients, interacting with holes attracted at the AlGaN surface by the negative polarization charge. Index Terms—AlGaN–GaN HEMTs, current collapse, device simulation. I. INTRODUCTION R F drain-current ( ) collapse is the major factor limiting the output-power density at microwave frequencies in GaN-based FETs [1]. Its detrimental consequences on device performance have been shown by using different experimental techniques, including measurements of pulsed versus drain–source-voltage ( ) characteristics, gate- and drain-lag transients, transconductance ( m ) frequency dispersion, and RF response [1]–[6]. Several studies suggest that surface states can play a predominant role in originating the observed device behavior. Indications in favor of surface-induced collapse are: 1) dispersion effects are reduced by SiN surface passivation [7] which decreases the density of deep levels [8], and/or it blocks electrons from getting trapped at the surface [9], or it traps positive charge at the interface, neutralizing the net neg- ative charge at the surface [10] and 2) in unpassivated devices Manuscript received March 10, 2004; revised June 30, 2004. This work was supported in part by Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR) under the PRIN project “GaN heterostructure FETs for wideband telecom systems,” in part by the Agenzia Spaziale Italiana (ASI), and in part by the European Research Office of the US Army under Contracts N68171-01-M-5822 and N62558-02-M-5600. The review of this paper was arranged by Editor M. Anwar. G. Meneghesso, R. Pierobon, F. Rampazzo, and E. Zanoni are with the De- partment of Information Engineering and INFM, University of Padova, 35131 Padova, Italy (e-mail: gauss@dei.unipd.it). G. Verzellesi and C. Canali are with the Department of Information Engi- neering and INFM, University of Modena and Reggio Emilia, I-41100 Modena, Italy. A. Chini and U. K. Mishra are with Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106 USA. Digital Object Identifier 10.1109/TED.2004.835025 dispersion is also strongly reduced by adding p or n layers on access regions [11], [12]. The mechanism by which a high-density distribution of sur- face states can cause the collapse has been clearly delineated in [2]: the ungated surfaces between gate and source/drain con- tacts act as “virtual gates,” modulating the underlying deple- tion region through changes in the trapped charge density; as the gate–source voltage ( ) is changed abruptly, these vir- tual gates respond with the times characteristic of carrier cap- ture/emission phenomena, this leading to delayed switching, hence, the RF collapse. Owing to the potential barrier at the AlGaN–GaN heterointerface and within the AlGaN barrier, low energy electrons in the channel can not interact with deep states on the surface. There are, however, two mechanisms enabling electrons to modulate the surface charge: 1) Charging up of sur- face states in the drain access region can occur due to tunneling of electrons from the gate into the surface states, possibly fol- lowed by surface conduction provided by hopping effects or lossy dielectrics [2], [5], [9], and 2) during operation at high drain voltages, hot electrons in the channel can overcome the potential barrier at the interface and in the AlGaN layer, and be trapped at the surface, as it occurs in other III-V MESFETs and HEMTs [13]. As pointed out in [14], overall charge neutrality within the AlGaN–GaN structure requires the two-dimensional (2–D) electron gas (2DEG) at the AlGaN–GaN heterointerface to be balanced, at equilibrium, by an equal, positive charge in the AlGaN barrier and/or at the surface. Neglecting the con- tribution of doping in the AlGaN layer, this positive charge can in principle be provided either by ionized, donor-like surface traps or by holes attracted at the AlGaN surface by the negative polarization charge or by both. In the absence of hole charge, surface donors must be located at 1.65 eV below the conduction-band edge ( ), in order for a 2DEG with a sheet density cm to form at the AlGaN–GaN interface [14]. The presence of such a deep level can actually explain gate lag and the related -collapse phenomena characterized by time constants in the order of 1 s or more, which are most commonly observed [5], [9]. In this paper, the RF current collapse phenomenon is charac- terized in AlGaN–GaN HEMTs by means of pulsed, transient, and small-signal measurements. Measurements points out dis- persion effects characterized by relatively short-time constants (on the order of 10 to 100 s). Moreover, 2-D numerical de- vice simulations are adopted to analyze the influence of surface states on the pulsed characteristics of AlGaN–GaN HEMTs. 0018-9383/04$20.00 © 2004 IEEE