154 IEEE ELECTRON DEVICE LETTERS, VOL. 19, NO. 5, MAY 1998 New Aspects and Mechanism of Kink Effect in InAlAs/InGaAs/InP Inverted HFET’s Bogdan Georgescu, Student Member, IEEE, Marcel A. Py, Abdelkader Souifi, Georg Post, and G´ erard Guillot, Member, IEEE Abstract— The kink effect in InAlAs/InGaAs/InP composite channel heterojunction field effect transistors (HFET’s) was in- vestigated as a function of temperature and optical excitation. Drain source and gate current measurements show that above 325 K the kink effect disappears while the impact ionization process is still present. The kink at low temperatures is sup- pressed by illumination with photons of energy above 1 eV. These results prove that this parasitic effect is mainly related to the presence of traps in the top layers. I. INTRODUCTION L ATTICE-MATCHED InAlAs/InGaAs/InP composite channel transistors are promising devices for optoelec- tronic integrated circuits (OEIC’s) for optical communications at 1.3- m or 1.5- m wavelength [1], [2]. These devices are used in preamplifiers in pinFET photoreceivers. For OEIC’s, fast transistors with a very low-gate leakage current are needed; therefore, heterojunction field effect transistors (HFET’s) with undoped AlInAs barrier layers, InGaAs/InP double channel are suitable for these applications [2]. However, similarly to other III–V transistors, the perfor- mances of our InP-based FET’s are degraded by the kink effect [3]. This parasitic effect consists in a sharp increase in the drain-source current at a certain drain-to-source voltage , leading to high drain-source conductance and reduced voltage gain. Many studies of the kink in InGaAs channel transistors were reported, but the origin of this effect is still controversial. Some authors suggested that impact ionization is responsible for this effect [4], [5], while others attributed the kink effect to traps [3], [6]. Experimental results have confirmed that the kink effect has a complex behavior: it starts to decrease at low frequencies (10–100 Hz) and is absent at higher frequencies [7], [8]. It can be suppressed using low-temperature buffers [6] or using silicon nitride passivation [9], it disappears when the device is heated [7] and there is a correlation between impact ionization and the kink effect [10]. Composite InGaAs/InP/InAlAs FET’s are especially sensitive to kink effects and to frequency dispersion of drain conductance [11]. In order to clarify the role of the traps and that of impact ionization, measurements of and as function of and Manuscript received November 26, 1997; revised January 26, 1998. B. Georgescu, A. Souifi, and G. Guillot are with the Laboratoire Physique de la Mati´ ere (UMR CNRS 5511), Villeurbanne 69621, France. M. A. Py is with the Institute for Micro-Optoelectronics, Department of Physics, Swiss Federal Institute of Technology, Switzerland. G. Post is with France Telecom CNET–DTD, Laboratoire de Bagneux 92 225, France. Publisher Item Identifier S 0741-3106(98)03313-8. were performed in the 100–350 K temperature range, in dark conditions and under illumination. II. DEVICE FABRICATION The devices have a composite channel InGaAs/InP in order to combine the high mobility of InGaAs at low electric fields, with the low-impact ionization coefficient and the high electron drift velocity of InP at high electric fields [2], [12]. In order to minimize the gate leakage current and to obtain a higher breakdown voltage, an inverted lattice matched heterostructure was grown by MOVPE as follows: a 80-nm undoped InAlAs buffer, a 25-nm doped InP channel, a 2-nm InP spacer, a 4-nm InGaAs channel, a 50-nm undoped InAlAs barrier, and an InGaAs cap layer. FET’s with 0.8- m gate length were made using conventional lithography and a selective chemical etch for gate recess. The gate current is very low because of the non-doped InAlAs barrier. This result is required for baseband photodetection applications and motivates the use of MOVPE technology in OEIC’s design. III. RESULTS AND DISCUSSION The behavior of the characteristics in the 100–350 K range was studied and found to be similar to the one observed in InP channel HFET’s [7]. Fig. 1(a) and (b) shows, at 225 K and 325 K, respectively, the drain-source conductance as a function of for different . The kink effect corresponds to a peak on which is clearly observed around V at 225 K, but almost not present at 325 K. A common hypothesis states that the kink effect is induced by impact ionization which occurs in the InGaAs channel for high electric fields [4]. The holes generated from the impact ionization phenomenon flow towards the source region. Thus, a hole pile-up is formed reducing the ohmic drop at the source and leads to an increase of . However, we have also observed the kink effect in InP channel HFET’s [7], where the impact ionization component was not detected on the gate current. To clarify the role of the impact ionization in the kink effect, the gate current was measured as a function of temperature. In Fig. 2(a) and (b) the gate current is presented as a function of for different . The “bell” shape in is a clear evidence of impact ionization [13]. The gate leakage current has two principal contributions: near the pinchoff ( V) is dominated by the Schottky current, while in the normal operating zone, is dominated by 0741–3106/98$10.00  1998 IEEE