~ Pergamon Solid-State Electronics Vol. 39, No. 10, pp. 1423-1426, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved PII: S003g-l101(96)00066-4 0038-1101/96 $15.00 + 0.00 INFLUENCE OF KINK EFFECT ON NOISE MEASUREMENTS IN InP SUBSTRATE PHEMTs AT MICROWAVE FREQUENCIES P. ROUQUETTE I, D. GASQUET I, F. BARBEROUSSE ~, W. DE RAEDT 2 and Y. BAEYENS 2 'Centre d'Electronique de Montpellier, Universit6 Montpellier II, F-34095 Montpellier cedex 05, France 21MEC, Kapeldreef 75, B-3001 Leuven, Belgium (Received 6 February 1996; in revisedform 18 April 1996) Abstract--We present d.c. and r.f. measurements in AllnAs/GalnAs/InP high electron mobility transistors with different gate length. First we observed a kink effect in the I-V characteristics. Next we have extracted the different parameters of the r.f. equivalent circuit from scattering parameters. Finally we have deduced the current spectral density and time constants from the noise measurements. These results confirm the presence of an excess noise contribution related to impact ionization in the channel, which affects low noise applications even at microwave frequencies. Copyright © 1996 Elsevier Science Ltd 1. INTRODUCTION 3. d.c. MEASUREMENTS High electron mobility transistors (HEMTs) have attracted the interest of microelectronics engineers owing to their improved noise and high-frequency performance. Nevertheless, some GalnAs/InP pseu- domorphic HEMTs (PHEMTs) present anomalous characteristics linked with the so-called kink effect. Recent studies on kink effect in InA1As/InGaAs/InP HEMTs have been performed in order to understand the underlying mechanisms[l]. In this paper we show that the kink phenomenon induces excess noise even at high frequencies. The main purpose of this work is to investigate the nature of the noise caused by this parasitic effect. 2. DEVICE STRUCTURE We have investigated "T" gate PHEMTs structures with a gate length L~ = 0.3, 0.5, 1 and 2 pm and gate width W = 100 ttm. Their transition frequency is in the range 30-80 GHz depending on the length. The InA1As/InGaAs layers were grown by molecular beam epitaxy. The general structure of our samples is shown in Fig. 1. The Gaj_ JnxAs channel is made of a strained layer (x = 0.53) and the electron mobility has a value of 10,400 cm2/Vs. The planar doped layer with a doping density of 5 x l0 n cm -2 supplying the carriers is separated from the channel by a 60/~ InA1As spacer. The Schottky barrier is made from 200 A, undoped InAIAs and to increase the ohmic contact quality a cap layer is made of 70 A. strained GalnAs heavily doped with silicon (6 x 10Is cm-3). All measurements were performed on the wafer using a probe station. The output I-V characteristics (Fig. 2) clearly show a kink effect which can be attributed to impact ionization in the channel due to lowering of the bandgap of InGaAs (~0.76 eV)[2]. This phenomenon appears at a bias Vds = Vk~k of about 0.5 V. The high value of the electric field in the channel gives rise to an important flow of generated electrons towards the drain and so contributes to a sudden increase of the drain current. At the same time the generated holes remain confined in the AlInAs buffer layer due to the GaInAs/AllnAs heterojunction barrier. Holes drift parallel to the channel towards the gate and recombine with electrons of the channel. Nevertheless, some holes are able to surmount the potential barrier at the AlInAs/GaInAs interface. This real hole space transfer gives rise to a slight increase in the gate current, 4. INFLUENCEOF KINK EFFECT ON NOISE MEASUREMENTS The measurements of the noise figure were performed in the frequency range 200 MHz-18 GHz. First of all, the equivalent circuit of the transistors have been extracted from the measured scattering parameters using the analytical method developed in Ref.[3]. Table 1 gives the values of the equivalent circuit elements for each different gate length at the same bias conditions Fds = 1 V, Fss = 0 V. Then, using the theory developed in Ref.[4] we have calculated the noise factor of the transistors in the above frequency range only taking into account the 1423