2136 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 10, OCTOBER 2005 InP HEMT Downscaling for Power Applications at W Band Farid Medjdoub, Mohammed Zaknoune, Xavier Wallart, Christophe Gaquière, François Dessenne, Jean-Luc Thobel, and Didier Theron, Member, IEEE Abstract—We have developed new solutions for InP high-elec- tron mobility transistor (HEMT) scaling for power applications at W band. We have shown that the use of a small barrier thick- ness in order to respect the aspect ratio for a 70-nm gate length results in a significant kink effect and high gate source capaci- tances. We have also shown through a theoretical study that a struc- ture containing an InP layer between the cap layer and the barrier would support both the frequency performances and the break- down voltage. Thus, we propose an HEMT structure containing a thick InP/AlInAs composite barrier and where the gate is buried into the barrier. This enables us to respect the aspect ratio and si- multaneously to obtain an important drain current density without observing any kink effect. Moreover, we have applied this process to structures containing innovative large band-gap InP and InAsP channels. We have achieved the best frequency performances ever reached for an InP channel HEMT structure. Power measurements at 94 GHz were performed on these devices. The InAsP channel HEMT demonstrated a maximum output power of 260 mW/mm at 3 V of drain voltage with 5.9-dB power gain and a power-added efficiency of 11%. These results are favorably comparable to the state-of-the-art of InP-based HEMT at this frequency. Index Terms—Compound semiconductor, high-electron mo- bility transistor (HEMT), high frequency, InAsP channel, InP, microwave, power. I. INTRODUCTION A LL microwave systems such as radar, smart munitions, or communication satellites require a power stage in the emission part. To prepare future communication systems at 80 and 160 Gbits/s namely for military applications, it is necessary to imagine and realize devices able to operate at W band with high-power performances. GaAs-based high-electron mobility transistors (HEMTs) have demonstrated good performances at 94 GHz [1]. However, they suffer from a limited cutoff fre- quency compared to InP-based HEMTs [2] which leads to lower power gain and a power-added efficiency (PAE) of 94 GHz. InP HEMTs have higher gain, higher cutoff frequency, lower source resistance, higher maximum current densities, and higher sub- strate thermal conductivity compared to GaAs-based ones [3], [4]. The best reported data for an InP HEMT at W band was obtained on a 640- m single-stage monolithic microwave in- tegrated circuit amplifier using 0.15- m gate length InGaAs channel HEMTs. It has delivered a 203-mW/mm output power with 13% PAE and a 4-dB power gain at 94 GHz. In that frame, Manuscript received May 2, 2005; revised July 11, 2005. The review of this paper was arranged by Editor C.-P. Lee. The authors are with the Département Hyperfréquences et Semi-conducteurs, Institut d’Electronique de Microélectronique et de Nanotechnologie, 59652 Vil- leneuve d’Ascq Cedex, France (e-mail: faridmedjdoub@hotmail.com). Digital Object Identifier 10.1109/TED.2005.856176 our aim is to investigate novel structures in order to achieve higher power performances at 94 GHz. Improving gain at 94 GHz requires us to decrease the gate length of our HEMTs under 0.1 m to typically reach 70 nm. Nevertheless, in order to avoid short-channel effects, it is neces- sary to maintain an aspect ratio ( is the gate length and a is the gate to channel distance) [5]. Thus, the reduction of the gate length implies the decrease of the barrier thickness. That leads to the increase of the gate current by a tunneling ef- fect [6] and to the reduction of the current density due to the influence of the surface states which is directly detrimental for power performances. Moreover, a too-reduced gate length re- sults in the increase of the electric field in the high field region of the channel. Therefore, other changes are necessary for the management of the device breakdown. This can be done by in- creasing the bandgap of the channel using an InP [7] or InAsP channel for instance. Furthermore, previous work reported that the insertion of an InP layer between the cap layer and the barrier made it possible to passivate the semiconductor on each side of the gate and consequently to benefit from a shorter effective gate length which results in a better cutoff frequency [8], [9]. In addi- tion, our Monte Carlo simulations show that this InP layer would be favorable to the breakdown voltage. In other words, the in- sertion of an InP layer between the cap layer and the AlInAs barrier would not only be favorable to the cutoff frequency, but also to the breakdown voltage. In this paper, using a Monte Carlo model, we will show that the insertion of an InP layer between the cap layer and the bar- rier is favorable to the breakdown voltage. Then, we will high- light the limits of the barrier thickness reduction. Consequently, we will suggest a solution for InP HEMT scaling for power ap- plications in W band using an InP/AlInAs composite barrier. Finally, we will apply this solution to structures containing InP and InAsP channels. To our knowledge, these types of channels have never been used for power amplification at such frequen- cies. II. MONTE CARLO STUDY OF THE INFLUENCE OF AN InP LAYER BETWEEN THE CAP LAYER AND THE BARRIER OF AN InP HEMT The main features of our Monte Carlo HEMT model have already been published [10] and the reader will only be briefly reminded of those features with some significant improvements. Concerning the electron transport, the model incorporates a -L-X band structure for all III-V semiconductor compounds used (AlGaAs, AlInAs, GaInAs, InP, etc.). We use a regular bi-dimensional mesh size where the carriers move and where the 0018-9383/$20.00 © 2005 IEEE Authorized licensed use limited to: IEEE Xplore. Downloaded on February 5, 2009 at 13:21 from IEEE Xplore. Restrictions apply.