Device linearity and intermodulation distortion comparison of dual material gate and conventional AlGaN/GaN high electron mobility transistor

Abstract

In the work proposed, linearity performance of dual material gate (DMG) AlGaN/GaN HEMT has been analyzed and compared with the corresponding performance of Single Material Gate (SMG) AlGaN/GaN HEMT using ATLAS device simulation. Specifically, we investigate the linearity of DMG and conventional AlGaN/GaN HEMT based on the linearity metrics such as g_m, $g_{m_2}$, $g_{m_3}$, VIP₂, VIP₃, IIP₃, IMD₃ and 1-dB compression point. The impact of various device parameters on the device linearity such as the channel length, doping and thickness of the barrier and spacer layer, Al mole fraction and the work function difference of the two gate metals has also been investigated. It is observed that a suitably designed DMG AlGaN/GaN HEMT can considerably improve the linearity performance and minimize intermodulation distortion due to reduced drain induced barrier lowering and high-field effect; and a more uniform electric field for applications in 3-G mobile communication and low noise amplifiers.

Introduction

The advancement of wireless communication systems and the rapid growth of mobile services have led to tremendous growth of radio frequency (RF) power devices [1]. In wireless communication systems, devices must maintain linear operation even while receiving a weak signal in the presence of a strong interfering one. The strong interferer signal, otherwise, might swamp out the desired weak signal or might cause cross-modulation. RF circuit applications often require transistors with low intermodulation distortion. High band gap GaN devices are ideal candidates for applications requiring high power and linearity simultaneously [2]. AlGaN/GaN HEMTs comprise several key attributes towards realizing high-linearity devices. AlGaN/GaN HEMTs have mainly been developed as candidates for next generation RF/microwave power amplifiers and are very attractive devices for use in low noise amplifiers, LNAs [3].

The nonlinearity of a device is one of the key sources of nonlinear behavior of microwave circuits. The second and third order intermodulation (IM) products resulting from nonlinearity of the device can cause IM distortion and are a major source of noise in the communication system. To maximize sensitivity of the receivers and minimize interference from the transmitters, reliable prediction and minimization of IM distortion is very important in the communication systems such as mobile systems. Infact, there has been a significant ongoing research in the field of intermodulation distortion analysis and characterization due to increase in the demand for highly linear components for use in digital telecommunication systems [2]. Linearity is an essential requirement in all RF systems ensuring that intermodulation and higher order harmonics are minimal at the output [4]. With radical increase in the demand for mobile communication and wireless systems, linear and low noise systems have become potential solutions for achieving higher system performance. Thus, there is a compelling need for innovative designs with higher transconductance and improved linearity figure of merits (FOMs).

During the past decade, excellent high speed performance has been achieved through improved design and reduced gate length. However, reduction in gate length below 100 nm results in severe downside of poor modulation efficiency (ME) [5], [6] and increased short channel effects (SCEs) [7], [8]. ME is related to the average electron transport velocity traveling through the channel, which in turn is related to the electric field distribution along the channel. In SMG HEMTs, the electrons enter the channel with a low initial velocity, gradually accelerating towards the drain; such that maximum electron drift velocity is obtained near the drain [9]. Hence, the speed of the device is affected by a relatively slow electron drift velocity in the channel near the source region leading to poor ME in such devices. As the carriers move from source to drain, they can acquire enough kinetic energy in the high field region of the drain junction so as to cause impact ionization. This effect arising from the heating of carriers in short channel devices is the high-field effect [10]. It is substantially reduced in DMG HEMT due to the redistribution of the electric field in the channel [11]. Long et al. [9] applied DMG architecture on pseudomorphic InGaP/InGaAs n-channel heterostructures grown on semi-insulating GaAs substrate using the method of tilt angle evaporation (TAE) to deposit material 1, with the photoresist layer as a shadow. For the deposition of material 2, normal evaporation was used. In DMG architecture, the work function difference between the two gate metals results in a step in the channel potential profile which leads to reduced drain induced barrier lowering (DIBL); enhanced modulation efficiency (ME); substantially reduced high-field effect; increased drain current and transconductance ensuring that intermodulation and higher order harmonics are minimal at the output [11], [13].

Recently, we reported the performance enhancement in DMG Al_0.2Ga_0.8N/GaN HEMT [11] due to improved gate control which results in high drain current, transconductance and reduced high-field effect. The work also shows the effect of DMG incorporation on conventional structure in terms of electron concentration and on current of the device. The results obtained from the model agree well with the ATLAS [12] simulation results and show that the DMG architecture exhibits improved gate controllability over the channel. Furthermore, the step in the channel potential profile induces screening of the drain potential variations, thereby suppressing the SCEs. Moreover, due to the peak in electric field at the interface of the two gate metals, the average electric field in the channel is enhanced which leads to an increase in the carrier velocity in the channel leading to improved modulation efficiency and this has a direct bearing on the on current which is also enhanced concomitantly. Results also emphasize that the introduction of DMG structure over their single gate counterparts offers a new way of improving the short channel behavior of AlGaN/GaN HEMTs and also shows potential for many future applications where high performance HEMTs, with gate lengths down to sub-100 nm, are required. In this work, we investigate and compare the potential of DMG AlGaN/GaN HEMT with conventional Single Material Gate (SMG) HEMT for its improved linearity and reduced distortion performance. ATLAS device simulator [12] has been used for the study as physics based device simulators provide extra insight into the device physics than the traditional compact models. Further, to maximize the device performance for high linearity applications, the simulation study has been extended to examine the impact of variations in (1) total gate length while keeping the lengths of individual gate metals equal; (2) work function difference between the two gate metals; (3) thickness of the barrier layer ‘d’; (4) thickness of the spacer layer ‘d_i’; (5) doping of the barrier layer ‘N_d’ and (5) Al mole fraction ‘m’.