New investigation of an E-mode metal-insulator-semiconductor AlInN/AlN/GaN HEMT with an Au-T-gate

In a high electron mobility transistor (HEMT), the density of the two-dimensional electron gas (2DEG) channel is modulated by the application of a bias to a Schottky metal gate. These devices are depletion mode (D-mode), which means that a negative bias must be applied to the gate to deplete the electron channel and turn. The most challenging aspect in the present research activity on based-GaN devices is the development of a reliable way to achieve an enhancement-mode (E-mode) HEMT. Enhancement-mode GaN HEMTs would offer a simplified circuitry by eliminating the negative power supply. In this work, the aim is to investigate the different techniques which can influence the threshold voltage and shift it to a positive value. A novel E-mode metal-insulator-semiconductor (MIS) AlInN/GaN HEMT with an Au-T-gate has been investigated. The impacts of window-recess and deep-recess have been discussed, it was found that for dp=28 nm and wn=1.8 µm the threshold voltage achieves 0.7 V and the transconductance (Gm) peak value of 523 mS at Vgs=3.5 V. The drain current characteristic has been demonstrated.


INTRODUCTION
The wireless communication industry has continued to witness exponential growth due to recent advances in modern electronics and semiconductor (SC) technologies [1]. Since 1960, silicon (Si) has been the most frequent SC for power electronics devices. However, today the continuous demand for higher current, voltage and power density capability, as well as the need of better energy efficiency to decrease the global energy consumption, is the main causes to bring in new semiconductor technologies in power electronics and to rise over the inherent limitations of Si-based devices [2,3]. The wide bandgap SCs are the most promising technology for next-generation applications. Especially, the wide bandgap gallium nitride (GaN) has attracted attention as the highly bright material for electronic devices because of its excellent transport properties, high critical electric field, and thermal stability [4,5]. It demonstrates high electron velocity (2.5x107 cm/s) and good electron mobility (>1500 cm 2 /V.s). It has a high breakdown field (3 MV/cm) as well due to its wide bandgap (3.507 eV) [6,7]. Over other highly demanded SCs such as Si and GaAs, GaN-based SCs offer five keys advantages, which are: high operating temperature, high critical electric field, high current densities, high-speed switching, and low on-resistances. The development in GaN-based high HEMT technologies, radio-frequency power performance of conventional D-mode AlGaN/GaN has been given off and is approaching the limit of their material structures. The maximum output power of the device is limited by its maximum drain current and breakdown voltage. However, many power applications require the use of E-mode HEMT. The main challenge in the present research on GaN devices is developing an E-mode HEMT. The E-mode HEMT offers a simplified circuit; by eliminating the negative power supply. Several GaN HEMT structures have been proposed in the literature to meet the E-mode functionality [8][9][10][11]. To realize an E-mode HEMT and achieve a positive threshold voltage (Vth), many techniques have been presented: Increase the height of the Schottky barrier requires making a suitable choice of the material of the gate, the dielectric with high permittivity can be used [12,13], but this technique does not have a significant impact on Vth [14]. Decrease the discontinuity band between GaN and barrier layer, in effect, this method decreases the density of 2DEG confined in the quantum well, which results in lower drain current. Fluorine ions (F -) implantation under the gate has been used to achieve E-mode AlGaN/GaN HEMTs, all the ions should reside in the barrier layer, due to the channeling effect, and there is a high chance for (F -) to be deeply implanted into the 2DEG channel, which degrades the electron mobility [15]. The easiest method, reduce the thickness of the barrier layer under the gate by using a recessed gate structure [16]. The recessed gate with a high permittivity combination can improve and increase the threshold voltage.
In this work, we present the results of a simulation of the DC characteristics of an E-modde GaN HEMT obtained using a recess etch technique. This paper is organized as follow: the device structure and the physical properties of the AlInN/AlN/GaN HEMT structure have been presented. To highlight the advantage of the gate structure, the influences of the deep recess-gate and window-recess etching on the threshold voltage and transconductance have been investigated. Then, the current drain characteristic has been demonstrated. Figure 1 shows a cross-section of the epitaxial used to fabricate the device. The device under investigation was realized by TCAD-silvaco. The E-mode HEMT structure was grown on 1.5 µm sapphire as a substrate, which allows good thermal stability. The HEMT epitaxial begins with a 2 nm AlN nucleation layer allows to reduce strain, followed by an 800 nm undoped GaN layer, which maintains the rule of the channel. The optimized structure induced a 7 nm doped AlInN Barrier Layer, followed by a 30 nm high quality of dielectric layer within the gate recess; different properties of dielectric have been presented in Table 1. Finally, the electrodes have been deposited. Our choice of the gate material has been done after studied the resistivity and work function properties of several metals summarized in Table 2. The source-drain length is 8 µm; the source-gate length is 2 µm [17].

Physical properties
The AlInN/AlN/GaN HEMTs is characterized by channel formation [21]. High levels of polarization at the interface of AlInN/AlN/GaN heterojunction induce a polarization sheet charges σ in the high bandgap of AlInN, which causes the accumulation of attracted mobile carriers, electrons in the case of a positive sheet charge σ, confined in a quantum well along the heterojunction. This electron accumulation induced by polarization is called a 2DEG. The confined electrons present a very high mobility 1360 cm 2 /V.s, which is main characteristic of HEMTs. Therefore, the polarization modeling is critical for GaN-based devices. The induced sheet charge σ created at the interface between the AlInN layer and the GaN layer can be written as the function of spontaneous and piezoelectric polarization across the heterostructure interface, it is given by the following equation [22]: The primary parameter to enhance the performance of AlInN/AlN/GaN HEMTs is the concentration of 2DEG (ns) [21]. Understanding the physical phenomena governing the operation of the HEMT and the formation of a 2DEG requires solving the Schrödinger's and Poisson's equation [23]. The solution of Schrodinger's equation gives a quantized description of the density of states in the presence of quantum mechanical confining potential variations. In situations where there is strong quantum confinement in the SC, Atlas can include charge quantization effects. To do this, use the Schrodinger-Poisson solver. Once the carrier concentration is calculated using (2) To maintain self-consistency, we need to take into account that electrons are being emitted or captured by the donor and acceptor-like traps. Therefore, the concentration of carriers will be affected. This is accounted for by modifying the recombination rate in the carrier continuity equations. Phonon transitions occur in the presence of a trap (or defect) within the forbidden gap of the SC. This is essentially a two steps process, the theory of which was first derived by Shockley and Read [24] and then by Hall [25]. The Shockley-Read-Hall recombination is modeled as follows: where ETRAP is the difference between the trap energy level and the intrinsic Fermi level, TL is the lattice temperature in degrees Kelvin, and are the electron and hole lifetimes. We have taken into account the low and the high field mobility to consider various types of scattering mechanisms during the two-dimensional numerical calculation. Following the work of Albrecht, the low field mobility, as a function of doping and lattice temperature, can be given by following equation and the default parameter values for the Albercht Model are summarized in Table 3   The nitride specific field-dependent mobility model is based on a fit to Monte Carlo data for bulk nitride, which is described in the following equation and Table 4 presents the values of default nitride field dependent mobility mode parameter [27]: The AlInN/GaN heterojunction is used to take advantage of the 2DEG density. The total charge accumulated in the potential well and the threshold voltage are given simply by following equations [21]: To develop a E-mode HEMT, as we can see in (7), several techniques can shift the threshold voltage to positive value. First, increasing the height of schottky barrier, schottky gate metals with higher work function can be used to increase the schottky barrier height. However, the choice of appropriate gate materiel is limited and its impact is not significant. Second, decrease the discontinuity band by reducing the aluminum and indium molar fraction on the barrier layer in (8) [28]. However, the objective of using lattice matched AlInN/GaN will not be achieved and the 2DEG density will reduce which impact also the drain current. Third, reduce the barrier thickness under the gate; this will be achieved by using T-gate and varying the recess-window and recess-deep. the 2DEG can be completely depleted at zero gate bias and E-mode HEMTs are formed, with a deep-enough gate-recess etching. The disadvantage of this technique is that the drain current drops too. Fourth, Fluorine ions (F -) implantation under the gate is a smart option and has been used to achieve normally off AlInN/GaN HEMTs. Incorporated Fluorine ions act as immobile negative charges that can deplete the 2DEG and positively shift Vth. Ideally, all the Fions should reside in the AlInN layer. However, due to the channeling effect for the Fions in GaN lattice structure, as well as the non-uniformity in Fion energy distribution, there is a high chance for some Fions to be deeply implanted into the 2DEG channel, which degrade the electron mobility by impurity scattering. Generally, when converting from D-mode to E-mode, IDS, max drops by more than 40% [11].

RESULTS AND DISCUSSION
We present the results of a simulation of the DC characteristics of a normally-off GaN HEMT obtained using a recess etch gate technique. Figure 2 illustrates the polarization charge induced at the interface by spontaneous and piezoelectric polarizations; the addition of a thinner AlN layer allows increasing the value of the polarization charge at the interface AlN/AlInN. The AlInN/AlN/GaN HEMT achieves a very high value of the 2DEG, which is cannot only be accounted to the presence of high polarization. The large band discontinuity between AlInN and GaN is responsible too. Figure 3 (a) shows the conduction band energy for the AlInN/AlN/GaN heterojunction. The discontinuity in the band gap between the different layers forms a quantum well at the interface, which is illustrated by Figure 3 (b) the present of a thinner AlN layer contributes to improve the confinement of the 2DEG at the quantum well. In order to highlight the impact of deep-recess gate etch, the window-recess has been fixed at wn=1.7µm and the deep-recess has been varied from 24 nm to 28 nm by a step of 2 nm. Figure 4. displays the transfer function characteristics and transconductance of the AlInN/AlN/GaN HEMT at Vds=10 V. It was found that the threshold voltage shifted to a positive value for dp=28 nm, with a threshold voltage between +0.5 V et 1 V as extracted from the linear extrapolation of the I-V curve. The peak extrinsic transconductance of the studied device achieves a high value with a deeper etching gate, it was measured to be 523 mS at Vgs=3.5 V.  According to the previous result, the deep-recess has been fixed at dp=28nm and the window-recess gate etch has been varied from 1.6 µm to 1.8 µm by a step of 0.1 µm. Figure 5 shows the transfer function and the transconductance of the AlInN/AlN/GaN HEMT at Vds=10 V. This result shows perfectly that reducing the barrier layer under the gate leads to a positive value of the threshold voltage as has been presented in the bellow section. On the other hand, it was seen that the values of transconductance and the curves of Vg-Id decrease with the window-recess. The drain current characteristic is illustrated in Figure 6 the maximum saturation TELKOMNIKA Telecommun Comput El Control  New investigation of an E-mode metal-insulator-semiconductor … (Asmae Babaya) 537 current density for this device was almost 1A at Vgs=+4 V which is higher compared to the value reported in [17]. As has been shown the drain current is almost null for Vgs=0 V, which means that the proposed structure allows obtaining a normally-off AlInN/AlN/GaN HEMT and presents an excellent potential for power electronic applications.

CONCLUSION
In this work, a novel E-mode metal-insulator-Semiconductor AlInN/AlN/GaN HEMT has been investigated. The influence of the deep-recess and window recess gate etch on the threshold voltage has been studied. The optimal values to shift the threshold voltage to a positive value of 0.7 V are wn=1.8 µm and pd=28 nm. The transconductance peak value Gm=523 mS at Vgs=+3.5 V. The proposed structure of AlInN/AlN/GaN HEMT allows obtaining a drain current characteristic null for Vgs=0 V.