TCAD ANALYSIS AND SIMULATION OF T-Gate E-ODE GaN HEMT

Abstract

GaN-HEMT semiconductor technology has already solidified its position as the dominant contender in the realm of high-power and RF applications. In this thesis, performance of GaN HEMT device has been studied and various techniques have been proposed to overcome the major roadblock of GaN HEMT such as spilling of 2DEG, leakage current, threshold voltage (Vth), and parasitic capacitances. In this regard, firstly polarization induce doping in the buffer layer (buffer Engineering), heavily doped source/drain region, and recessed T-gate (Gate Metal Engineering) are integrated simultaneously on a GaN HEMT i.e. T-gate E mode polarization induced doped buffer HEMT has been proposed. The polarization-induced doping in the buffer layer bent the conduction band upwardly convex, which enhanced the 2DEG confinement, reduced the buffer leakage current, and significantly uplifted the breakdown voltage (33 V), which is 5 times higher than the conventional InAlN/AlN GaN buffer HEMT (8 V). The recessed gate engineering further enhances various performance parameters. It achieves an impressive on/off ratio of 109 , reduces the subthreshold swing (SS) to 78 mV/dec, and minimizes the drain-induced barrier lowering (DIBL) to 100 mV/V. The proposed device exhibits a high current density of 2.8 A/mm, transconductance 1.55 S/mm, cutoff frequency fT (583 GHz), and maximum oscillation frequency fmax (840 GHz). At room temperature, the carrier density and mobility measured are 2.8 × 1013 cm-2 and 1250 cm2 /Vs. The large Johnson figure of merit (fT. VBR) 19.23 THz and (fT. fmax) 1/2 699 GHz shows the potential of the proposed device for high-power millimeter wave applications. Additionally, high-k gate oxide engineering has also been conducted, where a specific region of the recessed gate is substituted with a high-k dielectric material. This integration of the high k dielectric brings about notable improvements in both interfacial and transport characteristics, Megha Sharma vi while concurrently reducing the gate leakage current. However, the insertion of gate oxide alone does not enable E-mode operation. As a result, the superior option for achieving E-mode operation is through the utilization of gate-recessed engineering technique. After analyzing the performance of T-gate E mode polarization induced doped buffer HEMT, the in-depth investigation of the influence exerted by the T-gate shape on both the DC and RF performance of the device have been analyzed. The impact of T-shaped gate geometry on parasitic capacitance and RF Figure of Merits (FOMs) such as maximum oscillation frequency (fmax), gain-bandwidth product (GBP), cut-off frequency (fT), maximum stable and available power gain (Gms and Gma), maximum transducer power gain (MSG) and stability factor (k) has also been investigated for different gate head length (Hlength), gate stem height (Sheight), and gate foot length (Flength). The simulated results confirm that the proper choice of Hlength (280 nm), Sheight (100 nm), and Flength (10 nm) significantly reduced the parasitic capacitance (Cgs = 350 fF/mm and Cgd = 140 fF/mm) and enhanced the fmax (840 GHz), GBP (636 GHz), fT (583 GHz) and also improve the power gains. The simulated results of the proposed device provided the detailed knowledge about the impact of T-gate geometry on RF FOMs at such aggressively scaled dimensions. The noise and scattering parameters are critical in determining the overall performance of a device. The noise parameters are important because they determine the amount of noise that is present in the signal as it passes through the device. On the contrary, the scattering parameters determine the magnitude of power dissipated during the transmission of the signal across the device. Thus to account the noise performance T-gate E mode polarization induced doped buffer HEMT, the in depth investigation of the small signal and noise behavior of proposed device has been studied. The results show that the polarization-induced doping in the buffer layer bent the conduction band upwardly convex, which enhanced the 2DEG confinement, reduced the buffer leakage current, and significantly enhance the transconductance (1.55 S/mm). The increment in transconductance leads to a reduction in the reflection coefficient (S11, S22) and an improvement in the transmission coefficient (S21) as compared to GaN buffer HEMT. Furthermore, noise parameters such as auto/cross-correlation factor, minimum noise figure, noise conductance, and optimal noise resistance and reactance were also evaluated for the proposed device. Simulated results reveal that the proposed device has a lower noise figure and noise conductance than the GaN buffer HEMT by 57% and 20%, respectively. This research demonstrates that the T-gate polarization doped buffer (PDB-HEMT) structure is an excellent choice for Low Noise Amplifiers (LNA) operating at higher frequencies. Megha Sharma vii Although using the back barrier engineering enhances the RF and noise performance of device. However, it also reduces the drain current density. Therefore, it is important to find a balance between RF performance and drain current density in device. In this regard, a double channel engineering is executed on T-gate HEMT by inserting the AlN layer below the InAlN/GaN interface developing a double channel HEMT. Simulation results showed that formation of double channel significantly enhance the drain current density of 2.5 A/mm. However, due to a lack of gate controllability over a lower channel, the high leakage current is observed in double channel HEMT. This issue has been addressed by using the InGaN as a back barrier, which improves the carrier confinement of 2DEG by raising the conduction band for the GaN buffer and considerably improves the gate controllability over a lower channel. The performance of the proposed device is compared with that of conventional double-channel HEMTs. The simulation results showed, the proposed double channel back barrier HEMT (DC BB-HEMT) exhibited substantial improvements in analog performance. The intrinsic gain, which indicates a higher signal amplification capability, improved by 50%. Similarly, the transconductance gain frequency (TGF) increased by an impressive 477%, while the early voltage was enhanced by 23%. These numbers suggest that the InGaN back barrier significantly contributes to the device's analog performance. Moreover, RF performance of (DC-BB-HEMT) also compared with conventional DC-HEMTs. Simulation results showed that both the cut-off frequency and maximum oscillation frequency increased by around 11.7% and 10%, respectively. Moreover, the study also investigated the device's linearity performance with varying back barrier distances. It has been observed that the linearity performance of proposed device improved at a back barrier distance of 30 nm. This result implies that careful optimization of the back- barrier distance can further enhance the device's linearity performance.