Jin-Shan Shi

Low-Temperature Ohmic Contact Formation in GaN High Electron Mobility Transistors Using Microwave Annealing

In this letter, a low-temperature microwave annealing (MWA) method is first reported for the formation of ohmic contact to AlGaN/GaN high electron mobility transistors (HEMTs). Compared with the traditional GaN devices annealed by rapid thermal annealing (RTA), MWA-HEMTs can achieve a low ohmic contact resistance with much smoother surface of ohmic contacts. The spike mechanism is observed in our ohmic contact annealed by microwave under a low-temperature condition. Besides, MWA-HEMTs have higher ION/IOFF ratio and lower gate leakage current than the fabricated RTA-HEMTs in this letter.

Impacts of Thermal Atomic Layer-Deposited AlN Passivation Layer on GaN-on-Si High Electron Mobility Transistors

Thermal atomic layer deposition (ALD)-grown AlN passivation layer is applied on AlGaN/GaN-on-Si HEMT, and the impacts on drive current and leakage current are investigated. The thermal ALD-grown 30-nm amorphous AlN results in a suppressed off-state leakage; however, its drive current is unchanged. It was also observed by nano-beam diffraction method that thermal ALD-amorphous AlN layer barely enhanced the polarization. On the other hand, the plasma-enhanced chemical vapor deposition (PECVD)-deposited SiN layer enhanced the polarization and resulted in an improved drive current. The capacitance-voltage (C-V) measurement also indicates that thermal ALD passivation results in a better interface quality compared with the SiN passivation.

AlGaN/GaN MISHEMTs with AlN gate dielectric grown by thermal ALD technique

Recently, AlN plasma-enhanced atomic layer deposition (ALD) passivation technique had been proposed and investigated for suppressing the dynamic on-resistance degradation behavior of high-electron-mobility transistors (HEMTs). In this paper, a novel gate dielectric and passivation technique for GaN-on-Si AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MISHEMTs) is presented. This technique features the AlN thin film grown by thermal ALD at 400°C without plasma enhancement. A 10.6-nm AlN thin film was grown upon the surface of the HEMT serving as the gate dielectric under the gate electrode and as the passivation layer in the access region at the same time. The MISHEMTs with thermal ALD AlN exhibit enhanced on/off ratio, reduced channel sheet resistance, reduction of gate leakage by three orders of magnitude at a bias of 4 V, reduced threshold voltage hysteresis of 60 mV, and suppressed current collapse degradation.

Investigation of Temperature Dependence, Device Scalability, and Modeling of Semifloating-Gate Transistor Memory Cell

A semifloating-gate transistor had been proposed and its memory function has been demonstrated recently. In this paper, we further investigate its temperature dependency, device scalability, and device modeling. The high-temperature behavior is studied by measuring its endurance, retention, and disturbance immunity at 85 °C. The device scalability down to the 14-nm technology node is investigated by simulation. Its macrodevice model for circuit design is also developed. Finally, a memory array with the specific peripheral circuits is designed using the device model developed in this paper.

Investigation of a GaN-on-Si HEMT optimized for the 5th-generation wireless communication

In this paper, AlGaN/GaN heterostructure high electron mobility transistors (HEMTs) have been fabricated on GaN-on-Si substrate. These devices were studied for the application of the 5^th generation (5G) wireless communication. The geometric parameters such as gate length (L_g), cap length (L_cap) and gate-to-source distance (L_gs) are optimized so that the cut-off frequency reaches 38.8 GHz. It is found that decreasing L_g and L_gs can both improve drain current and peak transconductance (G_mmax). Highest drain current of 1 A/mm at V_g=2V and biggest G_mmax of 240 mS/mm was achieved. A fabricated device with L_g of 100 nm, L_cap of 300 nm and L_gs of 800 nm showed a f_T of 38.8 GHz and a f_MAX of 60 GHz at V_ds=5V which can meet the requirement of the 5G application.

Investigation of spin-on-dopant for fabricating high on-current tunneling field effect transistor

Spin-on-dopant and rapid thermal diffusion are used as the doping method for fabricating planar tunneling field effect transistor (TFET) in this paper to acquire abrupt doping profile and high surface doping concentration. Due to the heavy surface doping the on-current of TFET is enhanced. The ambipolar characteristics of TFET are also inhibited by reducing the drain doping concentration.

Impact of geometric dimensions on the behavior of GaN MIS-HEMT fabricated on patterned sapphire substrate

In this paper, an AlGaN/GaN based MIS-HEMT using Al2O3 dielectric as gate insulator was fabricated. We adopted the patterned sapphire as the substrate (PSS) of high-quality AlGaN GaN epitaxial layers. We also studied the influence of different gate-drain space (L_gd) on breakdown voltage (V_BD) and on-state resistance (R_on) of GaN HEMT fabricated on patterned sapphire substrate. A breakdown voltage of 105 V was obtained with the L_gd of 2 μm and gate width of 32 μm. The specific on resistance was 4.7 Ω·mm when L_gd equals 0.5 μm. Meanwhile, it is found that the devices fabricated on the wet-etched pyramidal patterned sapphire substrate GaN exhibit better drive current stability than that of the devices on the conventional GaN-on-Sapphire substrate.

Mechanism of ohmic contact formation in AlGaN/GaN high electron mobility transistors using microwave annealing

The formation of Ti/Al/Ni/Au ohmic contact in AlGaN/GaN high electron mobility transistor (HEMT) by microwave annealing (MWA) has been proposed and studied. In this paper, we investigated the electrical characteristics of this contact structure, as well as its transmission electron microscopy (TEM) images, to analyze the mechanism of MWA for the formation of ohmic contact. Our analysis indicates that the enhanced microwave energy absorption results in enhanced alloy reactions in the contact region. The rapidly changing electromagnetic field generated by the microwave coil is shielded by the thin metal sheet and the eddy current is induced in contact metal pads, which can generate more heat on the metal stacks than other areas. Due to the local heating effect, a direct path for electron flow can be formed between the two-dimensional electron gas (2-DEG) and the metal pad, at a much lower substrate temperature than the conventional thermal annealing process.

Effect of device geometry on static and dynamic performance of AlGaN/GaN-on-Si high electron mobility transistor

This paper discusses the effects of several geometric parameters in DC and RF performances of AlGaN/GaN high electron mobility transistors (HEMTs) grown on high-resistivity silicon substrates. Those parameters include the dependency of gate length (L g), gate cap length (L cap) and gate-to-source distance (L gs). It is shown that decreasing L g and L gs can both improve maximum drain current and transconductance behaviors. The fabricated 50 μm wide GaN-HEMT exhibits the maximum drain current of 1 A mm−1 at V g = 2 V and maximum extrinsic transconductance G mmax of 240 mS mm−1. Besides, decreasing L g and L cap also provides the improvement on current gain frequency (fT ) and maximum oscillation cut off frequency (f MAX). The fT of 40 GHz and f MAX of 55 GHz at V ds = 5 V are demonstrated by GaN-HEMT device featuring L g of 200 nm, L cap of 300 nm and L gs of 1.2 μm, which can realize the compact solid-state power amplifier used in S and C band. However, gate-to-source distance has little effect on RF performance of AlGaN/GaN HEMTs. Those results compared in our study are not only very essential for accurate GaN-based HEMT device modeling and fabrication, but are also vital to better understanding of their device physics.

Two-dimensional device simulation for radio frequency performance of AlGaN/GaN HEMT

In order to obtain better high frequency performance for GaN-based devices, we investigate the influence of the gate length L_g, the gate-source space L_gs, the gate-drain space L_gd, and the thickness of barrier AlGaN on frequency performance of AlGaN/GaN high electron mobility transistors (HEMTs) using silvaco TCAD simulation. Besides, the importance of Ohmic contact resistance R_c to current-gain cutoff frequency f_T is also presented by our simulation. It presents that the f_T increases dramatically with the decrease of L_g and R_c while L_gd and L_gs affect f_T lightly. Meanwhile, the optimized thickness of AlGaN barrier layer is obtained in our structure.