Kumud Ranjan

Demonstration of submicron-gate AlGaN/GaN high- electron-mobility transistors on silicon with complementary metal-oxide-semiconductor-compatible non-gold metal stack

We have demonstrated 0.15-µm-gate-length AlGaN/GaN high-electron-mobility transistors (HEMTs) with direct-current (DC) and microwave performances for the first time using a complementary metal–oxide–semiconductor (CMOS)-compatible non-gold metal stack. Si/Ta-based ohmic contact exhibited low contact resistance (Rc=0.24 Ωmm) with smooth surface morphology. The fabricated GaN HEMTs exhibited gmmax=250 mS/mm, fT/fmax=39/39 GHz, B Vgd=90 V, and drain current collapse <10%. The device Johnsons figure of merit (J-FOM = fT × B Vgd) is in the range between 3.51 to 3.83 THzV which are comparable to those of other reported GaN HEMTs on Si with a conventional III–V gold-based ohmic contact process. Our results demonstrate the feasibility of realizing high-performance submicron GaN-on-silicon HEMTs using a Si CMOS-compatible metal stack.

High-Frequency Microwave Noise Characteristics of InAlN/GaN High-Electron Mobility Transistors on Si (111) Substrate

We report for the first time high-frequency microwave noise performance on 0.17-μm T-gate In_0.17Al_0.83N/GaN high-electron mobility transistors (HEMTs) fabricated on Si(111). The HEMTs exhibited a maximum drain current density of 1320 mA/mm, a maximum extrinsic transconductance of 363 mS/mm, an unity current gain cutoff frequency (f_T) of 64 GHz and, a maximum oscillation frequency [fmax ^(U)/ f_max (MSG)] of 72/106 GHz. The product f_max(U) × L_g=12.24 GHz· μm is the highest value ever reported for InAlN/GaN HEMTs on Si substrate. At V_d=4 V and V_g=-2.25 V, the device exhibited a minimum noise figure (NF_min) of 1.16 dB for 10 GHz and 1.76 dB for 18 GHz. Small variation of NF_min (<;0.5 dB) from 8% to 48% with I_Dmax (100-636 mA/mm) was observed.

Electron velocity of 6 × 107 cm/s at 300 K in stress engineered InAlN/GaN nano-channel high-electron-mobility transistors

A stress engineered three dimensional (3D) Triple T-gate (TT-gate) on lattice matched In0.17Al0.83N/GaN nano-channel (NC) Fin-High-Electron-Mobility Transistor (Fin-HEMT) with significantly enhanced device performance was achieved that is promising for high-speed device applications. The Fin-HEMT with 200-nm effective fin-width (Weff) exhibited a very high IDmax of 3940 mA/mm and a highest gm of 1417 mS/mm. This dramatic increase of ID and gm in the 3D TT-gate In0.17Al0.83N/GaN NC Fin-HEMT translated to an extracted highest electron velocity (ve) of 6.0 × 107 cm/s, which is ∼1.89× higher than that of the conventional In0.17Al0.83N/GaN HEMT (3.17 × 107 cm/s). The ve in the conventional III-nitride transistors are typically limited by highly efficient optical-phonon emission. However, the unusually high ve at 300 K in the 3D TT-gate In0.17Al0.83N/GaN NC Fin-HEMT is attributed to the increase of in-plane tensile stress component by SiN passivation in the formed NC which is also verified by micro-photolumines...

Effect of OFF-state stress induced electric field on trapping in AlGaN/GaN high electron mobility transistors on Si (111)

The influence of electric field (EF) on the dynamic ON-resistance (dyn-RDS[ON]) and threshold-voltage shift (ΔVth) of AlGaN/GaN high electron mobility transistors on Si has been investigated using pulsed current-voltage (IDS-VDS) and drain current (ID) transients. Different EF was realized with devices of different gate-drain spacing (Lgd) under the same OFF-state stress. Under high-EF (Lgd = 2 μm), the devices exhibited higher dyn-RDS[ON] degradation but a small ΔVth (∼120 mV). However, at low-EF (Lgd = 5 μm), smaller dyn-RDS[ON] degradation but a larger ΔVth (∼380 mV) was observed. Our analysis shows that under OFF-state stress, the gate electrons are injected and trapped in the AlGaN barrier by tunnelling-assisted Poole-Frenkel conduction mechanism. Under high-EF, trapping spreads towards the gate-drain access region of the AlGaN barrier causing dyn-RDS[ON] degradation, whereas under low-EF, trapping is mostly confined under the gate causing ΔVth. A trap with activation energy 0.33 eV was identified in...

Thermally stable device isolation by inert gas heavy ion implantation in AlGaN/GaN HEMTs on Si

Multiple energies of heavy ion implantation with inert-gas ion (84Kr+) were carried out on AlGaN/GaN high-electron-mobility transistors (HEMTs) for planar device isolation. Thermal stability of the implantated samples were also investigated by isochronal annealing at 500, 600, 700, and 800 °C (each temperature for 1 h.). Due to the damages created by heavy ions (84Kr+) in the GaN lattice, the implant-isolated Al0.27Ga0.73N/GaN HEMT samples exhibited better thermal stability than 40Ar+-implant-isolation. This was also confirmed by Rutherford backscattering spectrometry in channeling condition and ultraviolet micro-Raman spectroscopy measurements. With reference to mesa-isolated AlGaN/GaN HEMTs, the buffer breakdown voltage is also stable in the implant-isolated AlGaN/GaN HEMTs. An enhanced OFF-state breakdown voltage was also realized in the implant-isolated AlGaN/GaN HEMTs. The inert gas heavy ion implantation (84Kr+) is a viable solution for the fabrication of thermally stable planar AlGaN/GaN HEMTs even...

Record-low contact resistance for InAlN/AlN/GaN high electron mobility transistors on Si with non-gold metal

We have demonstrated 0.17-µm gate-length In0.17Al0.83N/GaN high-electron-mobility transistors (HEMTs) on Si(111) substrates using a non-gold metal stack (Ta/Si/Ti/Al/Ni/Ta) with a record-low ohmic contact resistance (Rc) of 0.36 Ω mm. This contact resistance is comparable to the conventional gold-based (Ti/Al/Ni/Au) ohmic contact resistance (Rc = 0.33 Ω mm). A non-gold ohmic contact exhibited a smooth surface morphology with a root mean square surface roughness of ~2.1 nm (scan area of 5 × 5 µm2). The HEMTs exhibited a maximum drain current density of 1110 mA/mm, a maximum extrinsic transconductance of 353 mS/mm, a unity current gain cutoff frequency of 48 GHz, and a maximum oscillation frequency of 66 GHz. These devices exhibited a very small (<8%) drain current collapse for the quiescent biases (Vgs0 = −5 V, Vds0 = 10 V) with a pulse width/period of 200 ns/1 ms. These results demonstrate the feasibility of using a non-gold metal stack as a low Rc ohmic contact for the realization of high-frequency operating InAlN/AlN/GaN HEMTs on Si substrates without using recess etching and regrowth processes.

In 0.17 Al 0.83 N/AlN/GaN Triple T-shape Fin-HEMTs with g m =646 mS/mm, I ON =1.03 A/mm, I OFF =1.13 µA/mm, SS=82 mV/dec and DIBL=28 mV/V at V D =0.5 V

We report the first 3D Triple T-gate InAlN/GaN nano-channel (NC) Fin-HEMTs on Si substrate with record high device performances at V_D as low as 0.5 V. Utilizing a T-gate approach on NC Fin-HEMT with stress engineered techniques, enhanced device transport properties with g_m=646 mS/mm, I_on=1.03 A/mm, I_OFF=1.13 μA/mm, I_ON/I_OFF~106, SS=82 mV/dec at V_D=0.5 V were achieved. In addition, the Fin-HEMT also exhibited 3.2 times lower DIBL of 28 mV/V. The dramatic improvement of device performance is due to the tensile stress induced by SiN passivation in the NC Fin-HEMT.

High Johnson’s figure of merit (8.32 THz·V) in 0.15-µm conventional T-gate AlGaN/GaN HEMTs on silicon

AlGaN/GaN high-electron-mobility transistors (HEMTs) with a 0.15-?m gate were fabricated on a Si substrate with an 8-nm-thick AlGaN barrier. The device exhibited a unity current gain cutoff frequency fT of 63 GHz and maximum oscillation frequency fmax of 124 GHz. Its three-terminal OFF-state breakdown voltage BVgd is as high as 132 V. The estimated Johnson?s figure of merit (=BVgd ? fT) is 8.32 ? 1012 V/s (8.32 THz?V), which is the highest value ever reported for a conventional SiN-passivated T-gate AlGaN/GaN HEMTs on a Si substrate without an additional field plate or gamma gate.

Investigation of gate leakage current mechanism in AlGaN/GaN high-electron-mobility transistors with sputtered TiN

The gate leakage current mechanism of AlGaN/GaN Schottky barrier diodes (SBDs) and high-electron-mobility transistors (HEMTs) with sputtered TiN is systematically investigated. The reverse leakage current (JR) of TiN SBDs increases exponentially with the increase of reverse voltage (VR) from 0 to −3.2 V (Reg. I). This conduction behavior is dominated by Poole-Frenkel emission from TiN through an interface state of 0.53 eV to the conductive dislocation-related continuum states. The obtained interface state of 0.53 eV may be due to the plasma damage to the surface of the AlGaN/GaN HEMT structure during the TiN sputtering. When the TiN SBDs are biased with −20 < VR < −3.2 V, JR saturated due to the depletion of the 2-dimensional electron gas (2DEG) channel (Reg. II). This conduction behavior is dominated by the trap-assisted tunneling through the interface state at ∼0.115 eV above the Fermi level. The three terminal OFF-state gate leakage current of AlGaN/GaN HEMTs exhibited an activation energy of 0.159 eV,...

Low k-dielectric benzocyclobutane encapsulated AlGaN/GaN HEMTs with Improved off-state breakdown voltage

The impact of low-k dielectric benzocyclobutane (BCB) encapsulation on the electrical performance and structural stability of AlGaN/GaN HEMTs on Si were investigated. After BCB encapsulation, devices exhibited no degradation in their drain current density, extrinsic transconductance and small signal microwave performances. The curing temperature (280 °C) of BCB layer had no influence on the device electrical performances. Compared to devices without BCB encapsulation, the BCB encapsulated devices achieved ~2 orders of magnitude lower gate- and drain-leakage current. An order of magnitude lower surface leakage current was also observed by BCB encapsulation between the two adjacent mesas. Due to the reduction of leakage currents, ~2-times increase of OFF-state breakdown voltage was observed. In addition, the 9-µm-thick BCB encapsulation layer also helps to have structurally stable air bridges. This work demonstrates the low-k dielectric BCB as a viable solution for the complete encapsulation of GaN HEMTs and ICs.