Zhihong Feng

Schottky Source/Drain InAlN/AlN/GaN MISHEMT With Enhanced Breakdown Voltage

In this letter, we present the deployment of Schottky source/drain (SSD) in InAlN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MISHEMTs) for off-state breakdown voltage improvement. The improved breakdown voltage relies on the suppression of electron injection into the buffer under the Schottky source contact. A VBD of 460 V is obtained in an SSD MISHEMT with an LGD = 10 μm, at a 170% improvement compared with that of the control MISHEMT featuring ohmic source/drain. Despite the SSD contacts, an SSD MISHEMT with a gate length of 1 μm exhibits a respectable drain current density of 416 mA/mm and a transconductance of 113 mS/mm.

Schottky-Contact Technology in InAlN/GaN HEMTs for Breakdown Voltage Improvement

In this paper, we demonstrate a 253% improvement in the off-state breakdown voltage (BV) of the lattice-matched In_0.17Al_0.83 N/GaN high-electron-mobility transistors (HEMTs) by using a new Schottky-contact technology. Based on this concept, the Schottky-source/drain and Schottky-source (SS) InAlN/GaN HEMTs are proposed. The proposed InAlN/GaN HEMTs with an <i>L</i>_GD of 15 μm showed a three-terminal BV of more than 600 V, while the conventional InAlN/GaN HEMTs of the same geometry showed a maximum BV of 184 V. Without using any field plate, the BV of 650 V was measured in the SS InAlN/GaN HEMTs with <i>L</i>_GD = 15 μm, which is the highest BV ever achieved on InAlN/GaN HEMT. The corresponding specific on-resistance (R_sp, _on) is as low as 3.4 mΩ·cm². A BV of 118 V was also obtained in SS InAlN/GaN HEMTs with <i>L</i>_GD = 1 μm, which is the highest BV in GaN-based HEMTs featuring such a short <i>L</i>_GD with GaN buffer. The improvement of the BV relies on the effective suppression of source carrier injection into the GaN buffer under the SS due to the smooth metal morphology and elimination of metal spikes in the Schottky metallization.

Fabrication of 150-nm T-Gate Metamorphic AlInAs/GaInAs HEMTs on GaAs Substrates by MOCVD

Metamorphic AlInAs/GaInAs high-electron-mobility transistors (HEMTs) of 150-nm gate length with very good device performance have been grown by metal-organic chemical vapor deposition, with the introduction of an effective multistage buffering scheme. By using a combined optical and e-beam photolithography technology, submicrometer mHEMT devices have been achieved. The devices exhibit good dc and RF performance. The maximum transconductance was 1074 mS/mm. The nonalloyed ohmic contact resistance Rc was as low as 0.02 Ω·mm. The unity current gain cutoff frequency (fT) and the maximum oscillation frequency (fmax) were 279 and 231 GHz, respectively. This device has the highest fT yet reported for 150-nm gate-length HEMTs. Also, an input capacitance to gate-drain feedback capacitance ratio Cgs/Cgd of 3.2 is obtained in the device.

Isoelectronic indium-surfactant-doped Al0.3Ga0.7N/GaN high electron mobility transistors

We report enhanced performance of Al0.3Ga0.7N∕GaN high electron mobility transistors (HEMTs) grown on sapphire substrates incorporating isoelectronic indium surfactant doping in the two-dimensional electron gas channel, the AlGaN spacer, and the AlGaN cap layer. We found that the isoelectronic In doping plays the role of surfactant and can effectively reduce the defect density. The In surfactant also lowers the interface roughness scattering and improves the surface morphology, leading to higher electron mobility at 300 and 77K. More than 30% increase in the drain saturation current and peak transconductance were observed in the In-doped devices that also showed negligible current collapse and high cutoff frequencies. The on-wafer output power, linear gain and power-added efficiency of an unpassivated 1×100μm device measured at 2GHz were 3.01W∕mm, 20.9dB, and 35.7%, respectively.

Enhanced-performance of AlGaN-GaN HEMTs grown on grooved sapphire substrates

We report significantly improved dc characteristics and RF performance of AlGaN-GaN HEMTs grown on grooved sapphire substrates. Grooves 60 nm deep with 2-/spl mu/m-wide ridges and 4-/spl mu/m-wide trenches were created along the <101~0> orientation of the substrate by inductively coupled-plasma reactive ion etching. Device mesas were defined over the trench regions where superior crystalline quality was observed by other characterization techniques. Compared to conventional HEMTs grown on the planar area, the devices on the grooved substrate show increased drain saturation current and peak transconductance. Their reverse gate leakage current is over three orders of magnitude lower. These devices also show increased off-state breakdown voltage with hard breakdown characteristics. For nominal 1-/spl mu/m-gate-length HEMTs, the best current gain and power gain cutoff frequencies were 15 and 54 GHz, respectively. The on-wafer output power, gain, and power-added efficiency of an unpassivated device measured at 4 GHz were 3.26 W/mm, 25.7 dB, and 55.6%. The enhanced performance is attributed to low-density mixed dislocations and high crystalline quality over the trench regions.

Schottky source/drain Al 2 O 3 /InAlN/GaN MIS-HEMT with steep sub-threshold swing and high ON/OFF current ratio

Al2O3/InAlN/GaN MISHEMTs with Schottky source and drain were experimentally demonstrated with steep subthreshold swing (SS) at room temperature as well as high temperature up to 150 °C. The mechanism for the steep SS was proposed to be based on the dynamic discharging process of the interface states at the Al2O3/InAlN interface and the resultant positive feedback in the turn-on of the drain current. The model was validated by temperature-dependent characterization. The use of the Schottky source/drain contacts can effectively prevent the off-state leakage current, leading to high ON/OFF current ratio of ∼1010 in the proposed devices. At room temperature, SS as low as 6.6 mV/dec was observed and SS lower than 60 mV/dec was obtained over a wide drain bias range of 3∼10 V during the forward sweep of VGS. The proposed device delivers a maximum drain current density of 416 mA/mm and peak extrinsic transconductance of 113 mS/mm.

Schottky Source/Drain InAlN/GaN Metal–Insulator–Semiconductor High-Electron-Mobility Transistor with High Breakdown Voltage and Low On-Resistance

In this work, we present a novel device technology of using Schottky source/drain (SSD) in InAlN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MISHEMTs) for off-state breakdown voltage VBD improvement. The Schottky source/drain design can effectively prevent the source carrier injection compared to the conventional MISHEMTs, leading to enhanced VBD in the SSD MISHEMTs. A VBD of 460 V is obtained in an InAlN/GaN SSD MISHEMTs with low specific Ron of 2.27 mΩcm2, at a 170% VBD improvement compared to conventional MISHEMTs. Despite the Schottky source/drain used, a SSD MISHEMT with a gate length of 1 µm exhibits respectable drain current density of 416 mA/mm and transconductance of 113 mS/mm.

Observation of trap-assisted steep sub-threshold swing in schottky source/drain Al 2 O 3 /InAlN/GaN MISHEMT

Devices with steep subthreshold swing (SS) are of great interest and significance in view of increasing subthreshold leakage current with the continuous MOSFET scaling. The standby power dissipation has grown due to the nonscalability of the SS to below 60 mV/dec at room temperature (RT). To circumvent this obstacle, novel devices that employ various turn-on mechanisms have been proposed1–4. In this work, we report the observation of steep SS∼20 mV/dec in Schottky source/drain (SSD) Al2O3/InAlN/GaN MIS-HEMTs over a drain bias range of 0.1 to 5 V. The devices also feature high ION/IOFF ratio (∼109) and appreciable current drive of IDmax=230 mA/mm at room temperature. The devices are also characterized at elevated temperature (T) up to 155 °C. Steep SS lower than the theoretical diffusion limit is consistently observed over the testing temperature range. It is suggested that the steep switching behavior is obtained through the means of a dynamic de-trapping process at the Al2O3/InAlN interface. The dynamic de-trapping enables a dynamic negative shift in the threshold voltage during the gate upswing and effectively facilitates the formation of a sub-threshold swing as steep as 18 mV/dec.

Doping Concentration and Structural Dependences of the Thermal Stability of the 2DEG in GaN-Based High-Electron-Mobility Transistor Structures

The dependences of the stability of the two-dimensional electron gas (2DEG) on the doping concentration and the heterostructure in GaN-based high-electron-mobility transistors were investigated, in an annealing experiment at 500°C under N2 ambient. Both the 2DEG density and mobility decreased after annealing due to the strain relaxation in an AlGaN layer. Moreover, a decrease in 2DEG conductivity was found in samples with different doping concentrations in a Si-doped AlGaN layer. A significant decrease in 2DEG conductivity occurred in a sample with a thicker AlGaN layer due to a larger strain relaxation. We found that a GaN cap layer could enhance the stability of the 2DEG up to 75 h of aging. Relaxation suppression by the GaN cap inducing dislocation pinning is proposed to interpret this effect.

High breakdown voltage InAlN/AlN/GaN HEMTs achieved by Schottky-Source technology

In this paper, we demonstrate 253% improvement in the off-state breakdown voltage (BV) of the lattice-matched In_0.17Al_0.83N/GaN high-electron-mobility transistors (HEMTs) by using a new Schottky-Source technology. Based on this concept, the Schottky-Source (SS) InAlN/GaN HEMTs are proposed. The SS HEMTs with a L_GD of 15 μm showed a three-terminal BV of 650 V, while conventional InAlN/GaN HEMTs of the same geometry showed a maximum BV of 184 V. Without using any field-plate the result measured in the proposed device is the highest BV ever achieved on InAlN/GaN HEMTs. The corresponding specific on-resistance (R_{sp, on}) is as low as 3.4 mΩ·cm². A BV of 118 V was also obtained in an SS InAlN/GaN HEMTs with L_GD=1 μm, which is the highest BV in GaN-based HEMTs featuring such a short L_GD with GaN buffer.