Evaluation of AlGaN/GaN‐HFET with HfAlO gate insulator

Abstract

Metal-insulator-semiconductor (MIS) heterostructure field-effect transistors (HFETs) fabricated with HfAlO as a gate insulator with high dielectric permittivity are demonstrated to achieve true enhancementmode operation with no gate leakage and high drain current density. Insertion of the HfAlO layer allows for forward gate bias voltages of up to +8 V without gate leakage (maximum gate current density of 1.2×10-5 mA/mm at +8 V gate bias) under direct current operation. Utilizing the extended forward gate bias swing, a maximum drain current density of 767 mA/mm is achieved, substantially higher than that of the non-insulated HFET (427 mA/mm). A recessed-gate MIS-HFET with the HfAlO gate insulator exhibits a threshold voltage of +2.0 V and zero transconductance below a gate bias of +0.9 V, demonstrating true enhancement-mode operation for the first time in recessed-gate AlGaN/GaN-HFETs with a maximum drain current density of 253 mA/mm at +5 V. 1 Introduction High power handling is a distinguishing feature of AlGaN/GaN heterostructure fieldeffect transistors (HFETs), and many attempts have been made to enhance this characteristic. One promising method for achieving higher power handling is the augmentation of channel carrier density, and this approach has been investigated extensively, primarily through the design of novel epitaxial structures [1, 2] and improvement of crystalline quality. An alternative approach is to extend the forward gate bias swing, which would cause carrier accumulation in the two-dimensional electron gas (2DEG) channel and thereby achieve higher maximum drain current density. Such an approach is also a key factor in realizing enhancement-mode (E-mode) operation [3][4][5], which is strongly desired for the purposes of simplifying circuit design through the use of a single-polarity voltage supply, and for reducing power consumption by facilitating complimentary logic circuit design.To achieve greater forward bias swing, gate leakage should be maintained at a low level throughout the range of gate bias control, particularly in the region of forward gate bias. Although a gate insulator is useful for this purpose, the insertion of a gate insulator not only prevents gate leakage but also reduces the electric field over the 2DEG channel, resulting in poor channel modulation. To achieve both low gate leakage and effective channel modulation, a gate insulator with high dielectric permittivity (high-k) and large band offset to both the gate metal and the AlGaN layer is desired.Many high-k materials have been examined to date as a gate insulator, in single or laminated configuration achieved by various formation techniques [6][7][8]. However, previous insulators are imperfect in terms of compatibility in current blocking performance and gate bias translation capability.

A low‐leakage and reduced current collapse AlGaN/GaN heterojunction field effect transistor with AlOx gate insulator formed by metal‐organic chemical vapor deposition

“…Many materials have been shown to achieve good results in this blocking role. Metal oxides with wide-band discontinuity with respect to AlGaN (e.g., SiO 2 [6], HfAlO [7] and Al 2 O 3 [8]) have yielded particularly good results, providing high signal translation and current blocking capability. However, due to its resistance to current collapse [9], silicon nitride (Si 3 N 4 ) [3] remains the most widely accepted insulator for AlGaN/GaN HFET fabrication, despite having only moderate currentblocking capability.…”

Improved Electrical Performance and Thermal Stability of HfO[sub 2]/Al[sub 2]O[sub 3] Bilayer over HfO[sub 2] Gate Dielectric AlGaN/GaN MIS–HFETs

“…In addition, this dielectric can be used to passivate the surface traps, thus minimizing the current collapse problem. To date, various dielectrics, [4][5][6][7][8][9][10][11][12][13][14][15] such as SiO 2 , Si 3 N 4 , AlN, Ga 2 O 3 , Al 2 O 3 , Sc 2 O 3 , HfO 2 , ZrO 2 , HfAlO, and Pr 2 O 3 , and dielectric stacks, 16,17 such as an Al 2 O 3 /Si 3 N 4 bilayer and an Al 2 O 3 -HfO 2 laminated layer, have been explored as the gate insulators. Their corresponding MIS-HFETs have reported enhanced performance in terms of higher saturation drain current, higher breakdown voltage, lower gate leakage, and lower off-state drain current in comparison to corresponding unpassivated SG-HFETs, generally at the expense of a decrease in the device transconductance and a negative shift in the threshold voltage.…”

“…Their corresponding MIS-HFETs have reported enhanced performance in terms of higher saturation drain current, higher breakdown voltage, lower gate leakage, and lower off-state drain current in comparison to corresponding unpassivated SG-HFETs, generally at the expense of a decrease in the device transconductance and a negative shift in the threshold voltage. 6,[8][9][10][13][14][15][16][17] Therefore, the use of dielectrics with high dielectric constant ͑high-k͒ has been suggested [8][9][10][11][12][13][14][15]17 to mitigate these issues.…”