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AlGaN/GaN heterostructure field-effect transistors (HFETs) with different floating gate lengths and floating gates annealed at different temperatures, are fabricated. Using the measured capacitance–voltage curves of the gate Shottky contacts for the AlGaN/GaN HFETs, we find that after floating gate experiences 600 °C rapid thermal annealing, the larger the floating gate length, the larger the two-dimensional electron gas electron density under the gate region is. Based on the measured capacitance–voltage and current–voltage curves, the strain of the AlGaN barrier layer in the gate region is calculated, which proves that the increased electron density originates from the increased strain of the AlGaN barrier layer.
Because of the high electron mobility and high breakdown electric field, AlGaN/GaN heterostructure field-effect transistors (HFETs) have been an outstanding candidate for applications in high-frequency and high-power fields.[1,2] In order to take full advantage of the material properties, advanced device structures and device processing, such as field plate, passivation, T-gate, recessed-gate, and back barrier, have been widely used.[3–7]
A floating gate (FG), located in free-contact areas, can be used to enhance the breakdown voltage.[8,9] An appropriate gate annealing temperature can change the strain of the AlGaN barrier layer and improve the electrical properties of AlGaN/GaN heterostructure field-effect transistors.[10] However, the AlGaN/GaN heterostructure field-effect transistors with post-annealed FG have hardly been reported.
In this paper, a novel AlGaN/GaN heterostructure field-effect transistor by employing a post-annealed floating gate is fabricated. Using the measured current–voltage (I–V) and capacitance–voltage (C–V) characteristics, the effect of the post-annealed floating gate on the performance of the AlGaN/GaN heterostructure field-effect transistor is investigated.
The AlGaN/GaN heterostructure was grown by molecular beam epitaxy (MBE) on a sapphire substrate. The epitaxial structure consists of an Al0.24Ga0.76N (14.9 nm) barrier layer, an AlN (1 nm) interlayer, a GaN (
As shown in Fig.
Using the transmission line method (TLM), the values of specific resistivity
The I–V output characteristics for the four samples after FG RTA processing are shown in Fig.
![]() | Fig. 3. (color online) Variations of measured I–V output characteristics for the four samples after FG RTA processing. |
![]() | Fig. 4. (color online) Measured transfer characteristics for the four samples after FG RTA processing. |
![]() | Fig. 6. (color online) Obtained values of ![]() |
The 2DEG is induced by the spontaneous and piezoelectric polarization in the AlGaN/GaN heterostructure.[18,19] The alloy composition, barrier layer thickness, and barrier layer strain can influence the value of
Using the measured I–V and C–V characteristics of the Schottky contacts, we can calculate the total polarization sheet charge density
![]() | Fig. 7. (color online) Forward I–V characteristics of the gate Schottky contacts for the AlGaN/GaN HFETs after FG 600 °C RTA. |
Based on the obtained
The measured and calculated parameters of the four samples after FG 600 °C RTA are shown in Table
![]() | Table 1.
Measured and calculated parameters of the four samples after FG 600 °C RTA. . |
During FG 600 °C RTA, the gate metal atoms (Ni and Au) are at high temperatures and have large energy. The high-energy metal atoms can diffuse into the AlGaN barrier layer and then destroy the lattice structure in the FG region. This destruction of the lattice structure means that the spontaneous and piezoelectric polarization in the AlGaN barrier layer decrease.[10,23] The decrease of the polarization effect makes the tensile strain in the FG region weaker. Consequently, due to the decrease of the tensile strain, the lattice constant of the AlGaN barrier layer in the FG region decreases. Because of the lattice continuity, the lattice in the gate region can be stretched by the contractive lattice in the FG region. The stretched lattice in the gate region results in the increase of the strain. As the FG length increases, the number of FG metal atoms increases with the FG area increasing. During the annealing processing, there are more FG metal atoms which can diffuse into the AlGaN barrier layer. The increase of the diffused metal atoms can further destroy the lattice structure and reduce the lattice constant in the FG region, which can enhance the lattice stretch in the gate region and increase the strain of the AlGaN barrier layer. Therefore, the strain in the gate region can increase with the FG length increasing. For sample 2 with
The increase of the strain can cause the increase of the electric field across the AlGaN barrier layer. The electrons in the surface states are identified as the source of the 2DEG electrons in the channel.[19] The surface electric potential energy can be lifted by the increased electric field. More electrons in the surface state are then able to transfer from occupied surface states to empty conduction band states at the interface, leading to the fact that the electrons in the triangular potential well are increased.
In this work, the AlGaN/GaN HFETs with different FG lengths and different FG annealing temperatures are fabricated. It is found that for the AlGaN/GaN HFETs after FG 600 °C RTA, the larger the FG length, the larger the value of
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[3] | |
[4] | |
[5] | |
[6] | |
[7] | |
[8] | |
[9] | |
[10] | |
[11] | |
[12] | |
[13] | |
[14] | |
[15] | |
[16] | |
[17] | |
[18] | |
[19] | |
[20] | |
[21] | |
[22] | |
[23] |