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 Al_0.24Ga_0.76N (14.9 nm) barrier layer, an AlN (1 nm) interlayer, a GaN ( ) channel layer, and an AlN (100 nm) buffer layer. Hall measurement shows a sheet electron density ( of and an electron mobility (μ) of . Device processing began with device isolation by utilizing Cl₂/BCl₃ inductively coupled plasma (ICP) etching. Ti/Al/Ni/Au was deposited and annealed at 850 °C for 40 s in a rapid thermal annealing (RTA) system to form the source and drain ohmic contacts. After that, Ni/Au FGs were deposited and subsequently annealed in the same rapid thermal annealing system for 40 s. For the remaining property of the ohmic contact (especially considering that the melting point of aluminum in the ohmic contact is about 660 °C), the floating gate (FG) annealing temperatures were chosen as 200 °C, 400 °C, and 600 °C, respectively. Finally, the Ni/Au gate Schottky contact was located in the middle of the source and drain contacts.

As shown in Fig. 1, the gate length ( is , the gate width (W) is , the source-drain spacing ( is , and the FG-gate spacing ( is . The floating gates are symmetrically located on each side of the gate, and their lengths ( are , , , and , which are marked as sample 1, sample 2, sample 3, and sample 4, respectively. The C–V measurements were performed using an Agilent B1520A at 1 MHz; the I–V measurements were performed using an Agilent B1500A semiconductor parameter analyzer.

Fig. 1.

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	Fig. 1. (color online) Top views for the four samples with the sizes being in units of micrometer.

Using the transmission line method (TLM), the values of specific resistivity for the samples after FG RTA processing (0 °C, 200 °C, 400 °C, and 600 °C) are obtained and shown in Fig. 2. The increase of the is caused by the degradation of the ohmic contact surface morphology.^[11] The increase of the can be attributed to forming more TiN after further annealing, which can further reduce the barrier height of the ohmic contact and also the ohmic contact resistance.^[11]

Fig. 2.

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	Fig. 2. Variation of specific resistivity with floating gate annealing temperature.

The I–V output characteristics for the four samples after FG RTA processing are shown in Fig. 3. The transfer characteristics for the four samples after FG RTA processing are measured as shown in Fig. 4. It is apparent after FG RTA processing that the samples retain excellent output and transfer characteristics. Figure 5 shows the C–V curves of the gate Schottky contacts for the samples after FG RTA at different temperatures. , where C₀ is the measured capacitance value at zero gate bias for the Schottky contact, ε₀ is the vacuum permittivity, is the relative permittivity of the AlGaN barrier layer, is the Schottky contact area, and is the thickness of the AlGaN barrier layer.^[12] The relative permittivity, which is closely related to the spontaneous polarization and piezoelectric polarization in the AlGaN/GaN HFETs, can be changed by the device processing, such as ohmic contacts and Schottky contacts.^[12,13] The thermal stress can also affect the relative permittivity.^[14–16] Hence the capacitance difference among the four samples after FG RTA processing should originate from the variation of the relative permittivity. This also means that the FG RTA processing can change the relative permittivity of the AlGaN barrier layer. By integrating C–V curves, the two-dimensional electron gas (2DEG) electron density in the gate region can be obtained as shown in Fig. 6.^[17] For the devices without FG RTA (also called as-deposited) or after FG 200 °C RTA, the values of for the four samples are almost the same. After FG 400 °C RTA, the values of for these samples begin to show a little difference. This difference in value of becomes significant after FG 600 °C RTA, and at this annealing temperature the increases with FG length increasing. This means that the FG structure has an important influence on after FG 600 °C RTA. This variation of after FG 600 °C RTA can be explained as follows.

Fig. 3.

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	Fig. 3. (color online) Variations of measured I–V output characteristics for the four samples after FG RTA processing.

Fig. 4.

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	Fig. 4. (color online) Measured transfer characteristics for the four samples after FG RTA processing.

Fig. 5.

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	Fig. 5. (color online) Measured C–V curves for the four samples after FG RTA.

Fig. 6.

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	Fig. 6. (color online) Obtained values of under different gate biases for the four samples after FG 200 °C.

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 . Because these samples are fabricated on the same AlGaN/GaN heterostructure material, the alloy composition and the barrier layer thickness are uniform. Therefore, the difference in the value of in the gate region after FG 600 °C RTA likely originates from the variation of the strain of the AlGaN barrier layer.

Using the measured I–V and C–V characteristics of the Schottky contacts, we can calculate the total polarization sheet charge density as follows:^[10,20] 1 Here is the 2DEG electron density corresponding to 0 V gate voltage, is the flat-band voltage, n₁ is the ideality factor. The V₀ and n₁ can be obtained from the forward I–V characteristics of the gate–source Schottky diode.^[21,22] The forward I–V characteristics of the gate Schottky contacts for the four samples after FG 600 °C RTA are measured as shown in Fig. 7.

Fig. 7.

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	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 and the polarization effect of the AlGaN/GaN heterostructures, the strain of the AlGaN barrier layer can be determined from^[18] 2 3 4 5 Here, and refer to the spontaneous polarization and piezoelectric polarization, respectively; a₀ and a represent the a-axis lattice constants of the unstrained AlGaN and of the strained AlGaN, respectively; e₃₁ and e₃₃ are the piezoelectric coefficients; C₁₃ and C₃₃ are the elastic constants; ε is the a-axis in-plane strain of the AlGaN barrier layer.

The measured and calculated parameters of the four samples after FG 600 °C RTA are shown in Table 1. Table 1 shows that with the increase of the floating gate length, the strain of the barrier layer rises, even reaches a value more than twice the strain of the barrier layer when . This means that the floating gate after 600 °C RTA can enhance the strain of the AlGaN barrier layer in the gate region. The larger the floating gate length is, the larger the strain under the gate region is.

Table 1.

Table 1.

Table 1. Measured and calculated parameters of the four samples after FG 600 °C RTA. . After FG 600 °C RTA n1 (C/m−2) (C/m2) Sample 1 6.48 4.05 1.004 3.349 1.17 −1.612 0.107 Sample 2 6.77 4.18 0.535 2.528 1.18 −1.746 0.116 Sample 3 7.41 4.52 1.158 3.649 1.32 −3.319 0.220 Sample 4 7.49 4.57 0.989 3.108 1.35 −3.468 0.230

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 FG length is not big enough to affect the strain in the gate region, and the increase of strain is less. For sample 3 with , the FG length is big enough to change the strain in the gate region, and the sample has the largest strain increase. Because the AlGaN barrier layer is clamped by the GaN channel layer, the increase of strain can be limited after reaching a big enough value, leading to the fact that the increase of strain for sample 4 with is smaller than that for sample 3 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 in the gate region is. Based on the measured C–V and I–V curves, the strain of the AlGaN barrier layer is determined, which proves that the increased originates from the increased strain. It offers a possible approach to improving the performance of AlGaN/GaN HFET with a post-annealed floating gate structure.