Gallium nitride-based transistors for use in high-performance applications

Transistors are the elementary building blocks of modern electronic devices and are used in a vast array of applications, including televisions, computers, telephones, and most other electronic systems. They function to amplify or switch electronic signals, much like an electronic valve, and are made of a semiconductor material with at least three electrodes—the source, drain, and gate terminals—connected to an external circuit. The conductivity between the source and drain terminals is varied by the electric field produced when a very small voltage is applied between the gate and source terminals, effectively ‘pinching’ the channel through which the drain-to-source current flows.

Metal-oxide-semiconductor (MOS) transistors have an insulator between the gate terminal and the channel to prevent current leakage. For silicon (Si)-based MOS devices, high quality insulating layers of silicon dioxide (SiO₂) grown directly on Si using wet or thermal methods play an important role in obtaining high performance devices. However, many semiconductor materials with superior electrical properties relative to Si, such as aluminum gallium nitride (AlGaN), do not form good semiconductor-to-insulator interfaces and thus have not been suitable for high-frequency or high-power applications where current loss must be minimized.

Many insulator materials (i.e., dielectrics) have been tested for use in AlGaN/GaN MOS-high electron mobility transistors (HEMTs) with varying results. Recently, a novel photoelectrochemical (PEC) oxidation method has been developed to oxidize GaN semiconductors^1,2 in an effort to reduce the influence of contaminants on the semiconductor surface. In our work, we have adapted this PEC oxidation method to grow gate insulators for AlGaN/GaN MOS-HEMTs for the first time, and our results have been promising.

Figure 1. The photo-assisted capacitance-voltage characteristics of AlGaN MOS diodes. (Xe: xenon)

A helium-cadmium light source with a wavelength of 325nm and an electrolytic solution of phosphoric acid (pH 3.5) were used in our PEC experiments. Unfortunately, it was found that the oxidized AlGaN films dissolved easily in developer, acid, and alkaloid solutions, making it difficult to use this kind of oxide film in subsequent device processes. However, after annealing them at 700^° in ambient oxygen for two hours, the film was converted to monoclinic gallium oxide (β-Ga₂O₃) and corundum (α-Al₂O₃) crystalline phases,³ which demonstrate substantial resistance to chemical etching. AlGaN MOS diodes that were assembled with a 45nm-thick gate insulator formed by PEC oxidation demonstrated an average interface state density of 5.1 × 10¹¹ cm⁻²eV⁻¹, as estimated by using the photo-assisted capacitance-voltage characteristics shown in Figure 1. The forward and reverse breakdown field was 2.2MV/cm and 6.6MV/cm, respectively.

In light of these results, AlGaN/GaN MOS-HEMTs were fabricated using PEC-derived oxide films as gate insulators. First, a surface-sulfidation treatment⁴ was used to remove the native oxide layer on the AlGaN surface to decrease the specific contact resistance. Ohmic metals (titanium, aluminum, platinum, gold), which demonstrate high thermal stability, were then deposited.⁵ A 45nm-thick oxide film was then grown using the PEC oxidation method and annealed. Finally, nickel and gold were deposited as gate pads. The gate length and width were 3μm and 300μm, respectively. The electrical performance of these devices was measured at room temperature using an HP 4145B semiconductor parameter analyzer. The cutoff voltage and drain-source saturation current at V_GS (voltage, gate-to-source) = 0V of the AlGaN/GaN MOS-HEMTs were −5V and 200mA/mm, respectively. When operated at V_DS (voltage, drain-to-source) = 10V, the maximum extrinsic transconductance (g_m(max)) of 50mS/mm was obtained at V_GS=−2.09V. The gate-source current-voltage characteristics were also measured. The gate leakage current was only 50pA and 2pA when the gate-source bias was 10V and −10V, respectively.⁶

Figure 2. The I_DS-V_DScharacteristics and transfer characteristics of AlGaN/GaN MOS-HEMTs (t_ox: gate oxide thickness, W/L: width/length).

The high-frequency performances and low-frequency noise characteristics of AlGaN/GaN MOS-HEMTs with a 40nm annealed oxide layer and 1μm-long and 50μm-wide two-finger gate pads were also measured and analyzed. The I_DS (current, drain-to-source)-V_DS characteristics and transfer characteristics of the devices are shown in Figure 2. The cutoff voltage and drain-source saturation current at V_GS=0V was −9V and 580mA/mm, respectively. A g_m(max) of 76.72mS/mm was obtained at V_GS=−5.1V when the devices were operated at V_DS=10V. The breakdown voltage was 25V and larger than −100V (the limit of the semiconductor parameter analyzer). The gate leakage current was only 960nA and 102nA when V_GS=20V and −60V, respectively.⁷ The short-circuit current gain (h₂₁) and maximum available power gain (G_max) as a function of frequency were derived from the S-parameter measurement at V_DS=10V. The current gain cutoff frequency (f_T) and max oscillation frequency (f_max) were 5.6GHz and 10.6GHz, respectively, at the h₂₁ and G_max values of 0dB, as shown in Figure 3.

Figure 3. The short-circuit current gain (h₂₁) and maximum available power gain (G_max) of AlGaN/GaN MOS-HEMTs derived from the S-parameter measurement.

The low frequency noise (LFN) is an important parameter of MOS-HEMTs when they are used as mixers and oscillators in communication systems. Figure 4 shows the normalized noise power spectra of the AlGaN/GaN MOS-HEMTs in the saturation region (V_DS=10V). It was found that the LFN was well fitted by the 1/f law from 4Hz to 10kHz. The Hooge's coefficient (α) can be estimated from the data in Figure 4 using the following equation:

where S_I(f) is the noise power density, f is the frequency, N is the total carrier number, L_g is the gate length (1μm), q is the elementary charge, μ is the mobility, and R_ch is the resistance of the channel. The resulting value α = 1.25 × 10⁻³ at f = 100Hz and V_GS= 0V in the saturation region (V_DS= 10V) was comparable to other reports.

Figure 4. The normalized low frequency noise spectra in the saturation region of AlGaN/GaN MOS-HEMTs.

Overall, our results are very promising. The AlGaN/GaN MOS-HEMTs with gate insulators fabricated using the PEC oxidation method demonstrated significant performance gains. We feel that this approach overcomes the limitations of other semiconductor-to-insulator interfaces and represents an important advance towards developing transistors with substantially better performance characteristics in high-power and high-frequency applications. In the future, the radio frequency power performance and the LFN characteristics of these transistors will be evaluated and discussed in further detail.