Volume 7 Issue 1 - December 19, 2008
High frequency and low frequency noise of AlGaN/GaN metal-oxide-semiconductor high-electron mobility transistors with gate insulator grown using photoelectrochemical oxidation method
Li-Hsien Huang, Su-Hao Yeh, Ching-Ting Lee*

Institute of Microelectronics, College of Electrical Engineering and Computer Science, National Cheng Kung University
ctlee@ee.ncku.edu.tw

Appl. Phys. Lett., vol. 93, 043511 (2008).

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Recently, GaN-based semiconductors are widely used in electronic evices and optoelectronic devices owing to their high-electron mobility, wide and direct energy bandgap, better thermal and chemical stability. Metal-semiconductor field-effect transistors and metal-semiconductor high-electron mobility field-effect transistors using Schottky gate have been used in high-frequency applications successfully. Those devices have some disadvantages such as large gate leakage current and small breakdown voltage when they are applied in high-power applications. Therefore, the metal-oxide-semiconductor high-electron mobility field-effect transistors (MOS-HEMTs) have better performances in high-frequency and high-power applications.

Up to now, several dielectrics have been used for gate insulators of GaN-based metal-oxide-semiconductor high-electron mobility field-effect transistors. In Si-based MOS devices and integrated circuits, using wet oxidation method or dry oxidation method can obtain high quality SiO2 films and SiO2/Si interfaces on Si wafers directly. To obtain high quality insulators and interfaces at insulator/GaN-based semiconductors, growing oxide films on GaN-based semiconductors surface directly would be a promising method to fabricate high performance GaN-based metal-oxide-semiconductor devices.

Photoelectrochemical (PEC) oxidation method has been used to oxidize GaN-based semiconductors directly to form oxide films successfully. In addition to high quality oxide films and oxide/semiconductors interfaces, the dc performances of related GaN-based MOS devices and MOS-HEMTs have been reported recently. In this study, we measured and analyzed the high frequency performances and low frequency noise at large drain-source bias of AlGaN/GaN MOS-HEMTs with gate insulators grown using PEC oxidation method.

Figure 1. The schematic configuration of AlGaN/GaN MOS-HEMTs.
Figure 1 shows the schematic configuration of Al0.15Ga0.85N/GaN HEMTs epitaxial structure used in this study. First, the reactive ion etching system and Ni/Au metal mask were used to perform mesa patterns. Before depositing Ti/Al/Pt/Au (25/100/50/200 nm) as ohmic metals, surface sulfide-treatment was used to remove native oxide films on AlGaN surface. The ohmic contacts were performed at 850˚C in N2 ambient for 2 mins using rapid thermal annealing system. Then, the PEC oxidation method was used to grow insulators between source and drain regions for gate insulation and surface passivation. The as-grown oxide films do not dissolve in developer, alkaloid solutions and acid solutions and are suitable for the following device process after annealed at 700˚C in O2 ambient for 2 hrs. The thickness and interface-state density of the annealed oxide films was 40 nm and 5.1×1011 cm-2eV-1, respectively. Finally, 1-μm-long and 50-μm-wide two-finger Ni/Au (20/100 nm) were deposited as gate metals (GSG forms).   

Fig. 2 shows the drain-source current-voltage characteristics at different gate-source biases and the transfer characteristics. The saturation current at VGS=0 V is 580 mA/mm and the threshold voltage is –9 V. The maximum extrinsic transconductance (gm(max)) of 76.72 mS/mm was obtained at VDS=10 V and VGS=–5.1 V. The forward breakdown voltage and reverse breakdown voltage was 25 V and larger than -100 V. The gate leakage current was only 960 nA and 102 nA when VGS= 20 V and -60 V.

The current gain and maximum available power gain derived from s-parameter measurement as a function of frequency of AlGaN/GaN MOS-HEMTs were shown in Fig. 3. The fT=5.6 GHz and fmax=10.6 GHz were obtained when VDS=10 V.   
Figure 3. The current gain and maximum available power gain as a function of frequency derived from S-parameter measurement.
Figure 2. The drain-source current-voltage characteristics under various gate-source biases and the transfer characteristics of AlGaN/GaN MOS-HEMTs.


The low frequency noise of AlGaN/GaN MOS-HEMTs operated at VDS=10 V was measured at room temperature with the frequency range from 4 Hz to 10 kHz and the gate-source voltage varied from -8 V to 0 V (by a step of 2 V). Figure 4 shows the normalized noise power spectra and they were fitted well by 1/f law. Using mobility fluctuation model, the Hooge’s coefficient (α) can be estimated from equation (1) as following:

α=SI(f) × f × N/I2=[SI(f) × f × (Lg2/q × μ × Rch)]/I2               (1)

Figure 4. The normalized noise power spectra in saturation of AlGaN/GaN MOS-HEMTs.
where SI(f) is the noise power density,f is the frequency, I is the drain-source current, Lg is the gate length, μis carrier mobility, Rch is channel resistance. The α=1.25×10-3 was estimated at 100 Hz when MOS-HEMTs operated at VDS=10 V and VGS=0 V. As shown in Fig. 4, the normalized noise power densities increased when gate-source bias decreased. Owing to the gate leakage current is five orders of magnitude smaller than drain-source current and the specific contact resistance is only 8.7×10-6 Ω-cm2, the low frequency noise was dominated by bulk noise in our MOS-HEMTs. The total noise can be expressed as equation (2):

SRt=SRch+SRs                                             (2)

where SRt is total noise, SRch is the noise originated from channel under gate, SRs is the noise originated from un-gated region. The total resistance can be expressed as equation (3):

Rt = Rs + Rch = Rs+Lg|Voff|/(WqμnchVG)               (3)

where Rt is total resistance, Rs is the resistance of the un-gated region, Rch is the channel resistance, Lg is the gate length, Voff is the cutoff frequency, W is the gate width, μis carrier mobility, nch is the concentration of 2DEG at VGS=0 V, VG=VGS-Voff is the effective gate bias. When gate-source bias is negative, the bulk noise was dominated by the channel noise because the Rch is larger than Rs. The total low frequency noise can be expressed as following:

SI(f)/I2 =SRt/Rt2 = (SRch+SRs)/(Rch+Rs)2 SRch/Rch2         (4)

SI(f)/I2 = α/fN ∝ VG–1                                (5)

According to the equations mentioned above, it can explain that the normalized noise power densities increased as gate-source bias decreased.

In this work, we fabricated AlGaN/GaN MOS-HEMTs with gate insulators grown using PEC oxidation method. The saturation current at VGS=0 V and maximum extrinsic transconductance is 580 mA/mm is 76.72 mS/mm, respectively. The forward breakdown voltage and reverse breakdown voltage is 25 V and larger than -100 V, respectively. The fT and fmax is 5.6 GHz and 10.6 GHz, respectively. The low frequency noise of AlGaN/GaN MOS-HEMTs can be fitted well by 1/f law. The Hooge’s coefficient of 1.25×10-3 was estimated at 100 Hz when MOS-HEMTs were operated at VDS=10 V and VGS=0 V. According to those data, the PEC oxidation method can be expected a promising method to fabricate high performance GaN-based MOS devices and integrated circuits.
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