Two-Dimensional Electron Gas Transport Properties in AlGaN/(In)GaN/AlGaN Double-Heterostructure Field Effect Transistors
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EXPERIMENTAL DETAILS AND DISCUSSION Comparison of GaN DH and GaN SH To examine the influence of the enhanced polarization effects in DH-FETs on the transport properties, we have fabricated both conventional AlGaN/GaN SH-FETs and AlGaN/GaN/AlGaN DH-FETs, and have compared their electrical properties. The sample structures of (a) conventional GaN SH-FETs and (b) GaN DH-FETs are (a) Al0.15Ga0.85N (300 Å) /GaN (1 µm), and (b) Al0.15Ga0.85N (300 Å) /GaN (200 Å) /Al0.15Ga0.85N (1000 Å), respectively. The samples were grown on SiC(0001) substrates using AlN buffer layers by metalorganic vapor-phase epitaxy (MOVPE) at 300 Torr. In all samples, only the surface-side 300 Å Al0.15Ga0.85N barrier layers were uniformly doped with Si. We have performed Van der Pauw Hall effect measurements on the GaN SH- and GaN DH-FET samples, both of which are doped with the same Si doping concentration of 4x1018 cm-3. Figure 1 shows the temperature dependencies of the 2DEG mobility and the 2DEG density from 20 to 380 K in the two samples. The striking feature shown in Fig. 1 is a drastic increase in the 2DEG mobility in the DH-FET sample. The 2DEG mobility in DH-FET reaches 8900 cm2/Vs at 20 K, whereas that in SH-FET reaches 4600 cm2/Vs. The 2DEG densities at 20 K in DH- and SH-FET samples are 6.5x1012 and 7.5x1012 cm-2, respectively. This difference in the 2DEG density is not so large as to explain the observed large difference in the 2DEG mobility. Hence, the observed mobility enhancement in DH-FET is a unique phenomenon specific to nitride HFETs. To clarify the dependencies of the 2DEG mobility on the 2DEG density in DH- and SH-FETs, we have performed Hall effect measurements under the gate-voltage application. In this experiment, we have used DH- and SH-FET samples with the Si doping concentration of 5X1018 cm-3. The grown structures were mesa-etched to form a 400X400 µm2 rectangular Van der Pauw geometry device, by using low-damage reactive ion etching (RIE) technique using chlorine-nitrogen mixed plasma [17]. The Al/Au ohmic metals were deposited on the corners of the rectangular mesa structure, and the Ni/Au Schottky gate metals was deposited on almost all the area of the device except for the ohmic terminals. 10 000 2DEG mo bility (cm2/Vs)
SH 1 .0
80 00
0 .8
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0 .6 40 00 0 .4
SH 20 00
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200 300 Temp erat ure (K)
2DEG densit y ( X1013 cm-2)
1 .2
DH
0 .0 400
Figure 1. Temperature dependencies of 2DEG mobility and density in GaN SH- and DH-FETs.
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2DEG mo bility (cm2/Vs)
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4 .2 K
2000 0 4.0
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8.0
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2DEG density ( X1012 cm-2)
Figure 2. Dependencies of 2DEG mobility on 2DEG density in GaN SH- and DH-FETs.
For the above devices, we have performed Hall effect measurement at 4.2 K under the gate voltage (Vg) application. The Vg was stepped from 0 to –2.5 V in –0.25 V increments. In this Vg region, the 2DEG density has shown to be almost linearly controlled by Vg both in DH- and SH-FET samples. Figure 2 shows the dependence of
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