The linearity characteristics of the AlGaN/GaN HEMT and Al2O3 MOS-HEMT on Si substrates are investigated in this study. Although the MOS-HEMT device demestrated gigher maximum Gm of 132 mS/mm at VDS= 7V, flatter Gm distribution was achieved for the MOS-HEMT device. To further investigate the linearity performance of the two devices, polynomial curve fitting technique was applied to the transfer characteristic functions. It shows that the MOS-HEMT device has highest a1 and lowest a3 and a5, as compared with the regular HEMT. Therefore, the devices linearity improvement can be achieved by using Al2O3 insulated-gate.
The AlGaN/GaN HEMTs on Si substrates with Al2O3 gate insulator to improve the device linearity is demonstrated. The reduction of gate leakage current in MOS-HEMT results in the improvement of the Gm vs. VGS curve flatness, and leads to lower overall IM3 and higher IP3 for MOS-HEMT devices, even though the regular HEMT device exhibits higher peak Gm. These DC characteristics lead to higher IP3 levels and lower IM3 for the MOS-HEMT device as compared to the regular HEMT devices studied. The experimental
50
results in this work show that Al2O3 insulated-gate can be practically used on GaN HEMT devices for the development of high linearity devices for wireless communication applications.
51
(a) (b)
Fig. 5.1 Cross section of the (a) 1.5μm a Schottky-gate AlGaN/GaN HEMT (b) MOS-HEMT with 10nm Al2O3.
52
0 2 4 6 8 10
VGS= +1V to -6V step -1V
VGS (V)
Drain-Source Voltage V
DS[V]
HEMT
Drain cur rent densi ty I
DS[mA/mm]
0 100 200 300 400 500 600 700 800
-4V -3V -2V -1V 0V +1V
-5V -2V -1V 0V +1V
GaN 1.5um Al
2O
3 MOS-HEMT and HEMT
MOS-HEMT
Fig. 5.2 DC ID versus VDS characteristics at VGS= 1 to -6 V of the AlGaN/GaN HEMT and Al2O3 MOS-HEMT.
53
-8 -6 -4 -2 0 2 4 6
0 200 400 600 800 1000
HEMT MOS-HEMT
ID,max= 747mm/mA @VGS=3.6V ID,max= 880mm/mA @VGS=6V
D rain curr ent density I
DS[mA/mm]
Gate-Source Voltage V
GS[V]
Fig. 5.3 IDS versus VGS curve for the AlGaN/GaN HEMT and Al2O3 MOS-HEMT at the VDS bias is 7 V.
54
-8 -6 -4 -2 0 2 4 6 8
0 40 80 120 160 200
HEMT
MOS-HEMT
Gm,max= 132mm/mS @VGS=-1.8V Gm,max= 171mm/mS @VGS=-1.1V
Drain current density I
DS[mA/mm]
Gate-Source Voltage V
GS[V]
Fig. 5.4 Transconductance gm versus gate-source bias VGS at the same drain bias Vds = 7V in the saturation region for the AlGaN/GaN HEMT and Al2O3 MOS-HEMT with gate length =1.5μm.
55
-8 -6 -4 -2 0 2 4 6
2x10
-52x10
-32x10
-12x10
12x10
3V
DS= 7V
Gate-Source Voltage V
GS[V]
Ga te c urren t I
G[mA/mm]
HEMT
MOS-HEMT
Fig. 5.5 Gate leakage currents for the AlGaN/GaN HEMT and Al2O3 MOS-HEMT with the same device dimensions.
56
Drain-Source Voltage VDS [V]
ID
IGS
Drain leakage current ID [μA/mm]
Gate leakage current I g [μA/mm]
0.0
Drain-Source Voltage V
DS[V]
D rain leakage current I
D[
μA/mm]
G ate leakage current I
g[
μA/mm]
100
Fig. 5.6 Off-state drain-source breakdown characteristics of Al2O3 MOS-HEMT and regular-HEMTs.
57
-6 -4 -2 0 2 4
Drai n current densi ty I
DS[mA/mm]
Gate-Source Voltage V
GS[V]
0 200 400 600 800 1000
HEMT MOS-HEMT VDS= 4V
VDS= 5V VDS= 6V VDS= 7V
Fig. 5.7 IDS versus VGS curve for the AlGaN/GaN HEMT and Al2O3
MOS-HEMT at the VDS bias is from 4 V to 7 V.
58
-6 -4 -2 0 2 4
Transconductance G M [mS/mm]
Gate-Source Voltage V
GS[V]
0 40 80 120 160 200
HEMT MOS-HEMT VDS= 4V
VDS= 5V VDS= 6V VDS= 7V
Fig. 5.8 Gm versus VGS curve for the AlGaN/GaN HEMT and Al2O3
MOS-HEMT at the VDS bias is from 4 V to 7 V.
59
-4 -2 0 2 4 6
0 200 400 600 800
GaN 1.5um MOS-HEMT
D rain current I
DS[mA/mm]
Gate-Source Voltage V
GS[V]
4 V 5 V 6 V 7 V 8 V 9 V 10 V
Fig. 5.9 IDS versus VGS curve for the Al2O3 MOS-HEMT at the VDS bias is from 4 V to 10 V.
60
-6 -4 -2 0 2 4 6
0 20 40 60 80 100 120 140
GaN 1.5um MOS-HEMT
Gate-Source Voltage V
GS[V]
Transconductance G M [mS/mm] 4V 5V
6V 7V 8V 9V 10V
Fig. 5.10 Gm versus VGS curve for the Al2O3 MOS-HEMT at the VDS bias is from 4 V to 10V.
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0.2 0.3 0.4 0.5 0.6 0.7 0.8 4
8 12 16 20
IP3max= 18.93 dBm for MOS-HEMT
T h ird order intercept point IP3 [dBm]
Idss [%]
HEMT
MOS-HEMT
Fig. 5.11 IP3 versus IDS curve of the the AlGaN/GaN HEMT and Al2O3 MOS-HEMT, and the test frequency is 2GHz and VDS =7V.
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Table 5.1 Comparison of the IP3 of Al2O3 MOS-HEMT and HEMT device at drain bias is 7V.
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Chapter 6 Conclusion
In this dissertation, 1.5-μm-length Al2O3 AlGaN/GaN MOS-HEMTs and HEMTs grown on Silicon substrate were successfully fabricated and the DC and RF characteristics were evaluated. The maximum IDS of 810 mA/mm and peak extrinsic Gm of 747 mS/mm were obtained for the GaN HEMT fabricated.
However, the AlGaN/GaN HEMT suffers from high leakage current. In order to solve this problem, an insulated Al2O3 gate was applied to the device to suppress the high gate leakage current. The innovative MOS-HEMT technique not only reduces the leakage current but also successfully improved the linearity characteristics of the AlGaN/GaN HEMTs on Si substrate. It is found that MOS-HEMT effectively reduced the high electric field, in the forward bias, it enables the electrons to maintain a stable electron velocity under the gate domain and further to suppress high gate leakage current. The MOS-HEMT devices demonstrates increased drain current and maintain stable Gm value under a larger gate bias domain; the MOSHEMTs also demonstrated improvement in the Gm vs. VGS curve flatness and thus leads to lower overall IM3 and higher IP3 for even through the regular HEMT exhibits higher peak Gm. For the MOS-HEMT, the maximum third order intermodulation point (IP3) of 18.93 dBm was achieved in the MOS-HEMT devices. The IP3 was higher them the regular HEMT device.
In addition, the third order intermodulation points of the MOS-HEMT and Schottky-gate device under the different gate biases are also compared in this study. By polynomial curve fitting technique, It successfully demonstrate that
64
the multi-gate device have the higher IP3 in the larger IDSS % region. Therefore, it indicated that multi gate technique could effectively improve the linearity performance.
In conclusion, the experimental results in this study shows that, in the future, the AlGaN/GaN HEMTs with Al3O3-insulated gate have great potential to be used in the RF power amplifier, with improved linearity for the modern wireless communication system.
65
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