Ohmic contact with contact resistance of 2.8×10-6 (Ohm-cm2) was evaluated by TLM method. Fig. 5.2 shows the typical output current-voltage (I-V) characteristics of the 1.5μm gate length AlGaN/GaN HEMT and Al2O3 MOS-HEMT. The Schottky-gate device has a maximum drain current of 404 mA/mm at VGS = 0, while the MOS-HEMT devices have 544.2 mA/mm drain currents, respectively. Besides, the HEMTs and MOS-HEMTs were completely pinch-off at a gate voltage of -5 and -6.7V, respectively. The negative shift in the Vth was attributed to the decrease gate barrier capacitance. The experimental Vth for both HEMTs and MOS-HEMTs were in good agreement with the values obtained from Eq. (5-1), neglecting the residual doping in the AlGaN barrier layer [6]:
= ⋅
Cb s th
V en (5-1) Where e is the electronic charge, ns is the sheet charge density and Cb is the total unit area capacitance of the barrier layer and dielectric.
Fig. 5.3 shows the IDS versus VGS curves of HEMT and MOS-HEMT devices. From a comparison of these device performances, it can be seen that the
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HEMTs have lower IDS of 747 mA/mm at VGS = 3.6 V, but for MOS-HEMTs, its reaches 880 mA/mm at 6 V gate bias. In this sense, the good quality of both Al2O3 insulator and Al2O3/HEMT interface has rendered a higher applicable gate bias, which result in a higher driving current capacity of MOS-HEMTs compared to HEMTs. Moreover, the drain current at the same gate bias is also higher for MOS-HEMT. This difference arises, thereby making the MOS-HEMT channel depletion for the same gate voltage smaller than that for the HEMT. In Fig. 5.4, a slight transconductance decrease in MOS-HEMTs compared to HEMTs from 171 to 132 mS/mm was observed, which is consistent with a further separation between the control gate and the 2-DEG channel with the presence of an additional Al2O3 layer in MOS-HEMTs. However, due to the high dielectric constant of Al2O3, the degradation in gm,max of MOS-HEMT is only 22.8% relative to that of HEMT, much better than the serve transconductance deterioration in MOS-HEMTs using low-k gate dielectrics such as SiO2 (27.2%), Si3N4 (35.7%). This is in agreement with an estimated reduction of 20% by (5-1), assuming drift velocity saturation (at Lg = 1.5μm) with Vsat = 5x106 cm/s. In additional, the gate voltage swing (GVS), defined as the 10% drop from the gm,max increase from 0.3V for HEMTs to 3.1 V for MOS-HEMTs. The larger GVS suggests a better linear behavior for MOS-HEMTs compared to Schottky-gate HEMTs, from which a smaller intermodulation distortion, a smaller phase noise and a larger dynamic range could be expected, thus desirable for practical amplifier application.
Fig. 5.5 shows the gate leakage performance of the both HEMTs and MOS-HEMTs with the same device dimensions, from which the leakage current of MOS-HEMTs is found to be significantly lower than that of the Schottky-gate HEMTs. The gate leakage current density of MOS-HEMTs is almost 3 orders of
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magnitude lower than that of the HEMTs. Such a low gate leakage current should be attributed to the large band offsets in the Al2O3/HEMT and a good quality of both the reactive-sputtered Al2O3 dielectric and the Al2O3/HEMT interface. This leads to an increase of the two terminal reverse breakdown voltage (about 25%) and of the forward breakdown voltage (about 30%). This confirms that the Al2O3 dielectric thin film acts as an efficient gate insulator. To investigate the breakdown behavior of Al2O3-insulated gate device, the off-state three-terminal drain-source breakdown characteristics of the HEMT and Al2O3 MOS-HEMT were measured, the results are as shown in Fig. 5.6; the devices were measured at gate voltage Vgs of -8V. The breakdown voltage BVDS is defined as the drain voltage at a gate current of 1ma/mm, which is consistent with the rapidly increased currents caused by avalanche breakdown. The Al2O3 MOS-HEMT with 1.5μm gate length shows a higher breakdown voltage, while the conventional HEMT. The high breakdown voltage is related to the utilization of the Al2O3 gate insulator to reduce the leakage current.
Fig 5.7 shows IDS vs. VGS transfer curves for Al2O3-insulated gate and Schottky-gate AlGaN/GaN HEMTs with the different drain voltages from 4 to 7 V. With increasing the drain voltage, both of the HEMT and MOS-HEMT devices have higher maximum drain current, except the HEMT at VDS is 7 V. In addition, at forward gate bias beyond +2V, high drain current drops for the Schottky-gate HEMT was observed as compare to the MOS-HEMT. This is because the high Schottky-gate leakage current of HEMT, with results in the degradation of the ID. On the other hand, the slope of ID curve of MOS-HEMT is lower than regular HEMT; however, when increasing the gate bias to the positive voltage, the drain current increases at a stable rate in a large gate bias region. This is because Al2O3 gate insulator with larger bandgap that can afford much higher forward gate bias.
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The characteristics of the Gm dependence on the gate-bias of Al2O3-insulated and Schottky-gate AlGaN/GaN HEMTs with the different drain voltages from 4 to 7 V are shown in Fig. 5.8. With the increase of the drain voltage, both of the HEMT and MOS-HEMT devices show almost have the similar maximum transconductance. The maximum drain current depends Gm versus VGS curve, the MOS-HEMT device has a lower maximum Gm value, but a flatter Gm distribution as compared to that of regular HEMT. It represents that the drain current increased in a stable rate in a wider range of gate bias region, this may be due to that the gate leakage current was suppressed in the MOS-HEMT. As mentioned before, a lower IM3 level can be achieved by increasing the flatness of the Gm distribution across the gate-bias region, in our current case, it indicate, that MOS-HEMT may have better device linearity performance.
For linearity assessment, nonlinear transfer function based analysis method is used. Previously published results revealed that, the Gm were to remain constant over the operating range of gate bias for minimizing third-order distortion. Hence, improving the flatness of the extrinsic Gm profile will result in lower IM3 levels and higher third-order intercept point (IP3), and thus improve the device linearity [30]. Eq. (5-2) shows the relationship between Gm and drain-source current (IDS). To maintain Gm constant with different gate-source voltage (VGS), the IDS as a function of VGS should be straight and large.
GS DS
dV
Gm= dI (5-2)
To further investigate the linearity performance of the three devices, polynomial curve fitting technique was applied to the transfer characteristic functions of these devices as equation (5-2).
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Hence, the relationship between IM3, IP3 and Gm, Gds are shown in equation (5-3) and (5-4) [31-33].
( )
6 In order to improve the device linearity, IDS should increase linearly with VGS. Therefore, a1 should be larger and the higher order constants, while a3 and a5 should be minimized [34-35]. Table 5.1 shows the coefficients of HEMT and MOS-HEMT devices. It shows that the MOS-HEMT device has higher a1 of 6.64×10-3, while regular HEMT device has lower a1 of 5.53×10-3. In addition, the lower a3 is 1.5646×10-4 and the lower a5 is 4.33061×10-6 from MOS-HEMT device. From the data analysis, the devices linearity improvement can be achieved by using MOS-HEMT with the Al2O3 as gate insulator approach.To evaluate the device linearity, the measurement of IM3 and IP3 of these devices were necessary. The IM3 and IP3 measurements were carried out by injecting two signals with the same amplitude but at two different frequencies:
2.0 GHz and 2.001GHz with the devices biased at VDS = 7V, and adjust the IDS to get the IP3 vs. IDS curve. Furthermore, the load impedance was firstly tuned for maximum gain in input side and maximum power in output side for each individual device. The measurement results of the IP3 versus IDS curves for these three different 80nm × 50μm devices are shown in Fig.5.11. It shows that
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MOS-HEMT devices possess higher IP3 value, and wider high IP3 region versus different IDS. The tuning at Γsource and Γload of MOS-HEMT and HEMT devices were Γsource = 33.5∠88.71º and 74.8∠89.47º, and Γload = 12.22∠86.62º and 12.22∠86.62º, respectively. From the data in Fig. 5.11 and Table 5.1, it can be concluded that Al2O3-insulated gates can achieves flatter Gm distribution versus VGS bias and thus lower overall IM3 and higher IP3 of these devices even though Schottky-gate gate device exhibits higher peak Gm.