Fig.3 displays the high resolution x ray diffraction (HRXRD) ω-2θ scan result of the AlGaN double channel heterostructure from symmetric (0004) reflection. The diffraction intensity from AlN nucleation layer, GaN buffer, AlGaN graded buffer, AlGaN channel and AlGaN barrier layers can be observed. Moreover, the spectrum scan from 71.0° to 73.2° is presented in Fig.2(b) with a magnification for clarity, and Lorentz function is applied to fit the multi-peaks. The diffraction peaks of GaN buffer, AlGaN channel and AlGaN barrier locate at 71.6°, 72.2° and 72.8°, and the AlGaN graded buffer results in a platform between the peaks of GaN buffer and AlGaN channel. These results indicate the distinct multi-layer structure and the sophisticated control of the growth process, and the AlN compositions of 10% and 31% in the AlGaN channel and barrier can be extracted.
Capacitance-voltage (C-V) measurement with mercury-probe configuration was performed to investigate the double channel characteristics of the heterostructure. As shown in the inset of Fig.4, two distinct capacitance steps can be observed at around -2.5 V and -10 V with the applied voltage swept from 0 V to -15 V, corresponding to the depletion of 2DEG at AlN/Al0.10Ga0.90 interfaces. In addition, the carrier distribution properties can be extracted from C-V curve and the result is illustrated in Fig.4. Two carrier concentration peaks locate at 24 and 78 nm with the values of 6.1 × 1019 and 2.5 ×1019 cm-3, which is in accordance with the calculated result as shown in Fig.2. Specially, no parasitic conduction channel can be observed until the test depth reaches 1 μm, suggesting the successful achievement of double channel properties of the heterostructure. In addition, the 2DEG sheet density and mobility were determined to be 1.3 ×1013 cm-2 and 1130 cm2/V∙s by the Hall effect measurement.
The standard HEMTs fabrication process was carried on the AlGaN double channel heterostructure. The device fabrication process started with ohmic contact formed with Ti/Al/Ni/Au multilayer metal stack deposited by electron beam evaporation, followed by a rapid thermal anneal at 850 °C for 30 s in N2 atmosphere. Then, the mesa isolation was performed by Cl2/BCl3 inductively coupled plasma etching to a depth of 200 nm, and 100-nm-thick SiN passivation layer was formed by plasma enhanced chemical vapor deposition. Afterwards, a gate area with a length (LG) of 0.8 μm was defined by photolithography after etching the top SiN with CF4 plasma, and then a Ni/Au schottky gate electrode was deposited. The gate-source (LGS) and gate-drain (LGD) distances are 0.8 and 1 μm, respectively. For comparison purposes, two additional HEMTs samples based on the conventional AlGaN single channel and GaN double channel heterostructures were also fabricated, and the cross section schematics are shown in Fig.1(b) and 1(c). The device process and characteristic parameters of the additional HEMTs samples are exactly the same with the AlGaN double channel HEMTs. The output and transfer properties of the devices were carried out with Keithley 4200 semiconductor parameter analyzer, and the breakdown characteristics were performed using Agilent B1505A high-voltage semiconductor analyzer system.
The typical output characteristics of the HEMTs are illustrated in Fig.5, for which the VGS and VDS were swept from 0 ~ -10 V and 0 ~ 10 V. The maximum drain current density (Imax) and differential on-resistance (Ron) of the AlGaN single channel sample are 265.3 mA/mm and 27.1 Ω∙mm. These results are in accordance with the previous reports, suggesting the deficiency of AlGaN channel HEMTs in current drive capacity. For the AlGaN double channel HEMTs, the Imax dramatically increases to 473 mA/mm, which is 1.8 times higher than that of AlGaN single channel HEMTs. We attribute the improvement in Imax to the superior transport properties of the AlGaN double channel heterostructure. On one hand, double channel structure possesses two potential wells along the vertical direction, and the carrier storage capability of the AlGaN conduction channel is promoted. On the other hand, although the total channel carrier density is increased, the average electron density in each channel is reduced. As a result, the carrier-carrier scattering effect is suppressed and the channel electron mobility is elevated. However, it can be observed that the Ron of AlGaN double channel HEMTs is 12.5 Ω∙mm, still larger than that of GaN double channel HEMTs. This phenomenon is related to the large contact barrier height of the AlGaN barrier layers, for which the AlN composition is as high as 31%. Due to the self-heating effect resulted from the high power dissipation, the negative differential resistance effect can be observed for the GaN double channel HEMTs when VGS>-4V and VDS>6V. Nevertheless, for the AlGaN channel HEMTs (both single channel and double channel), this negative differential resistance effect is significantly suppressed, manifesting the superior performance of AlGaN channel HEMTs in elevated temperature conditions.
Fig.6 illustrates the typical transfer properties of the HEMTs with VDS of 10 V. The AlGaN single channel HEMTs exhibit a threshold voltage (VT) of -3.8V, together with an inferior peak extrinsic transconductance (Gm,max) of 80.5 mS/mm in the vicinity of VGS=-1.5V. For the AlGaN double channel and GaN double channel HEMTs, the VT remarkably decreases to -9.2 and -11.2 V, which is resulted from the high channel carrier density and the relatively long distance from the gate electrode to the lower 2DEG channel. The high VT may result in high power loss of the devices at off state, and this issue can be improved by further optimize the growth parameters of double channel structures, such as properly reducing the thickness of barrier and upper channel layers. Specially, double-hump characteristics can be observed of the transconductance curves of AlGaN double channel and GaN double channel HEMTs. For the AlGaN double channel HEMTs, two peak values of 97.9 and 42.5 mS/mm can be extracted at VG=-1.0 and -6.0 V. The sub-peak value reaches 43% of the Gm,max, indicating the decent gate-control ability and linearity of the AlGaN double channel HEMTs. Moreover, based on our previous research achievement [21], the results can be further improved by modulating the structure parameters, such as the thickness and composition of the AlGaN double channels, and the coupling effect between the double channels and the device linearity can be enhanced.
The off-state breakdown characteristics of the HEMTs based on different heterostructures are measured and shown in Fig.7. The Vb is defined by the criteria of leakage current reaching 5 μA/mm. It can be observed that all the three samples present hard breakdown characteristics, and the breakdown performance of AlGaN channel HEMTs is obviously better than that of the GaN channel HEMTs. The Vb of the AlGaN double channel HEMTs is 143.5 V, more than two times higher than that of the GaN double channel HEMTs. Taking the LGD=1 μm into consideration, the Vb,standard (defined by Vb/LGD) is as high as 143.5 V/μm for the AlGaN double channel HEMTs. The Imax and Vb,standard results of the AlGaN double channel HEMTs in this work are benchmarked against the GaN channel and AlGaN channel HEMTs reported by other groups in Fig.8, and the results of depletion-mode (DM) and enhancement-mode (EM) devices are distinguished. In addition, the core indexes of the AlGaN channel HEMTs (heterostructures) in previous reports and this work are summarized in Table I. As Fig.8 shown, it is obvious that the breakdown performance of AlGaN channel HEMTs is generally better than that of GaN channel HEMTs, accompanying with the deterioration in Imax. Noticeably, the Imax of the AlGaN double channel in this work is comparable to most results of the GaN channel HEMTs. Moreover, it is necessary to note that the Imax value in this work is obtained at VGS=0 V, which is conservative and can be further improved by applied positive gate voltage. Therefore, these results convincingly demonstrate the enormous potential of AlGaN double channel HEMTs in microwave power device applications.
Table I Core indexes of AlGaN channel HEMTs (heterostructures) in previous reports and this work
Institution
|
μ
(cm2/Vs)
|
ns
(1013cm-2)
|
IMAX
(mA/mm)
|
VT
(V)
|
Vb,standard (V/μm)
|
Mitsubishi [6]
|
|
0.53
|
114
|
|
153
|
Mitsubishi [7]
|
645
|
0.22
|
145
|
-1.0
|
180
|
Mitsubishi [8]
|
460
|
0.79
|
340
|
-4.0
|
170
|
Sandia National Laboratories [9]
|
250
|
0.60
|
2
|
-4.9
|
82
|
USC [10]
|
284
|
1.15
|
250
|
-10
|
99
|
XDU [11]
|
801
|
0.39
|
200
|
-4.0
|
104
|
Sandia National Laboratories [12]
|
390
|
0.72
|
160
|
-6.0
|
186
|
XDU [13]
|
801
|
0.39
|
275
|
-2.8
|
110
|
XDU [14]
|
807
|
0.61
|
849
|
-4.3
|
82
|
XDU [15]
|
1179
|
0.61
|
768
|
1.0
|
103
|
This work
|
1130
|
1.30
|
460
|
-9.2
|
142.5
|