Group III nitride based high electron mobility transistors (HEMTs) have be identified as the most promising candidate for next-generation microwave power electronics owing to their fast switching speed and low switching loss [1-5]. Lately, the most advanced nitride HEMTs have achieved initial commercialization up to 650 V. However, with the maturity of device fabrication technology, it has become increasingly difﬁcult to further scaling up the breakdown voltages (BV) and improving the device reliability at high temperatures. Therefore, in view of the larger bandgap and superior thermal stability of AlGaN over GaN, AlGaN channel devices have been proposed as promising candidate to further improve the performance limits of nitride HEMTs in high-voltage and high-temperature applications [6-13]. Recently, the successful implementation of AlGaN channel metal-oxide-semiconductor field-effect-transistors (MOSFETs) have also been reported .
However, the limitations of the previously reported AlGaN channel devices are equally obvious. On one hand, on account of the ternary alloy disordered scattering effect, the two dimensional electron gas (2DEG) mobility in AlGaN channel is much lower than that in GaN channel. As a result, the current drive capacity of AlGaN channel devices are much lower than that of the traditional GaN channel devices. On the other hand, in order to induce the same amount of 2DEG in AlGaN channel, the Al component in AlGaN barrier layer should be higher than that of conventional GaN channel heterostructures, which will increase the difficulties in material growth process. These contradiction seriously inhibit the widespread application of AlGaN channel devices, and the optimization of heterostructure layout are urgently needed.
Double channel (DC) technique is an intriguing approach to promote the channel carrier density of nitride based heterostructures without any adverse impact on the electron mobility, and the current conduction capability of the devices will be improved [15-17]. However, there is few report on the AlGaN double channel heterostructures or electron devices up to now. In this work, for the first time, AlGaN double channel heterostructure is proposed and grown to resolve the contradiction between the current drive capability and breakdown performance of nitride based electron device. Further, high performance AlGaN double channel HEMTs based on the novel heterostructures are fabricated and investigated in detail.
The cross section schematic of the AlGaN double channel heterostructure is shown in Fig. 1, and the growth processes can be summarized as follow. Firstly, 1600 nm GaN buffer layer was grown on the sapphire substrate. Then, 500 nm graded AlGaN buffer layer with Al composition increasing from 0 to 10% was grown, which was beneficial to suppress the formation of parasitic channel. Whereafter, 100 nm lower AlGaN channel, 1 nm AlN interlayer and 23 nm lower AlGaN barrier were grown successively, and the Al compositions in the channel and barrier layers are 10% and 31%, respectively. Finally, 30 nm upper AlGaN channel, 1 nm AlN interlayer and 23 nm upper AlGaN barrier layers were grown, for which the compositions were the same with the lower layers. The conduction band diagram of the AlGaN double channel heterostructure can be calculated by self-consistently solving the one dimensional Poisson-Schrödinger equation, and the electron density distribution is also extracted as shown in Fig.2. Two deep potential well are formed at the interface of AlN interlayer and Al0.10Ga0.90N channel layers, corresponding to the double 2DEG channels. In addition, the lower AlGaN barrier also acts as back barrier of the upper channel, which is beneficial to promote the confinement of the 2DEG.
Fig.3 displays the high resolution x ray diffraction (HRXRD) ω-2θ scan result of the AlGaN double channel heterostructures from symmetric (0004) reflection. The diffraction peaks of AlN nucleation layer, GaN buffer, AlGaN graded buffer, AlGaN channel and AlGaN barrier layers can be observed at 75.3°, 71.6°, 72.2° and 72.8°, indicating the distinct multi-layer structure and the sophisticated control of the growth process. Capacitance-voltage (C-V) measurement with mercury-probe configuration was performed to investigate the double channel characteristics of the heterostructures. 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 0V to -15V, 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 23 and 78 nm with the values of 6.1 ×1013 cm and 2.5 ×1013 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 reached 500 nm, 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 single 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 HEMT 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 ~ -10V and 0 ~ 10V. The maximum drain current density (Imax) and differential on-resistance (Ron) of the AlGaN single channel sample are 265.3 mA/mm and 27.1Ω∙mm, much inferior than that of the GaN single channel devices. This result is in accord with the previous reports, suggesting the deficiency of AlGaN channel HEMTs in current drive capacity. However, for the AlGaN double channel HMETs, the Imax dramatically increases to 473 mA/mm, which is 1.8 times higher than that of AlGaN single channel HEMTs and even higher than the results of GaN single channel HEMTs. We attribute the improvement in Imax to the superior transport properties of the AlGaN double channel heterostructures. 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 single channel HEMTs. This phenomenon is related to the large contact barrier height of the AlGaN barrier layers, for which the Al composition is as high as 31%. Moreover, due to the self-heating effect resulted from the high power dissipation, negative differential resistance effect can be observed for the GaN single channel HEMTs when VG>-2V and VD>5V. 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 condition.
The off-state breakdown characteristics of the HEMTs based on different heterostructures are measured and shown in Fig.7. The breakdown voltage (Vb) is defined by the criteria of leakage current reaching 0.5 μA/mm. It can be observed that all the three samples present hard breakdown characteristics, and the breakdown performance of AlGaN channel HEMTs are 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 single 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. 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 in this work is the highest result in AlGaN channel HEMTs, which is comparable to most results of the GaN channel HEMTs. Moreover, it is necessary to note that the Imax value in our experiments 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 with requiring 1200 V.
In summary, AlGaN double channel heterostructure is proposed to fabricate high performance HEMTs. The superior transport properties of AlGaN double channel heterostructsure is reveled, leading to the improved current drive capability of the HEMTs. In addition, the excellent breakdown performance of the AlGaN double channel HEMTs is demonstrated. The results in this work show the great potential of AlGaN double channel devices in microwave power applications in the future.