† Corresponding author. E-mail:
Project supported by the Natural Science Foundation of Hebei Province, China (Grant No. F2013202256).
In this work, we use a 3-nm-thick Al0.64In0.36N back-barrier layer in In0.17Al0.83N/GaN high-electron mobility transistor (HEMT) to enhance electron confinement. Based on two-dimensional device simulations, the influences of Al0.64In0.36N back-barrier on the direct-current (DC) and radio-frequency (RF) characteristics of InAlN/GaN HEMT are investigated, theoretically. It is shown that an effective conduction band discontinuity of approximately 0.5 eV is created by the 3-nm-thick Al0.64In0.36N back-barrier and no parasitic electron channel is formed. Comparing with the conventional InAlN/GaN HEMT, the electron confinement of the back-barrier HEMT is significantly improved, which allows a good immunity to short-channel effect (SCE) for gate length decreasing down to 60 nm (9-nm top barrier). For a 70-nm gate length, the peak current gain cut-off frequency (fT) and power gain cut-off frequency (fmax) of the back-barrier HEMT are 172 GHz and 217 GHz, respectively, which are higher than those of the conventional HEMT with the same gate length.
The lattice-matched In0.17Al0.83N/GaN high-electron mobility transistors (HEMTs) show outstanding performances for high-frequency applications as a result of the congenital material advantages of the strain-free barrier layer and the large spontaneous polarization.[1–6] A major obstacle that limits the performances of In0.17Al0.83N/GaN HEMTs for further high-frequency operations is short-channel effect (SCE). Alternatively, since the SCE mainly originates from the poor electron confinement and the lack of the buffer isolation,[7] the insertion of a potential barrier such as an AlGaN[8–14] or an InGaN[15–18] at the backside of GaN channel is a good solution for suppressing SCE without excessively reducing top-barrier thickness.
Inserting a thin AlGaN layer between the channel and the buffer layer, an unwanted parasitic electron channel will be created at the GaN buffer layer.[8,11] In order to avoid the generation of the parasitic electron channel, an AlGaN buffer is adopted as a back-barrier, generally. However, growing high-quality AlGaN buffer layer is more difficult than GaN.[11,12] Meanwhile, the use of an AlGaN buffer instead of a GaN buffer also degrades the thermal conductivity of the structure.[10] Although an InGaN back-barrier seems to have no above-mentioned shortcomings, since the band gaps of InGaN are narrower than those of GaN, a thin parasitic electron channel is formed in the InGaN layer. Electrons in the InGaN well are hard to pinch off and deteriorate the device transconductance,[19] which is more pronounced for the ultrashort-gate device with a thick GaN channel and a relatively high indium composition(In ∼ 0.15).[17] In 2016, He et al. proposed the idea of using AlInN thin layer as a back-barrier,[19] which not only creates a barrier below the GaN channel, but also eliminates the parasitic electron channel. Therefore, this new back-barrier may have a great potential for further improving the performances of highly scaled devices.
In this work, we present an improved bottom confinement by applying a 3-nm-thick Al0.64In0.36N back-barrier layer to short-gate InAlN/GaN HEMT. Two-dimensional device simulator (Silvaco Atlas)[20] is employed to calculate the band profile and the electron distribution of InAlN/GaN structure with or without an AlInN back-barrier. The influences of the AlInN back-barrier on the DC and RF characteristics of InAlN/GaN HEMT are investigated, theoretically. The results show that the AlInN back-barrier can effectively enhance the electron confinement, which can help InAlN/GaN HEMT to suppress the SCEs down to LG = 60 nm (9-nm top barrier). A 70-nm-gate-length device with an AlInN back-barrier also has a distinct improvement in pinch-off quality and RF performance compared with the conventional HEMT.
In this work, the conventional InAlN/GaN heterostructure employed was epitaxially grown by metal–organic chemical vapor deposition (MOCVD) on a SiC substrate. The epitaxial structure consisted of a 2-μm undoped GaN buffer/channel layer, a 1-nm AlN interlayer and an 8-nm undoped In0.17Al0.83N barrier layer. E-beam lithography was employed to define a 70-nm T-shape gate in the center of the 2-μm source-drain space. A Ni/Au metal stack was deposited for the Schottky contact and had 2 × 40-μm gate width (Wg). The ohmic contacts were formed with a Si/Ti/Al/Ni/Au stack deposition and the contact resistance was 0.32 Ω · m. Finally, the device was passivated with SiN deposited by plasma-enhanced chemical vapor deposition (PECVD). The conventional heterostructure and experiment are described in detail in Ref. [6]. Compared with the conventional InAlN/GaN heterostructure, the newly proposed AlInN back-barrier heterostructure is assumed to add a 3-nm AlInN back-barrier layer below a 10-nm GaN channel layer as shown in Fig.
![]() | Fig. 1. (color online) Cross-sectional view of AlInN/AlN/GaN HEMT structure with Al0.64In0.36N back-barrier. |
Concerning the choice of Al composition for AlxIn1−xN back-barrier material, we mainly consider the following principles.
A negative polarization charge is induced at AlxIn1−xN back-barrier/GaN buffer heterointerface to avoid forming parallel electron channel in GaN buffer; A larger band gap than GaN to avoid forming a parasitic electron channel in AlxIn1−xN back-barrier layer, i.e., Eg > 3.42 eV; The back-barrier height induced by AlxIn1−xN layer is highest.
By calculation, Al0.64In0.36N meets the requirements. The models for calculating the relevant parameters are described in the next section.
Due to the thermal effects that are not analyzed in this work, drift–diffusion transport model is invoked to complete the simulations, which can provide relatively fast simulation and an acceptable level of accuracy. Several important physical models such as polarizations, SRH, the low field electron mobility, and high field mobility are included in simulations.
The total polarization-induced sheet charge density for a top/bottom heterointerface is expressed as[21]
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In this work, we selected the following band gap calculation expression of AlxIn1−xN material based on LDA-1/2 approach:[22]
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![]() | Table 1.
Parameters of GaN, AlN, and AlxIn1–xN used in the simulation. . |
The low-field mobility is modeled as follows:[20]
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![]() | Table 2.
Low-field mobility model parameters for electron used in simulations. . |
The high-field mobility is modeled as follows:[20]
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The unintentional n-type doping concentration in each of the layers is assumed to be 1 × 1016 cm−3. The donor states existing at the surface of barrier layer may be the main source of the electrons in the two-dimensional electron gas (2DEG).[24] Donor-like traps are introduced at the SiN/In0.17Al0.83N interface with a single energy level of 0.42 eV below the conduction band and a density of 3.86 × 1013 cm−2. Usually, additional p-type doping is used to compensate for unintentional n-type doping. We also consider acceptor-like traps in the GaN buffer with an energy level of 0.5 eV below the conduction band and a trap density of 5 × 1016 cm−3. Using this setup, the simulated transfer characteristics (Fig.
Figure
![]() | Fig. 3. (color online) Conduction-band energy diagram and the electron distribution in InAlN/GaN structures with and without an AlInN back-barrier. |
Figure
For investigating the ability of the AlInN back-barrier to suppress the SCEs with the gate length scaling down, we simulate the threshold voltages (VTH’s) of HEMTs with and without AlInN back-barrier as a function of gate length (30 nm–200 nm) at VDS = 7 V and the results are shown in Fig.
![]() | Fig. 6. (color online) Threshold voltages as a function of the gate length of InAlN/GaN HEMTs with and without back-barrier at VDS = 7 V, respectively. |
The RF device performances of the two devices are also studied by small-signal AC simulation. Figure
In this work, we investigate the influences of a 3-nm-thick Al0.64In0.36N back-barrier on the DC and RF performance of InAlN/GaN HEMT, theoretically. Compared with the conventional InAlN/GaN HEMT, the device with an AlInN back-barrier can significantly enhance the electron confinement and the modulation efficiency of the gate, which help to suppress the SCEs down to LG = 60 nm (9-nm top barrier). In addition, the back-barrier device also shows distinct improvement on RF performance. So, this new back-barrier material may have great potential to improve the performances of highly scaled devices and the future InN-channel HEMTs. Our study provides a reference for research on AlInN back-barrier application. And, the new back-barrier material may also be conducive to suppressing the current collapse due to the better electron confinement but less total electron density than the conventional devices.
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