InAlN/GaN HEMT With n+GaN Contact Ledge Structure for Millimeter-Wave Low Voltage Applications

In this work, high performance InAlN/GaN HEMT based on the n+GaN regrown ohmic contact with n+GaN contact ledge structure is proposed. The regrown ohmic contact of InAlN/GaN HEMT is formed by MBE n+GaN regrowth and self-stopping etching, which makes the total ohmic contact resistance between the 2DEG channel and the ohmic metal decrease to $0.12 \Omega \cdot $ mm and forms n+GaN contact ledge structure. Owing to the n+GaN contact ledge on the InAlN barrier, with the increasing of drain-source voltage (VDS), an additional current path comes into being between the n+GaN contact ledge on the InAlN barrier and the 2DEG channel, which can “shorten” the device effective drain-source distance, thus further reducing the parasitic resistance. Compared with regrown InAlN/GaN HEMT without n+GaN contact ledge structure, the peak transconductance (Gm.max) of regrown InAlN/GaN HEMT with n+GaN contact ledge structure increases from 747 mS/mm to 874 mS/mm, and the saturation current density (ID.max) increases from 2.6 A/mm to 2.9 A/mm. Besides, the self-stopping etching on the access region does not induce extra defects, and negligible current collapse is obtained. As the results of low parasitic resistance, high output current density, low knee voltage and negligible current collapse, power-added-efficiency (PAE) of 44% together with output power density (Pout) of 2.5 W/mm is achieved at 30 GHz and $V_{\mathrm{ DS}}$ of 10 V, which indicates regrown InAlN/GaN HEMT with n+GaN contact ledge structure has great potential for millimeter-wave low voltage applications. Additionally, the transfer and schottky gate characteristics show negligible degradation after OFF/ON-state electrical stress tests.


I. INTRODUCTION
Benefitting from the large critical electric field (∼3.3 MV), the high electron saturation drift velocity (∼2.5×10 7 cm/s), and the high two-dimensional electron gas (2DEG) density (∼2×10 13 cm -2 ), GaN-based high electron mobility transistor (HEMT) shows great potential for high power and high frequency applications [1]. However, with the development of the 5G wireless communication technologies, future mobile terminals need to operate at millimeter-wave frequency with a lower supply voltage. For example, in order to achieve as long as possible standby time, mobile phones, laptops, and GPS locators are required to ensure high efficiency and low power consumption while powered by battery packs. Compared with Si-based and GaAs-based RF devices, GaN HEMTs have advantages in power-addedefficiency (PAE) and output power density (P out ) at low supply voltage [2], [3], exhibiting great consumption space of low voltage GaN RF applications.
In order to achieve high PAE and large P out for low voltage RF devices, it is important to increase the maximum output current density (I D.max ) and reduce the knee voltage (V knee ) [4], [5], [6]. Therefore, reducing the parasitic resistance, including access resistance and ohmic contact resistance, is a key technology. The access resistance is determined by the drain-source distance and the sheet resistance of the heterostructure. In order to reduce the access resistance, self-aligned gate technology can be used to scale down the drain-source distance and strongly polarized heterojunction can be utilized to reduce the sheet resistance of the 2DEG channel [7], [8]. To reduce ohmic contact resistance, at present, n + GaN regrown ohmic contact technology is commonly used [9], [10]. Furthermore, TiN-based and n + GaN cap contact ledge structures have been used to provide a current path between the contact ledge and the 2DEG channel to further increase the maximum output current density [11], [12]. However, annealing process needs to be introduced to form the TiN-based ledge, which leads to worse surface morphology and causes the ohmic metals edge to expand outward [13], which makes the minimum drain-source spacing can only scaled to 3μm, and it is difficult to further reduce the drain-source spacing to submicron dimension. For previous n + GaN cap contact ledge structure device in [12], n + GaN cap did not interact with the 2DEG channel. The ohmic contact resistance still needed to be reduced. Besides, it was difficult to control the etching time accurately and make the etching stop on the surface of the barrier by Cl-based etching. Therefore, if self-stopping etching technology was used, it is advantageous for non-alloyed n + GaN regrown ohmic contact combined with the n + GaN contact ledge structure to scale down the access dimension and reduce the parasitic resistance.
Based on the above, regrown InAlN/GaN HEMT with n + GaN contact ledge structure has been fabricated by regrown ohmic contact and self-stopping etching technology in this work, and its ohmic contact resistance reaches 0.12 ·mm. An additional current path between n + GaN regrown contact ledge and 2DEG channel is formed, which "redefines" the device drain-source distance thus reducing the effective drain-source spacing. Although this structure has been reported in previous work [14], the GaN cap layer is inevitably etched during the self-stopping etching process, resulting in output current degradation and large gate leakage current. This work not only optimizes the device structure to 10 nm InAlN barrier replacing the previous InAlN layer combined with GaN cap layer to solve the above problems, but also evaluates the long-term reliability of this newly proposed structure. Therefore, with the assistance of optimized barrier structure and n + GaN regrown contact ledge, the regrown InAlN/GaN HEMT could obtain I D.max of 2.9 A/mm. Although self-stopping etching is performed on the access region, current collapse of the device can be ignored under low voltage operating conditions. The device with the gate length of 150 nm achieves f T /f MAX of 93 GHz/142 GHz. Notably, due to the lower parasitic resistance and larger output current density, output power density (P out ) of 2.5 W/mm and PAE of 44% are realized at 30 GHz and V DS of 10 V for regrown InAlN/GaN HEMT with n + GaN contact ledge structure. After OFF-state and ON-state electrical stress measurements, the drain current and the schottky gate current of the proposed device show negligible degradation, indicating that the regrown HEMT with n + GaN contact ledge structure has decent stability.

II. DEVICE FABRICATION PROCESS
Device fabrication process of regrown InAlN/GaN HEMT with contact ledge structure is demonstrated in Fig. 1. As shown in the Fig. 1(a), the epi-layers of InAlN/GaN heterojunction were grown by metal-organic-chemical vapor deposition (MOCVD) on SiC substrate, containing 10 nm In 0.17 Al 0.83 N barrier and GaN buffer. The carrier density of 2.4×10 13 cm −2 and mobility of 1638 cm 2 /V·s were measured by room temperature Hall measurement.
The device fabrication process began with the definition of the regrowth region via stepper lithography. Next, BCl 3 /Cl 2 ICP plasma dry etching was used to form a steep etching side wall and wet treatments were performed to remove surface impurities to decrease the interface resistance between the regrown n + GaN and 2DEG channel. The drain-source distance (L DS ) was 600 nm, defined by the spacing of the two etching wells as shown in Fig. 1(b). Then, 80 nm n + GaN with heavily doped Si (> 2×10 20 cm −3 ) was regrown on the whole wafer by MBE. After that, device isolation was achieved by boron ion implantation. Subsequently, partial n + GaN on the access region was defined by Electron-Beam-Lithography (EBL) and removed via BCl 3 /SF 6 ICP plasma dry etching to form the contact ledge. Previous work reported that BCl 3 /SF 6 mixed gas plasma dry etching can etch the regrown n + GaN and spontaneously stop on the Al-containing barrier, while ensures the smoothness of the barrier surface. When the SF 6 plasma reaches the InAlN barrier surface, the fluorine ion reacts with the Al atoms and forms a thin AlF 3 etching stop layer [15]. After BCl 3 /SF 6 ICP plasma self-stopping etching, the byproduct was removed by ammonia water treatment at 55 • C for 10 mins. As shown in Fig. 1(e), n + GaN contact ledges overhung on the InAlN barrier from both drain and source etching wells to gate side. Afterwards, the nonalloyed Ti/Al/Ni/Au ohmic contact metals of 20/160/55/45 nm were deposited by electron beam evaporation and a 120 nm SiN layer was deposited by PECVD to provide a surface passivation. Then, 150 nm gate foot was defined by EBL and removed SiN by CF 4 plasma dry etching. Finally, gate head was defined by stepper lithography and Ni/Au gate metals were deposited by electron beam evaporation.
As shown in Fig. 1(j), regrown InAlN/GaN HEMT with n + GaN contact ledge structure had been fabricated. The distance between the drain etching well and the source etching well was defined as drain-source spacing (L DS ), which was shrunk to 600 nm, as well as the gate width was 2×50 μm. T-gate was in the middle of the drain and source region. The T-gate length (L G ) was 150 nm, while the n + GaN contact ledge length (L_ n+Ledge ) on each side was also 150 nm.

A. TLM MEASUREMENT AND DISCUSSION
As shown in Fig. 2(a) and (b), in order to characterize ohmic contact resistance, two kinds of TLM patterns are designed [16]. TLM results are shown in Fig. 2(c). By measuring TLM1, the resistance R 1 between ohmic metal and n + GaN is 0.03 ·mm, as well as the sheet resistance of n + GaN (R SH_n+GaN ) is 47 / . According to TLM2 measurement, 2DEG channel sheet resistance (R SH_2DEG ) is 162 / , and the total ohmic contact resistance R C of the device is 0.12 ·mm, including the resistance between ohmic metal and n + GaN (R 1 of 0.03 ·mm), resistance of n + GaN access region (R 2 of 0.035 ·mm) calculated by multiplying R SH_n+GaN by the distance from the ohmic metal to the etching side wall, and the interface resistance between n + GaN and 2DEG channel (R 3 of 0.055 ·mm). Fig. 2(d) shows that the TLM pattern with n + GaN regrown contact ledge structure has a higher maximum output current density than the pattern without n + GaN regrown contact ledge, both with the same drain-source distance of 3 μm, however, the on-resistance of the two devices is basically the same. This phenomenon could be explained by Fig. 2(e) based on Silvaco TCAD simulation. With the assistance of Silvaco TCAD simulation, the energy band between source n + GaN regrown contact ledge and 2DEG channel are shown in Fig. 2(f) to further explain the phenomenon mentioned above. As shown in Fig. 2(f), at lower drain-source voltage (such as 0∼1 V), the InAlN barrier width between source n + GaN contact ledge and 2DEG channel are still large, which is difficult for electrons to transfer from source n + GaN contact ledge to 2DEG channel in source region, so that only current Path 1 has been turned on. As drain voltage increases, the barrier width and height between source n + GaN contact ledge and 2DEG channel become thinner and lower, and electrons could easily transfer from source n + GaN contact ledge to 2DEG channel through tunneling or thermal electron emission, so that the current Path 2 has been gradually turned on. When the device is biased at a higher drain-source voltage (such as V DS = 10 V), both the current Path 1 and Path 2 contribute to the drain current increment, corresponding to the reduction of drain-source effective spacing.

B. DC CHARACTERISTICS
The InAlN/GaN HEMT adopting regrown ohmic contact without contact ledge structure has been fabricated as well. The device has the same size and adopts the n + GaN regrowth technology, but there is only no n + GaN contact ledge structure. The schematic figures of the regrown InAlN/GaN HEMT with and without n + GaN contact ledge structure were shown in Fig. 3(a) and (b).
The DC characteristics of the regrown InAlN/GaN HEMT with and without n + GaN contact ledge structure were measured by Keithley 4200 semiconductor parameter analyzer. The transfer characteristics of the two regrown InAlN/GaN HEMTs were measured at V DS of 10 V, as shown in Fig. 4(a). Benefitting from the n + GaN contact ledge structure providing an additional current path and scaling down the effective drain-source spacing, the peak extrinsic transconductance (G m.max ) of the regrown HEMT with n + GaN contact ledge is 127 mS/mm larger than the one without n + GaN contact ledge, reaching 874 mS/mm. Fig. 4(b) shows the output characteristics with the maximum gate voltage (V GS.max ) of 2 V, with the assistance of n + GaN contact ledge structure, the maximum output current density (I D.max ) of the HEMT is 2.9 A/mm, which is 246 mA/mm larger than the regrown HEMT without n + GaN contact ledge structure. Both regrown n + GaN ohmic contact and contact ledge structure are responsible for the improved DC performance.

C. PULSED I-V & SMALL SIGNAL CHARACTERISTICS
As shown in Fig. 5(a), the pulsed I-V characteristics were measured with quiescent bias points of (V GSQ , V DSQ = 0 V, 0 V) and (V GSQ , V DSQ = −6 V, 10 V). The pulse period was 1 ms and the pulse width was 500 ns. The current collapse is only 2%, and the negligible current collapse indicates that the self-stopping etching on the access region does not induce extra damage.
The Agilent 8363B vector network analyzer was used to measure the small signal characteristics. Benefitting from the n + GaN contact ledge providing an additional current path   and reducing the effective drain-source spacing, the access resistance is reduced. Therefore, the regrown InAlN/GaN HEMT with n + GaN contact ledge has larger f T and f MAX than regrown HEMT without n + GaN contact ledge. As shown in Fig. 5(b), by extrapolating the short circuit current gain (|H 21 |) and the Mason's unilateral gain (UPG) curves using −20 dB/decade slopes, f T and f MAX of the regrown InAlN/GaN HEMT with n + GaN contact ledge at V DS of 10 V are 93 GHz and 142 GHz, respectively. The small-signal performance can fully meet the requirement of millimeter-wave low voltage applications.

D. RF POWER CHARACTERISTICS
The on-wafer load-pull system was used to measure the RF power characteristics of the regrown HEMTs with and without n + GaN contact ledge at 30 GHz, and both input and output impedances of the devices were tuned to optimize the power-added-efficiency (PAE). Two kinds of the InAlN/GaN HEMTs were both biased at V DS of 10 V and I DQ of 100 mA/mm. What's more, with the assistance of the n + GaN  contact ledge reducing the effective drain-source spacing, the regrown InAlN/GaN HEMT with n + GaN contact ledge structure has the lower parasitic resistance and larger output current density, which makes the regrown InAlN/GaN HEMT with n + GaN contact ledge structure deliver a higher peak PAE and larger P out . The regrown InAlN/GaN HEMT with n + GaN contact ledge structure delivers a PAE of 44% with a P out of 2.5 W/mm, as shown in Fig. 6(a).
Within the Ka band, the regrown InAlN/GaN HEMT with n + GaN contact ledge structure has decent power performance by comparing the power characteristics of GaN HEMTs under similar low voltage bias, as shown in Fig. 6(b), which indicates great advantage of regrown InAlN/GaN HEMT with n + GaN contact ledge structure for millimeter-wave low voltage terminal applications. However, the maximum applicable V DS is reduced due to the shorter effective gate-drain spacing. But considering that the device works for low voltage terminal applications, the operating voltage of the device is usually 5∼15 V, so that the reduction of the applicable V DS would be tolerated. In our subsequent work, the reduction of the maximum applicable V DS caused by the shorter effective gate-drain spacing of the device will be considered and handled. The n + GaN contact ledge on the drain side will be removed to increase the applicable voltage range of the device.

E. ELECTRICAL STRESS RELIABILITY
Reliability is critical for RF devices. Therefore, the electrical stress reliability of this newly proposed device with n + GaN contact ledge structure formed by self-stopping etching was evaluated. As shown in Fig. 7, the OFF-state and ON-state electrical stress tests have been adopted to assess the reliability of the regrown InAlN/GaN HEMT with n + GaN contact ledge structure. The OFF-state electrical stress condition was V GS = −6 V and V DS = 10 V, and the ON-state electrical stress condition was V GS = 0 V and V DS = 10 V. The threshold voltage, drain current, and schottky gate current have negligible change after OFF-state and ON-state electrical stress measurements. It can be seen that the regrown HEMT with n + GaN contact ledge structure has decent stability by electrical stress evaluation.

IV. CONCLUSION
In conclusion, high performance regrown InAlN/GaN HEMT with n + GaN contact ledge structure for low voltage application has been fabricated. The fabricated device has low on-resistance and ohmic contact resistance. In particular, attribute to the n + GaN contact ledges shortening the effective drain-source distance, the G m.max of the device reaches 874 mS/mm, I D.max reaches 2.9 A/mm. Ignorable current collapse indicates that no excessive etching damage was introduced on the self-stopping etching region. It is worth noting that regrown InAlN/GaN HEMT with n + GaN contact ledge structure exhibits PAE of 44% and P out of 2.5 W/mm at 30 GHz and V DS of 10 V, indicating great potential of regrown InAlN/GaN HEMT with n + GaN contact ledge structure for 5G millimeter-wave band low voltage applications. Besides, by OFF/ON-state electrical stress measurements, the regrown HEMT with n + GaN contact ledge can be proved to have good reliability.