Improved Linearity in AlGaN/GaN HEMTs for Millimeter-Wave Applications by Using Dual-Gate Fabrication

An effective method of improving the linearity of AlGaN/GaN HEMTs by using dual-gate technology is demonstrated. In this letter, we compare the DC characteristics and device linearity of the dual-gate AlGaN/GaN HEMTs with conventional single-gate AlGaN/GaNHEMTs.Thecorrelationbetweentheextrinsictransconductance(G m )withthird-orderintermodulationdistortion(IM3) and third order intercept point (IP3) suggests that the broader G m distribution as a function of gate-bias, causes a lower IM3 level and higher IP3 values for the device. The improved device linearity demonstrates that dual-gate AlGaN/GaN HEMT design is a good approach for high-linearity RF device applications.

AlGaN/GaN based high-electron mobility transistors (HEMTs) have been widely studied for microwave power applications, such as wireless base station and satellite communication owing to the high power handling capability at high frequencies for GaN devices. [1][2][3][4] In modern wireless communication systems, multichannel transmissions are extensively used for signal transmission. During signal transmission, there are many operating frequencies and the neighboring frequencies are located closely to each other, hence the device used in this system will produce the intermodulation distortion and lead to the degradation of the system signal-to-noise ratio (SNR). Among all intermodulation distortions, third-order intermodulation distortion (IM3) dominates the linearity performance of the devices. Hence, IM3 performance is one of the most important criteria for evaluating device performance used in wireless communication systems.
Recently, improvement of linearity from the device level is attracting lots of attention. A nonlinearity transfer-function-based analysis method was used for the evaluation of device linearity. 5 In order to reduce IM3, the transconductance needs to remain constant during a wide operating range of the gate bias. It indicates that the flatter transconductance profile results in a lower IM3 levels and a higher third order intercept point (IP3), and thus improves the device linearity. Several methods have been reported in the past to improve the linearity of the GaN HEMTs, including optimizing epitaxial structure 6,7 and utilizing ALD-Al 2 O 3 8 or nanolaminate La 2 O 3 /SiO 2 9 or ferroelectric material of Pb(Zr 0.52 Ti 0.48 )O 3 (PZT) 10 as gate dielectric.
In addition, there have been several reports of monolithic microwave integrated circuit (MMIC) low-noise amplifier (LNA) using GaN dual-gate HEMT devices because the two gates allow higher bias operation and increase the transistor output impedance. Moreover, the dual-gate transistors also exhibit smaller feedback capacitance compared to a single-gate transistor. 11,12 Furthermore, the dual-gate configuration can be fabricated at the same time as the single-gate configuration using the same process. However, to our best knowledge, there are very few reports on linearity characteristics of dual-gate GaN HEMTs. In this letter, the DC measurements and linearity test were performed on the single-gate and dual-gate GaN HEMTs, and the device linearity performance of these two devices are compared for communication applications. z E-mail: edc@mail.nctu.edu.tw

Experimental
The AlGaN/GaN HEMT heterostructure structure was grown by metal organic chemical vapor deposition (MOCVD) on SiC substarate. The epitaxial structure consisted of a AlN nucleation layer, 1.3 μm GaN buffer layer, 0.5 nm AlN spacer, and 22 nm undoped AlGaN barrier layer. The device fabrication began with mesa isolation of the active areas by inductively coupled plasma (ICP) etching using Cl 2 based gas mixture. The ohmic contacts were formed by Ti/Al/Ni/Au metal stack which was deposited by electron beam evaporator and subsequently annealed by RTA at 800 • C for 1 min in N 2 ambient. Then, the electron beam (EB) lithography with dual-layer photoresists was used for the fabrication of the single-gate and dualgate devices. Ni/Au was deposited by electron beam evaporator as gate metal. Finally, the Si 3 N 4 film was deposited by PECVD at 300 • C as passivation layer. Fig. 1a shows the cross-sectional schematic of the conventional single-gate device structure. The gate-to-drain spacing L GD , gate-tosource spacing L GS , and gate length L G were 3.35 μm, 3.35 μm, and 0.3 μm, respectively. Fig. 1b shows the cross-sectional schematic of the dual-gate device structure. The gate-to-drain spacing L GD , gate-togate spacing L GG , gate-to-source spacing L GS , and gate length L G were 2.5 μm, 1.4 μm, 2.5 μm, and 0.3 μm, respectively. These two kinds of devices were fabricated by the same process and the single-gate device is referred as the control sample.

Results and Discussion
DC and RF measurements.-The output I DS -V DS characteristics of the single-gate and dual-gate devices are shown in Fig. 2. The I DSS (I DS @V GS = 0 V) for the 300 nm single-gate and 300 nm/300 nm dual-gate devices were 740 mA/mm and 620 mA/mm respectively. The relationship between the gate length and drain current has been reported. 13 When the Schottky contact is deposited on the strained Al-GaN/GaN heterostructures, the two-dimensional electron-gas (2DEG) electrons under the Schottky contact are extracted to the empty surface donor states. It suggests that with the increased area of Schottky contact, more surface states of AlGaN barrier layer are affected by the electrons from the Schottky contact metals, and more 2DEG electrons are extracted to the empty surface donor states. Thus, the 2DEG sheet carrier concentration is decreased. Due to the larger schottky contact area of the dual-gate device, the 2DEG carrier concentration decreased, resulting in lower drain current for the dual-gate device.  Fig. 3 shows the I DS -V GS curves of these two devices under study. It can be observed that the single-gate device has a higher pinch off voltage of −3.7 V and the dual-gate device has a lower pinch off voltage of −3.2 V, indicating the threshold voltage dependence on the gate length. 14,15 As the device gate length reduced, the electron velocity overshoot effect becomes more significant because of there are more electrons travel ballistically. 16 It can be observed that the slope of the I DS curve of single-gate device is higher than double-gate device at V GS = −3 V to −2 V. This is due to the high electric field at the edge of gate, the high velocity electrons reduce the transit time and lead to higher drain current and transconductance. However, when increasing the gate bias to the positive voltage, the drain current of the dual-gate device increases in a stable rate owing to the fact that the second gate could effectively reduce the electric field, and thus suppress the electron overshoot effect. 16,17 Therefore, the maximum currents of these two different types of devices reach almost the same value when biased at V GS = 2 V and V DS = 10 V.
The maximum extrinsic transconductance (G m,max ) of the singlegate and dual-gate GaN HEMTs were 174 mS/mm and 169 mS/mm, respectively, as shown in Fig. 4. The gate voltage swing (GVS) was defined as the 10% drop from the G m,max . It can be observed that the dual-gate device has a larger GVS, suggesting a better linear behavior compared with the single-gate device. 5,8 Fig. 5 shows the OFF-state drain leakage currents of the single-gate and dual-gate GaN HEMTs. There is an obvious reduction in the drain leakage current of the dual-gate device, indicating the electric field alleviated by the second gate. 18 The microwave small-signal performances were measured from 100 MHz to 50 GHz using an Agilent N5245A network analyzer. The S-parameter measurement system was calibrated using the standard LRRM calibration method. The single-gate device biased at Linearity test.-In order to investigate the device linearity performances, we used the polynomial curve fitting to the transfer characteristics of these devices. A polynomial equation was used to express device I DS -V GS transfer curves, as shown in the following: For a device with good linearity, I DS should increase linearly with V GS , therefore, a 1 should be larger and the higher order constants should be minimized. The coefficients of these two kinds of devices extracted from the measurement data with V DS = 10 V are listed in Table I, and it is clearly observed that the dual-gate device has a larger a 1 and smaller a 3 /a 1 and a 5 /a 1 values, indicating a better linearity for the dual-gate device. 5,19 In addition, the relationships between IM3, IP3 and G m , G ds are shown in Equation 2 and 3: where A is the signal amplitude, G m is the second derivative of the transconductance, and R L is the load impedance. It can be seen that the IM3 level is directly proportional to (G m ) 2 , while the IP3 value is inversely proportional to the G m and directly proportional to (G m ) 3 . Therefore, a lower IM3 level can be achieved by increasing the flatness of the G m distribution across the gate-bias region. 5,9 As shown in Fig.  4, the dual-gate device has a flatter G m curve, which leads to small absolute G m over a wide range of gate-source voltage shown in Fig. 6. As a result, the dual-gate device has lower IM3, indicating the device has a better linearity. 7 To further evaluate the device linearity, the IM3 and IP3 measurements were performed by injecting two-tone signals at 6 GHz with an offset frequency of 1 MHz and V DS was set at 10 V. For the IP3 measurement, the load impedance was first tuned for maximum power for each individual device. Then, the IM3 was measured and plotted as a function of input power under given dc bias conditions. IP3 was determined by the intercept point of the Pout and IM3 curves after extrapolation. Fig. 7 shows the IP3 of these two kinds of devices with   (Table II).

Conclusions
In summary, the single-gate and dual-gate AlGaN/GaN HEMTs were fabricated, and the device DC performance and linearity were compared. Even though the single-gate device has higher I DSS and G m,max , the dual-gate device shows much better device linearity with lower overall IM3 level and higher IP3 value. These results show the dual-gate GaN HEMTs have great potential to be used for future wireless communication systems.