# Continuously Beam-Steered Phased Array Antenna Using GaN Varactors for Millimeter-Wave Applications
## 2 Investigation of IEMN'S GaN Devices as Varactors  
The realized active device, GaN_Lg110 nm, studied here is based on the GaN process offered by the IEMN laboratory in Lille, France. The offered version of the IEMN-GaN HEMT structure that is used in this study features a gate length of 110 nm. This process is suitable for a 50 V drain voltage for power amplifier applications and yields current levels of \(\sim\)1.3 A/mm. Figure 1a illustrates a photograph of the fabricated and analyzed device, while Figure 1b shows a simplified cross-section of the epitaxial layers of the GaN HEMT process developed by IEMN. More information about IEMN's GaN process features and limitations can be found in [18; 19]. Many devices with different gate widths and lengths are available in the mask set and have been characterized in small and large signals [18]. However, only one sample called GaN_Lg110 nm, with a gate length of 0.11 \(\upmu\)m, a width of 50 \(\upmu\)m, and a number of fingers of 2, is studied and used for the proposed phased antenna array. This device is selected for the design as it provides a tunable capacitance range from 32 ff to 355 ff at 40 GHz, which is the highest range compared to the other studied devices. Temperature analysis has been performed on this device, and more details can be found in [19]. It is shown that over the 4 K to 400 K range, there is variation in the Hall electron mobility, sheet carrier density, and sheet resistance of the used HEMT structure. At liquid helium temperature (4 K), a state-of-the-art 2 DEG mobility of 7340 cm\({}^{2}\cdot\)V\({}^{-1}\cdot\)s\({}^{-1}\) combined with a sheet carrier density of 1.93 \(\times\) 1013 cm\({}^{-2}\) was measured, resulting in a remarkably low sheet resistance of 44 \(\Omega/\square\). Figure 1: (**a**) Photograph of fabricated device “GaN_Lg110 nm” and (**b**) schematic cross-section of IEMN’s AlN/GaN-on-SiC HEMTs devices, showing contacts and epitaxial layers [18]. Based on our previous works [15; 20] on GaN varactor diode modeling of small and large signals, here, the same proposed equivalent circuit is used as a model to analyze the behavior of IEMN's GaN HEMT devices as varactors. The on-wafer RF measurements were performed by using IEMN's facilities over a frequency range of 0.25-67 GHz with a small RF power and a Vd fixed to 0 V, while Vg was varied from \(-\)7 V to +2 V. This allows us to realize a virtual connection from drain to source, resulting in a single Schottky junction which can be considered a planar Schottky varactor diode (PSVD) [15]. From the measurement, the extracted equivalent capacitance values, CEq, versus frequency are shown in Figure 2a. For Vg ranging from \(-\)7 V to \(-\)2 V, it is seen that the CEq values remain almost constant over all frequencies up to 20 GHz. Meanwhile, for Vg ranging from \(-\)1 V to +2 V, from 20 GHz, the CEq values start to increase exponentially due to high-frequency parasitic elements that become self-resonant at this bias voltage. More information about the extraction method can be found in [15; 20]. Figure 2b illustrates the behavior of the extracted equivalent series resistance, \(\mathrm{R_{Eq}}\), versus frequency for various bias voltages Vg. Three ranges can be observed: in range_1, from \(-\)7 V to \(-\)2 V, the \(\mathrm{R_{Eq}}\) values are at their minimum and decrease versus frequency. In range_2, from 0 V to +2 V, the \(\mathrm{R_{Eq}}\) values increase compared to in range_1 and remain almost stable versus frequency. For Vg of \(-\)1 V, in range_3, the device exhibits its higher value of \(\mathrm{R_{Eq}}\), which drops slightly with frequency. Because our frequency of interest is 40 GHz, here, the analysis of \(\mathrm{C_{Eq}}\) and \(\mathrm{R_{Eq}}\) is performed only at 40 GHz. The extracted tuning characteristic of \(\mathrm{C_{Eq}}\) and \(\mathrm{R_{Eq}}\) is plotted in Figure 3a,b. Figure 3: (**a**) Extracted CEq and (**b**) \(\mathrm{R_{Eq}}\) vs. Vg at 40 GHz for GaN_Lg110 nm device.