Gallium nitride (GaN) low-noise amplifiers (LNAs) are of significant interest due to their robustness against high input power and moderately low noise performance [1]. GaN devices are increasingly employed not only in high-power/high-efficiency amplifiers operating in the microwave band but also in LNAs with high input ratings, owing to their superior power-handling capabilities and thermal stability compared to gallium arsenide (GaAs) devices. In typical applications, LNAs based on GaAs devices exhibit maximum input power ratings of several tens of milliwatts, necessitating the use of limiters to safeguard receivers from excessive input power. In contrast, LNAs based on GaN devices can handle input power levels of several watts, thereby eliminating the need for limiters [1–3].

This paper presents the design and fabrication of an X-band LNA monolithic microwave integrated circuit (MMIC) using 0.2 µm GaNon-silicon carbide (SiC) process technology developed by the Electronics and Telecommunications Research Institute (ETRI). The LNA applying two-stage structure is designed using a small-signal model with noise parameters. The suitability of the small-signal model is evaluated through the performance of fabricated LNA.

Increasing the gate width (i.e., the number of fingers) in MMICs significantly affects device characteristics. A larger gate width allows for a higher output current and power, making it suitable for highpower applications such as 5G base stations, radar systems, and satellite communications. Additionally, reducing on-resistance (R_on) improves power efficiency by lowering conduction losses. Linearity is enhanced by providing a more uniform current distribution, which reduces signal distortion and improves signal quality in radio frequency applications.

Aluminum GaN (AlGaN)/GaN heterostructures were grown on SiC substrates using metal-organic chemical vapor deposition. The structure consists of a 25 nm Al_0.25Ga_0.75N barrier, an aluminium nitride insertion layer, and a 1 µm GaN buffer. Ti/Al/Ni/Au was alloyed to form source and drain ohmic contacts. After device isolation, a 60 nm silicon nitride passivation layer was deposited. The source and drain contacts were opened, and the Ti/Au was evaporated to form the first metal layer. Subsequently, a 0.2 µm gate length was defined, and a Ni/Au was used to form the gate electrode. A diagram and crosssectional focused ion beam image of the fabricated device, which has a gate length (L_G) of 0.2 µm and source-drain spacing (L_SD) of 5 µm, are presented in Fig. 1.

Figure 1. (a) Diagram of AlGaN/GaN HEMT device structure. (b) Cross-sectional focused ion beam image of the fabricated AlGaN/GaN HEMT device with gate length (L_G) of 0.2 µm and source-drain spacing (L_SD) of 5 µm.

The X-band LNA was designed using the ETRI 0.2 µm GaN-on-SiC process technology. This design and layout were performed using Agilent Advanced Design System, and parasitic extraction-based models and electromagnetic simulations were conducted to analyze high-frequency operations. The GaN LNA was designed as a two-stage common-source structure, using a small-signal model with noise parameters.

The active device in the first stage of the LNA was a 4f50 (four fingers with a gate width of 50 µm) GaN high-electron-mobility transistor (HEMT) [Fig. 2(a)]. In the second stage a 4f100 (four fingers with a gate width of 100 µm) GaN HEMT was used.

Figure 2. Noise figures of 2f50, 4f50, and 4f100 GaN HEMT devices, along with a 0.2 µm GaN (4f50).

The noise figure characteristics of the 2f50, 4f50, and 4f100 GaN HEMT devices are shown in Fig. 2(b), where the 4f50 GaN HEMT exhibited a noise figure of 0.9 dB at 10 GHz. The noise characteristics of these devices were analyzed using a small-signal model that included noise modelling.

A schematic of the GaN LNA with a two-stage cascade structure is shown in Fig. 3. Since the input stage determines the LNA noise, the first stage was optimized using a source-degeneration structure with an inductor to enhance noise performance, bandwidth, and stability. A resistor (R1) was added to the gate bias line to improve stability, and a metal-insulator-metal (MIM) capacitor (C5) was connected in parallel to minimize internal noise due to resistance. In the second stage of the LNA with a common-source structure a 4f100 device was used to achieve higher gain and output power. A resistor (R2) was used in the gate bias line for improved stability.

Figure 3. Schematic of the X-band GaN LNA with a two-stage cascade structure.

Input, output, and inter-stage matching were accomplished using microstrip lines, MIM capacitors, and spiral inductors. The size of the circuit was minimized by incorporating a λ (wavelength)/4 bias line into the matching circuit. Direct current blocking was performed using MIM capacitors in the input, output, and interstage matching circuits to ensure bias separation between the amplifiers.

A photograph of the manufactured X-band GaN LNA chip, which features a two-stage cascade structure, is provided in Fig. 4. The chip size is 1.70 mm² (1.92 mm × 0.89 mm), including the input pad (left), output pad (right), and gate/drain bias pads (top and bottom). Small-signal (S-parameter) measurements were conducted using a network analyzer (S5180, Copper Mountain Technologies), while noise figure measurements were performed using a noise figure analyzer (N8975A, Agilent Technologies). The noise measurement setup for the X-band GaN LNA MMIC is shown in Fig. 5.

Figure 4. Photograph of the X-band two-stage GaN LNA MMIC (1.92 × 0.89 mm²).

Figure 5. Setup for the noise figure measurement of the X-band GaN LNA, including test instruments and the X-band GaN LNA MMIC chip.

The S-parameter characteristics of the GaN LNA (Fig. 6) were simulated and measured at a gate voltage of −3.5 V and a drain voltage of 20 V. In the 8 − 10 GHz range, the measured small-signal gain was 19 − 20 dB, input reflection loss was 15.8 − 25.0 dB, and output reflection loss was 9.2 − 24.8 dB. Although the measurement results exhibited a 2 − 3 dB downward shift in small-signal gain compared to the simulations, the trends were consistent. The simulation and measurement results of the noise figure characteristics of the GaN LNA at a gate voltage of −3.5 V and a drain voltage of 20 V are compared in Fig. 7. The measured noise figure was 1.70 − 1.99 dB in the 8 − 10 GHz range (below 2 dB). A 0.5 dB fluctuation was observed around 9.5 GHz. The noise error was within 0.08 dB in the 9 − 11 GHz range, demonstrating the high noise accuracy of the small-signal model. The simulation and measurement results for the X-band GaN LNA are compared in Table I.

Figure 6. Simulated and measured S-parameter results of the X-band GaN LNA MMIC.

Figure 7. Simulated and measured noise figure results of the X-band GaN LNA MMIC.

Table I. Small-signal and noise figure characteristics of the X-Band GaN LNA in this study..

Parameters	Unit	Simulated results	Measured results
Frequency	GHz	8 − 10	8 − 10
Small-signal gain	dB	21 − 23	19 − 20
Input return loss	dB	10.3 − 25.0	15.8 − 25.0
Output return loss	dB	10.3 − 10.6	9.2 − 24.8
Noise figure	dB	1.87 − 2.24	1.70 − 1.99

Table II benchmarks the study’s results against previously published GaN LNA performance in the 8 − 11 GHz range. The LNA of this study exhibited superior gain and excellent noise characteristics, particularly in terms of chip size compared with previously published twostage GaN LNAs.

Table II. Comparison of X-Band GaN LNA performances (*: simulated, CS: commonsource)..

Ref.	Tech (µm GaN)	Topology	Freq. (GHz)	Gain (dB)	Noise fig. (dB)	Chip size (mm2)
[2]	0.25	CS (2-stage)	7 − 12	>14.0	<2.5	5.0
[3]	0.25	CS (3-stage)	8 − 11	>22.0	<2.0	3.6
[4]	0.15	Cascode	8 − 11	>11.0	<2.2	-
[5]	0.15	CS (2-stage)	8 − 11	>16.8*	<1.7*	6.4
This work	0.20	CS (2-stage)	8 − 11	>14.6	<2	1.7

In this study, an X-band LNA was designed and fabricated using a 0.2 µm GaN-on-SiC process, and its performances was evaluated. The design utilized a small-signal model incorporating noise parameters, which was validated using the fabricated GaN LNA.

The GaN LNA measured in the 8 − 10 GHz range demonstrated a small-signal gain of 19 − 20 dB and a noise figure of 1.70 − 1.99 dB. Compared to the design results, the small-signal gain of the GaN LNA exhibited a downward shift of 2 − 3 dB, and the noise figure shifted downward by 0.5 dB and upward by 0.05 dB around 9.5 GHz. The error in the noise figure was within 0.08 dB in the 9 − 11 GHz range, confirming the model’s accuracy. A comparison with previously reported GaN LNAs highlighted the superior gain and noise characteristics of the LNA developed in this study, particularly considering the compact chip size.

This study was supported by the Civil Military Technology Cooperation Program (Grant No. 19-CM-BD-05). Also, we would like to thank Editage (www.editage.co.kr) for English language editing.

The authors declare no conflicts of interest.