**1 Introduction**
The GaAs MMIC control circuit has become a key component in modern electronic systems due to its compact size, lightweight design, fast switching speed, radiation resistance, high reliability, and near-zero power consumption. In advanced mobile communication systems, such as space diversity smart antennas and phased array systems, precise amplitude and phase control is essential. When phase adjustment is required, it's preferable for the amplitude to be as small as possible. Similarly, when adjusting amplitude, the phase shift should remain minimal. The paper presents a DC-50GHz ultra-wideband GaAs MMIC voltage-controlled E-attenuator that offers a low dynamic insertion range, excellent linearity in attenuation with respect to control voltage variation, superior input/output standing wave performance, and multiple functionalities. Its most notable feature is the low insertion phase shift, making it highly suitable for a variety of applications.
This circuit can be used in several ways:
a) As a DC-50GHz MMIC variable attenuator with minimal phase shift, forming an automatic loss control component;
b) In a multi-octave vector modulator, where it performs amplitude modulation while introducing minimal phase shift;
c) As a high-performance absorption (or matching) single-pole single-throw switch operating at DC-50GHz;
d) As a multi-octave pulse amplitude modulator;
e) When cascaded with a wideband amplifier, it can form an AGC (Automatic Gain Control) amplifier with a wide frequency band, large dynamic range, and high linearity.
**2 Design**
The overall design and implementation of this monolithic voltage-controlled variable attenuator are based on specific electrical performance requirements. The process involves selecting an appropriate circuit topology and designing a suitable switch MESFET. An equivalent circuit model parameter scaling equation is derived for the switching MESFET across the DC-50GHz frequency range. To simplify the circuit and reduce power consumption, load design techniques such as series and parallel switch MESFETs with two control terminals are employed.
The circuit operates using complementary control voltages that regulate the series and parallel switching MESFETs. During the ESC (Electronic Switching Control) process, the loads on both control terminals must be balanced to prevent significant degradation in standing wave characteristics. A DC parameter circuit is typically designed to balance these loads, improving the standing wave performance during ESC control. This method is also applied in the design to enhance the performance of the control terminal.
The design process includes simulation and optimization using mature microwave monolithic design software like AgilentEesof. After optimization, layout design is performed, followed by secondary simulations and adjustments. A reasonable fabrication process route is selected, and the final step involves microwave testing.
Figure 1 shows the electrical schematic of the circuit, which uses a wideband switching circuit topology with a matched SPST configuration. The switching arm is connected to an absorption resistor and a parallel branch of the switch MESFET. Multiple switch MESFETs are connected in parallel with the main transmission path, and low-dispersion, wideband coplanar waveguide transmission lines are used to broaden the frequency bandwidth.
The basic principle of achieving low insertion phase shift is through phase cancellation. The phase lag from the series inductive reactance and the phase lead from the parallel capacitive reactance cancel each other out. Key electrical performance indicators considered include a frequency range from DC to 50GHz, minimal attenuation within the band, low input/output standing waves at minimum attenuation, maximum attenuation with good flatness, low input/output standing waves at maximum attenuation, and minimal phase shift per unit of attenuation.
To achieve optimal performance, different gate widths are chosen for various MESFETs. Model parameter scaling techniques are used, and parameters are extracted using HPIC-CAP software. For frequencies up to 50GHz, frequency expansion and fitting techniques are also applied to obtain accurate design parameters for the switching MESFET circuit model.
**3 Production**
The chip was fabricated using the ion implantation wafer process line at the Nanjing Electronics Research Institute. This process ensures high yield and long-term stability. The fabrication steps include Au/Ge/Ni ohmic contact metallization, a 0.5μm gate length TI/Pt/Au Schottky barrier gate, N+ and N- ion implantation, ion implantation resistance, metal film resistance, SiO2 passivation, air bridge structures, through-hole grounding, back metallization, and plating. The process yield exceeds 80%, and the electrical performance of the chips matches well with that of the wafers. The chip dimensions are 2.33mm × 0.68mm × 0.1mm. The chip photo is shown in Figure 2. Signal input and output use coplanar waveguide interfaces with through-hole grounding and multi-chip passivation technology, ensuring high reliability. The control terminals are located on the side of the chip for easy installation and use.
**4 Performance**
Electrical performance tests were conducted using a microwave on-chip test system consisting of an HP8510C vector network analyzer and a Cascade Microtech microwave probe station. Various parameters were measured, including minimum and maximum attenuation, input/output standing waves at both states, and phase shift differences between them. When V1 is 0V and V2 is at the negative FET pinch-off voltage VP, the circuit is in the minimum attenuation state. When V1 is at VP and V2 is 0V, it is in the maximum attenuation state. By gradually adjusting V1 from 0V to VP and V2 from VP to 0V, the attenuation changes accordingly. The chip has passed rigorous tests, including high and low temperature storage, impact, operation, bonding, shearing, and a 1000-hour life test at 125°C.
**5 Conclusion**
The successful development of this low-phase-shift, multi-functional DC-50GHz high-performance MMIC voltage-controlled variable attenuator demonstrates excellent electrical performance, high process yield, and strong reliability. It meets all design requirements and has practical value in modern communication systems.
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