# Monolithically Integrated THz Detectors Based on High-Electron-Mobility Transistors
## 3 Design
### Integration of HEMT Terminals into the Design  
The HEMT terminals directly form the RF environment, eliminating clear separation between the antenna and the HEMT device. All metallic connection structures are integral to the detector design, ensuring scalability to large arrays with minimal cross-coupling between individual elements. For frequencies above 100 GHz, the detector structures are designed as closed metal entities, with antenna wings merging seamlessly into the structural border. This approach:  
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* Prevents crosstalk with neighboring structures.
* Preserves antenna impedance by isolating connection structures.  
Figure 5 presents the \(S\)-parameter simulation for two reference planes:  
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* Drain and gate terminals (red curve).
* Source and gate terminals (blue curve).  
Figure 5: Simulation of the impedance (Smith chart on the right with a 50 \(\Omega\) reference impedance) and isolation (graph on the left in dB as a function of the frequency) between the drain–gate terminal and the source–gate terminal. The inset on the top-left corner represents in a color-coded manner the simulated reference points on a scanning electron image of the antenna feeding point.  
The simulation covers frequencies from 50 GHz to 1.5 THz, which correspond to substrate wavelengths ranging from 1.9 mm to 63 \(\upmu\)m. The entire structure, including metallic surfaces and dielectric layers, was simulated using an electromagnetic (EM) full-wave solver (CST Microwave Studio).  
As concerns the impedance behavior at the drain gate terminal, it is important to note the following:  
* The real part of the impedance was less than 10 \(\Omega\) over the entire frequency range.
* The reactance increases with frequency due to the inductive behavior introduced by the distance between the MIM capacitor and the reference plane.  
In order to improve the performance, the design would have to be adjusted. The MIM capacitor should ideally encompass the drain terminal structure to mitigate inductive behavior. This design was excluded to avoid yield reductions from potential short circuits at metal edges.  
The simulation of the antenna feed point (between source and gate terminals) reveals three distinct frequency ranges:  
* _Low Frequencies_ (<80 GHz): The structure is small with respect to the operating wavelength, causing the impedance to be influenced by the external environment rather than the antenna.
* _Transition Range_ (80 GHz-400 GHz): The structure's edge acts as a short circuit, allowing the antenna to dominate impedance. However, the wavelength remains too large for efficient radiation.
* _High Frequencies_ (>400 GHz): The antenna structure reaches at least a quarter of the wavelength, where the impedance is determined by the radiation resistance in parallel with a capacitance determined by the metallic structures of the HEMT.  
Based on this principle, various antenna structures, including slot antennas, can be designed with appropriate guidance of metallic layers. All structures were developed using three-dimensional (3D) EM simulations.