# Monolithically Integrated THz Detectors Based on High-Electron-Mobility Transistors
## 5 Measurements
### THz Focal Plane Array  
To evaluate a signal, the THz detectors arranged in a square grid are read out individually in a defined sequence with fixed addresses. Given the extensive range of current measured by the detectors, spanning from a few \(nA\) to twenty \(\mu A\) or more, an 18-bit analog-to-digital converter is employed. The low current necessitates a transimpedance amplifier with high gain. The layout of the THz focal plane array consists of four identical 6 \(\times\) 6 detector sub-arrays. The detector features a gate-voltage-dependent sensitivity curve, as described in Section 5. A sufficiently high negative voltage at the gate switches off the GaN-HEMT detectors, causing them to behave like high resistance or an \"open contact\". Setting the voltage to the optimal detection value enables the GaN-HEMT detectors to detect THz radiation. Within a sub-array, gates are connected in columns, while all drain and source terminals are connected in rows. Only one transimpedance amplifier is required per sub-array row.  
To \"read out\" an individual detector, the optimal detection voltage must be supplied to the gate of that specific detector. Simultaneously, a sufficiently negative voltage must  
Figure 7: Image of the 144 pixel fabricated focal plane array of THz detectors, with a magnification of the first four rows in the first quadrant (inset on the right side).  
be supplied to all other detectors in the same row of that sub-array, ensuring that only the current from the chosen detector is sent to the connected transimpedance amplifier.  
Dividing the entire detector array into sub-arrays offers several advantages. It enables the recovery of operational sub-arrays from otherwise dysfunctional whole arrays and facilitates an interleaved readout scheme for the detection currents, enhancing readout speed. After the currents of a particular column of a sub-array have been digitized, the GaN-HEMT detectors in that column must be closed. Subsequently, the next column of detectors must be activated to detect radiation by supplying the optimal detection voltage to those gates. This change, however, affects the currents in the drain and source pairs and, consequently, the net currents provided to the transimpedance amplifiers (TIAs). Due to the characteristics of high-gain TIAs, these devices have limited bandwidth and thus require some time to stabilize with the newly supplied detection currents, meaning digitization cannot occur during this settling time. Splitting the rows into two parts, enabled by the division into sub-arrays, allows for the connection of an additional, equal set of transimpedance amplifiers, which can provide voltages for digitization in the meantime. Interleaving the switching of columns in one set of sub-arrays with the digitization of currents in the other set allows for doubled digitization speed, making it the preferred approach for digitizing the detector current.  
A more detailed examination of the readout electronics can be found at [18]. The readout speed of the current electronics is theoretically 100 frames per second (FPS). To improve the signal-to-noise ratio (SNR), dark current calibration was performed at the beginning of each measurement, followed by calculating an average across multiple recordings of the entire THz focal plane array, depending on the frequency and, thus, the power of the THz source. As the frequency rises, the averaging increases. At 100 GHz, only two frames were used, resulting in an FPS of 50 FPS. At 1.1 THz, 100 frames were utilized, which reduced the FPS to one.