# Research on Fast Detection Technology of Dark Currents in a Ge-Si Detector Array
## 2 Principle and System
### Scanning Detection Method and System
Due to the negative dark current of the PD detector under reverse bias and the positive dark current under forward bias, \(V_{2}\) can be either positive or negative, while the ADC inside the MCU can only read positive voltages. Therefore, we designed a third-stage amplification circuit, which outputs in-phase voltages on one side and reverses voltages on the other side; namely:  
\[\left\{ \begin{array}{l}V_{P}=V_{2}=-GR_{1}I_{dark}\\ V_{N}=-V_{2}=GR_{1}I_{dark}\end{array} \right. \tag{8}\]  
when \(I_{dark}<0\), \(V_{P}>0\), and \(V_{N}<0\), the ADC reads the voltage of \(V_{P}\). When \(I_{dark}>0\), \(V_{P}<0\), and \(V_{N}>0\), the ADC reads the voltage of \(V_{N}\). We used two ADC channels to read the \(I_{dark}\) for reverse and forward bias. Figure 4: Schematic diagram of controllable voltage source circuit. Figure 5: Dark current amplification circuit. Using the Toshiba semiconductor's optical relay TLP3320 to achieve the row and column selection of detector arrays and to replace mechanical relays with optical relays can reduce the installation area and achieve fast switch selection. Figure 6a shows the internal structure of an optical relay, which is similar in principle to an optocoupler. The switching speed of TLP3320 is less than 300 \(\upmu\)s. This system can achieve fast scanning of the array. The on-resistance of TLP3320 is about 10 \(\Omega\), and the off-resistance is about \(10^{14}\)\(\Omega\). Usually, the dark current of the PD detector is between 1 and 100 nA under a reverse-bias voltage of 1 V, and the equivalent resistance of PD is between \(10^{7}\) and \(10^{9}\)\(\Omega\). If the on-resistance \(R_{ON}\) of TLP3320 is set to 10 \(\Omega\), the off-resistance \(R_{OFF}\) is set to \(10^{14}\)\(\Omega\), and the equivalent resistance \(R_{PD}\) of PD is set to \(10^{8}\)\(\Omega\), then the equivalent circuit of the array is that shown in Figure 6b. The on-current \(I_{ON}\) is approximately \(10^{13}\) times the off-current \(I_{OFF}\), so the influence of the off-resistance can be ignored. The dark current \(I_{A}\) on the PD detector in the first row and column selected in Figure 6b is approximately equal to the dark current \(I_{dark}\) of the array. As shown in Figure 7, the hardware diagram of the detection system is composed of a Ge-Si detector array and its scanning circuit, a variable voltage source, a dark current detection circuit, a control circuit, and an LCD screen. The Ge-Si detector array is placed in the middle of the scanning circuit, and pixel selection is carried out by an optical relay. The variable voltage source and dark current detection circuit are integrated on a printed circuit board (PCB), and the bias voltage is output to the scanning circuit. The dark current is also connected to the dark current detection circuit from the scanning circuit. The control circuit is located below the LCD screen, and the MCU used is the ARM chip STM32F103 ZET6 from STMicroelectronics. The LCD screen can draw a bias voltage-dark current curve and display the magnitude of dark current under different bias voltages. The system detection includes two modules: single measurement and continuous measurement. In the single-measurement mode, rows and columns can be selected, and the bias voltage-dark current curve of the row and column pixels can be measured in a single measurement. By continuously measuring the modulus, we can automatically measure the bias voltage-dark current curve of all pixels at once and ultimately store all test data in the SD (secure digital memory) card. By inserting the SD card into the computer host, we can read all the data. Figure 6: Optical relays and their equivalent circuits. (**a**) Internal structure of optical relays. (**b**) Equivalent scanning circuit.