# Transimpedance Amplifier for Noise Measurements in Low-Resistance IR Photodetectors
## 4 Results
To reduce tunneling currents and decrease the maximum electric field occurring on the junction the additional graded Si-doped InAs\({}_{1-x}\)Sb\({}_{x}\) layer was sandwiched between the n+ contact and the absorber. Figure 4: Test results when using resistances at room temperature as DUTs. Tests were performed on 100 \(\Omega\) and 1 k\(\Omega\) resistances using two different values for the feedback resistance \(R_{R}\). The continuous black lines represent the expected noise, that is, the thermal current noise generated when using the resistances as DUTs. A Be-doped (p-type) AlSb barrier was used to cap the absorber layer. The Be-doped (\(8\times 10^{18}\) cm\({}^{-3}\) InAs\({}_{1-x}\)Sb\({}_{x}\) contact layer was applied to the top of the structure. Thanks to this construction both dark current and noise were reduced. The ohmic contact to the structures was performed by etching followed by Au/Ti metallization. The overall structure with the contacts was closed inside a metal TO-8 package. Typically, the resistance area product (\(R_{0}A\)) of commercially available InAs\({}_{x}\)Sb\({}_{1-x}\) diodes varies from about 4 to 60 \(\Omega\) cm\({}^{2}\) (with \(0\leq x\leq 0.36\), T = 300 K) [24]. The tested detector was optimized for the 5 \(\upmu\)m wavelength and mounted on a thermoelectric cooler to make it possible to improve its overall performance by operating at lower temperatures. In our experiments, we used a dedicated PID thermoelectric cooler controller to set up and precisely stabilize the temperature during measurements. Moreover, to dissipate heat from the \"hot\" side of the TEC, it was placed on a large-area aluminum radiator using thermoconductive paste. At zero bias voltage and \(A\) = 0.01 mm\({}^{2}\), the \(R_{0}A\) product of our sample at 300 K was about 7 m\(\Omega\)-cm\({}^{2}\). Further electro-optical details about the detector can be found in [25]. Figure 6 shows typical current-voltage characteristics of the investigated devices at different temperatures. These were measured using a precision SMU Keithley 236 using forcing voltage. While performing these measurements, the detector was covered to avoid any influence from background radiation coming from the environment. It was shown that for lower temperatures, the dependence of the current on reverse bias was much more visible than at higher ones. This confirms that the dynamic resistance of the investigated IR detector is a function of both the temperature and the bias. For this reason, all tests, including noise measurements, were performed while actively keeping the devices at a given temperature. To ensure proper temperature stabilization using TEC, the noise measurements reported in this paper were performed at 280 K, which is a temperature below, but not too far away from, ambient temperature. At this temperature, despite the cooling (which usually increases resistance), the detector is still characterized by relatively low resistance. The impedance of the device vs. frequency was measured at different bias voltages and different operating temperatures, as well. In all cases, the impedance of the device is essentially coincident with the differential resistance at DC up to frequencies as high as a few tens of kHz. The differential resistance vs. bias for the tested photodiode at an operating temperature of 280 K is reported in Figure 7.  
Figure 5: The architecture of the investigated InAsSb-based barrier IR detector. Reprinted with permission from Ref. [25]. 2023, SPIE. The noise characterization of the photodetector is performed in reverse bias. As can be noticed from Figure 7, in the case of the device to be measured, its differential resistance ranges from a few hundred ohms (for a reverse bias of a few tens of mV) to a maximum of about 20 k\(\Omega\), at about \(-\)250 mV. It can also be noticed that, due to the peculiar shape of the I-V curve, the device resistance decreases at biases below \(-\)250 mV.