# Transimpedance Amplifier for Noise Measurements in Low-Resistance IR Photodetectors
## 4 Results  
We built the amplifier presented in Figure 1 to experimentally verify that sensible direct low-frequency current noise measurements on low-impedance devices are indeed possible. We tested the amplifier with two different values for the feedback resistance \(R_{R}\), i.e., 10 k\(\Omega\) and 100 k\(\Omega\), as shown in Table 1. The test measurements were initially performed using known resistances as DUTs. In particular, we tested the system with both a 100 \(\Omega\) resistor and a 1 k\(\Omega\) resistor as the DUT. The 1 k\(\Omega\) resistor was taken as representative of the typical impedance we expect with actual devices in noise measurements. The test using a 100 \(\Omega\) resistor as a DUT was performed in order to more clearly evidence the noise contribution (background noise) introduced by the amplifier, since, as was shown in the previous section, its relative weight increases with decreasing DUT impedance. As can be observed in Figure 4, when testing a 100 \(\Omega\) resistance as a DUT, with \(R_{R}=10\) k\(\Omega\), the background noise due to the amplifier had a noticeable effect on the measurement results, although it can be observed that even in this configuration, it should be possible to perform sensible flicker noise measurements within the frequency range in which the flicker noise is much greater than the thermal noise of the device. It can also be noticed that, as can be deduced from Equation (18), the background noise in this case is mostly due to the first term in \(Q_{n1}\), since \(R_{D}/R_{R}=10^{-2}\). In other words, the background noise is set by the input JFETs, and increasing the value of the feedback resistance has a negligible effect on the background noise. When testing a 1 k\(\Omega\) resistance as the DUT, from Figure 4 it can be observed that the measured value of the equivalent input current noise was very close to the noise generated by the DUT, except for at very low frequencies, where the deviation is, however, very small. A careful examination of the two curves relative to \(R_{R}=10\) k\(\Omega\) (blue curve) and \(R_{R}=100\) k\(\Omega\) (red curve) indicates, as should be expected, that employing a higher feedback resistance is beneficial as far as the BN is concerned. On the other hand, no significant difference can be expected when even a moderate level of flicker noise is present, and therefore, in general terms, and unless the DUT impedance is considerably below 1 k\(\Omega\), there is no significant advantage to employing feedback resistances above 10 k\(\Omega\), as this will result in a limitation of the bias level that can be applied to the DUT. It can be observed from Figure 4 that the measured noise increases above 10 kHz. This can be explained by the fact that, because of the compensation network, the gain in the first stage decreases as the frequency increases, and the equivalent input noise increases because the relative weights of the noise introduced by \(R_{D1}\), \(R_{D2}\), and \(OA_{1}\) increase [14]. Following the preliminary tests on resistances, we performed noise measurements on advanced photodetectors to demonstrate the ability of the new amplifier to directly perform current noise measurements on low-impedance DUTs. The photodetector used in this investigation was an InAsSb (\(p^{+}\)B\({}_{p}\)\(\mu\)\(n^{+}\))-based barrier backside illuminated device. It was grown on a GaAs substrate with GaAs and InAs Si-doped layers using molecular beam epitaxy. The architecture details of the investigated structure are shown in Figure 5. It consisted of four main layers, which were additionally supplemented by gradient layers. The absorber, which is the main layer on which the radiation is absorbed, was non-intentionally doped (n.i.d.) with n-type conductivity. The bandgap barrier for electrons was made using AlAsSb. The n\({}^{+}\) contact placed at the bottom was made of a highly Si-doped InAs\({}_{1-x}\)Sb\({}_{x}\) layer.