# Trends in Performance Limits of the HOT Infrared Photodetectors
## 4 Interband Quantum Cascade Infrared Photodetectors (IB QCIPs)  
A low diffusion length, weak absorption and finally low dynamic resistance limit the performance of conventional p-n LWIR HgCdTe HOT detectors with doping concentrations in absorbers \(>10^{16}\) cm\({}^{-3}\). The QE is limited since the absorption depth of LWIR (\(\lambda>5\upmu\)m) is much longer than the diffusion length allowing charge carriers photogenerated at distance shorter than the diffusion length to be collected by the contacts. For example, estimates  
Figure 7: MWIR P-i-N HgCdTe photodiode performance with \(\tau_{\text{SRH}}\) = 1 ms and absorber doping \(5\times 10^{13}\) cm\({}^{-3}\): **(a)** diffusion and depletion current components versus temperature, **(b)** detectivity versus temperature. The thickness of active region: t = 5 \(\upmu\)m and consists of t\({}_{\text{diff}}\) = 2 \(\upmu\)m and t\({}_{\text{dep}}\) = 3 \(\upmu\)m. The experimental data is taken from different sources as marked. PV— Photodiode, CQD—Colloidal quantum dot, IB QCIP—Interband quantum cascade infrared photodetector. Experimental data is taken from [30; 33; 34]. show that for 10.6-\(\upmu\)m detector the absorption depth is \(\sim\)12 \(\upmu\)m while the ambipolar diffusion length is less than 2 \(\upmu\)m. In consequence, the QE is reduced to \(\sim\)15% [9]. To overcome above problems, the multiple heterojunction devices based on thin elements connected in series were proposed, where a proper example is a detector with junctions perpendicular to the substrate, introduced in 1995. The multi-heterojunction device shown in Figure 8a contains backside illuminated n+-p-P detectors connected in series. The advantages of such design are a high voltage responsivity, a short response time while on the other hand the response depends on polarization of incident radiation and is nonuniform across the active area. More promising design is the stacked tunnel junctions connected in series as shown in Figure 8b being similar to multi-junction solar cells. Potentially, this device can reach both good QE, high differential resistance, and fast response. As presented, each cell consists of lightly p-type doped absorber and N+/P+ wide-bandgap highly doped contact layers. The heterojunction contacts collect the photogenerated carriers absorbed in every active layer. However, practical problem is related to the resistance of the adjacent N+ and P+ layers. In the last decade, several designs of the multi-stage IR devices have been developed. They are based on III-V semiconductors and might be now divided into two classes: mentioned earlier interband (IB) ambipolar QCIPs and intersubband (IS) unipolar QCIPs. The first study on IS QCIPs began about two decades ago [11] as the photodetectors were developed from the quantum cascade lasers (QCLs). However, currently IB QCIPs show the higher performance in comparison with IS QCIPs due to the relativity much longer carrier lifetime. The IB QCIPs saturation current density is reported to be almost two orders of magnitude lower than for IS QCIPs [30]. Schematic illustration of IB QCIP is presented in Figure 9 where every single active layer is sandwiched between the relaxation and tunneling layers forming a cascade stage. The thickness of the single active layer should be thinner than the diffusion length to effectively collect all photogenerated carriers. The diffusion length restriction in traditional thick absorber detectors is bypassed by using the discrete absorber design imposing recombination of the photogenerated carriers in the next stage within short transport distance. The single thin absorbers are connected in series and the total thickness of the active layer can be even thicker than the absorption depth. The photocurrent is determined by carriers generated in the single absorber (one stage) and is independent of the number of stages meaning that the photons absorbed in following stages do not increase the net photocurrent but only provide the current continuity through the device.