## 3 The CBIRD Device Architecture  
The complementary barrier infrared detector (CBIRD) is a non-equilibrium device [41; 42; 43], where, under reverse bias, the minority density is reduced to below equilibrium values through exclusion and extraction [44]. The CBIRD structure consists of an absorber surrounded on the two sides by a pair of complementary unipolar electron and hole barriers, and capped at the two ends with top and bottom contact layers. The CBIRD absorber can be \(n\)-type (n-CBIRD), \(p\)-type (p-CBIRD), or even a combination of \(p\)-type and \(n\)-type (pn-CBIRD). The CBIRD was originally implemented for \(p\)-type InAs/GaSb T2 SL absorbers [45; 46], but can be implemented readily for InAs/InAsSb T2 SLS absorbers as well [47; 48]. Figure 4 shows schematic layer diagrams for a set of generic n-CBIRD, p-CBIRD, and pn-CBIRD structures with InAs/InAsSb T2 SLS absorbers. In each case, the etch-exposed absorber surface is degenerate \(n\)-type, but the electron surface leakage path is blocked by the unipolar electron barrier. We note that if the etch-exposed absorber surface is degenerate \(p\)-type, the hole surface leakage path would be similarly blocked by the unipolar hole barrier. The n-CBIRD illustrated in Figure 4a may be thought of as an nBn/XBn with a wide-gap emitter contact for reducing minority carrier injection. It has the same good dark current characteristics as the nBn/XBn, and is resilient with respect to surface issue as discussed in the last section. In the (V)LWIR, the attainable QE for n-CBIRD is limited by the short hole diffusion length. Absorber band gap grading, introduced by gradually varying the absorber superlattice period, can be used to create a built-in quasi-electric field that facilitates hole transport through drift in \(n\)-type absorber; this can provide a modest amount of QE enhancement. A more effective way to enhance QE is to take advantage of the thicker absorber afforded by the much longer electron diffusion length in the p-CBIRD, which is illustrated in Figure 4b. The drawback of the p-CBIRD is that it requires a deep etch through the \(p\)-type absorber and past the \(p\)-\(n\) junction, as depicted in Figure 4b. The exposed sidewalls, which run over the entire length of the \(p\)-type absorber, is a source of surface-related dark currents unless good surface passivation can be implemented. As an alternative to the p-CBIRD, we consider a CBIRD structure that incorporates both \(p\)-type and \(n\)-type absorber sections (pn-CBIRD) [47; 48; 49], as illustrated in Figure 4c. In the pn-CBIRD, a portion of the \(p\)-type absorber is replaced by an \(n\)-type absorber (for which the thickness is limited by the hole diffusion length). The \(n\)-type absorber can be either graded or non-graded. To delineated detector array pixels, we only need to etch past the absorber \(p\)-\(n\) junction, as illustrated in Figure 4c. The shallower mesa etch in the pn-CBIRD reduces fabrication demands and also decreases the exposed surface area of the \(p\)-type absorber. A drawback of the pn-CBIRD compared to the p-CBIRD is that it contains a \(p\)-\(n\) homojunction, which is a source of generation-recombination (G-R) and tunneling dark current. In Section 2, we discussed how the anisotropy in superlattice hole conductivity effective mass could lead to FPA crosstalk and degraded MTF. If this issue is a concern for a pn-CBIRD FPA (e.g., FPA with smaller pixel pitch), we could choose to fully reticulate the pn-CBIRD pixels. This would expose the sidewalls of the \(n\)-type absorber section of the  
Figure 4: Schematic generic device layer diagrams for (**a**) an n-CBIRD, (**b**) a p-CBIRD, and (c) a pn-CBIRD. pn-CBIRD. However, as discussed earlier, the exposed \(n\)-type InAs/InAsSb T2 SLS absorber surface is relatively benign from surface dark current considerations, so this would not be a problem. The same considerations apply to fully reticulated nBn and n-CBIRD pixels. Here, we summarize the device architecture discussions.