# Trends in Performance Limits of the HOT Infrared Photodetectors
## 3 The Ultimate HgCdTe Photodiode Performance
### SRH Carrier Lifetime  
The SRH generation-recombination mechanism determines the carrier lifetimes in both lightly doped n- and p-type HgCdTe in which SRH centres are related to residual impurities and native defects. [PERSON] et al. in 2005 [23] published that the experimental carrier lifetimes for n-type LWIR HgCdTe range from 2 up to 20 \(\upmu\)s at 77 K irrespective of doping \(<\)10\({}^{15}\) cm\({}^{-3}\). The MWIR carrier lifetime are substantially longer assuming 2 up to 60 \(\upmu\)s. However, several papers published in the last decade have shown \(\tau_{\text{SRH}}\) significantly higher in low temperature range and low doping concentrations, above 200 \(\upmu\)s up to even 50 ms versus cut-off wavelength [20] see Table 1. The range of low doping that can be reproducibly obtained in Teledyne Technologies HgCdTe epilayers grown by MBE is \(\sim\)10\({}^{13}\) cm\({}^{-3}\). [PERSON] et al. [24] published that for most tested MWIR photodiodes from CEA Leiti and Lynred by Sofradir & Ulis, the estimated SRH carrier lifetimes [from direct measurements (photoconductive or photoluminescence decay) and indirect estimates from current-voltage (I-V) characteristics], are in the range between 10 and 100 \(\upmu\)s. Those values are lower than the earlier assessed by US research groups [25][PERSON] However, they were estimated for devices with higher doping level in absorber \(>\)10\({}^{14}\) cm\({}^{-3}\). However, from just published announcement results, Teledyne Technologies confirmed fabrication of depletion layer limited P-i-N HgCdTe photodiodes with SRH recombination centers exhibiting carrier lifetimes in the range 0.5-10 ms [25][PERSON]  
All SRH lifetimes estimated for HgCdTe are usually carried out for temperatures below 300 K. Their extrapolation to 300 K to predict the photodiode operation behavior is questionable. In our estimates we assume \(\tau_{\text{SRH}}=1\) ms, which is supported by experimental data reached by Leonardo DRS and Teledyne Technologies research groups.  
Figure 2a shows a schematic P-i-N detector energy band profile for a reverse voltage. The active region consists of an undoped i-layer (often called as \(\upnu\) region exhibiting low n-doping) sandwiched between wider bandgap cap (P) and buffer (N) region (see Figure 2b) [26]. Very low doping in the absorber (below 5 \(\times\) 10\({}^{13}\) cm\({}^{-3}\)) is required to allow full depletion at zero or low reverse voltage. The surrounded wide gap contact layers are designed to reduce the dark current generation from these regions and to prevent tunneling current under reverse bias. Moreover, fully depleted absorber surrounded by wide bandgap regions theoretically reduces 1/f and burst noise. As previously mentioned,  
\begin{table}
\begin{tabular}{c c c c} \hline
**Spectral Range** & \(x\)**Composition** & **SRH Lifetime (\(\upmu\)s)** & **Temperature (k)** \\ \hline SWIR & 0.455 & \(>\)3000 & 180 \\ MWIR & 0.30 & \(>\)1000 & 110 \\ MWIR & 0.30 & \(\sim\)50,000 & 89 \\ LWIR & 0.225 & \(>\)100 & 60 \\ \hline \end{tabular}
\end{table}
Table 1: Summary of the SRH carrier lifetimes determined based on current-voltage characteristics (data after reference [20]).  
the fully depleted P-i-N architecture is compatible with the small pixel size, meeting the requirements of low crosstalk thanks to the built-in electric field [20; 26].  
In P-i-N design, the choice of absorber thickness should be a trade-off between the response time and QE (or responsivity). To reach short response times, the absorber thickness should be thin and fully depleted. For high QE the absorption region should be thick enough to effectively collect photogenerated carriers. However, to enhance QE while maintaining high response time, an external resonant microcavity was demonstrated [8]. In this design, absorber is placed inside a cavity so that more photons can be absorbed even in low detection volume.