Spitzer Documentation & Tools
IRS Instrument Handbook

2.8  Detectors

Each of the four IRS optical modules has a single 128x128 blocked impurity band focal plane array detector.  Two (SL and SH) are arsenic-doped silicon (Si:As) arrays, and two (LL and LH) are antimony-doped silicon (Si:Sb) arrays.  The Si:As arrays operate over a 5-26 micron wavelength window, while the Si:Sb arrays operate over a 14-40 micron window.

2.8.1             Bias and Detective Quantum Efficiency in LL and LH

The Detective Quantum Efficiency (DQE) is a function of wavelength () and bias voltage.  In Si:Sb arrays (LL and LH), an increase in bias voltage leads to a dramatic change in DQE at longer wavelengths (>36 microns); see Figure 2.10.  However, it also exacerbates the dark current associated with radiation-damaged pixels.  A performance optimization metric, developed from charged particle test data and infrared responsivity vs. wavelength measurements on Si:Sb detectors, was used to calculate the optimum bias voltage for a given cosmic ray dose.  

Figure 2.10:  DQE for the Si:Sb arrays as a function of wavelength for different bias settings (1-2.5 V).  The DQE declines linearly from 30 microns to 40 microns (reaching zero at approximately 40 microns) for the optimal bias voltage (1.6 V) that provides the best performance over the 14-36 micron interval.