Space Radiation Effects on Electronics-DDD Effects

In our previous post, Space Radiation Effects on Electronics-TID Effects, we dove deep into how TID affects spacecraft electronics and we provided examples of TID effects. As we mentioned in our last post, cumulative effects can be subcategorized into Total Ionizing Dose (TID) and Displacement Damage Dose (DDD). In this post, we will look into DDD effects on space electronics. 

What is Displacement Damage Dose?

Displacement damage refers to the structural damage imparted on the crystal lattice of the device by highly-energetic particles. This essentially involves the creation of crystal imperfections such as lattice atoms displaced to new defect locations and vacant lattice sites. This effect is detrimental to the device because the electrical properties at the defect’s region get altered by the introduction of new energy states inside the semiconductor’s energy band gap region. The defects can act in different undesired ways for the device function such as charge traps, recombination centres, generation centres of thermal charge etc.

Therefore, in contrast to TID, Displacement Damage Dose (DDD) encompases all the non-ionising dose effects on a device and is also known as Total Non-Ionising Dose (TNID). TNID effects are realised as an increased defect concentration throughout the device bulk as opposed to surface or interface regions in the TID case and are usually independent of the flux and the device biasing conditions. 

A few DDD Effects on electronic devices:

  • Gate-Oxide Breakdown
    In extreme cases, DDD-accumulated defects in the gate oxide bulk may result in a massive short through that insulating layer. This will melt the region locally and effectively destroy the structure.
  • BJT Gain Degradation
    Due to DD-induced recombination centres, the minority charge carriers have a shorter lifetime. This, in effect, implies that an increase in the input bias current is required to produce a specific collector current which amounts to a reduction in gain.
  • Solar Cells
    The main importance of the displacement defects produced by the irradiation of solar cells is in their effect on the minority carrier lifetime in the space charge region of the cell.
    The saturation current due to generation-recombination in the depletion region increases linearly as the carrier lifetime decreases (i.e. displacement damage increases). The increased leakage current of a solar cell reduces the cell Voc and the current available to an external load decreases. Consequently, Pmax will also decrease. Since solar cells are usually operated near the maximum power point, such changes have grave implications on in-flight performance. 
Post 2. Empirical relation between Maximum solar cell power output, normalised to its Beginning-of-Life state, and total proton fluence.
Figure 1. Empirical relation between Maximum solar cell power output, normalised to its Beginning-of-Life state, and total proton fluence.
  • Lasers
    Defects caused by irradiation act as recombination centres inhibiting the growth of stimulated emission. As a result, the threshold current is increased and the lasing wavelength is broadened.
  • Charge Traps and Hot Pixels on Camera Sensors
    Displacement damage has multiple effects on image sensors (CCD, CMOS etc.) Firstly, clusters of defects in the pixel array can act as locations with increased spurious signal (Dark Signal). These pixels appear distinctly bright in images and are called Bright Spots. Secondly, defects can act as traps of photogenerated charge, thus reducing the Charge Transfer Efficiency of the CCD. This is observed as signal streaks in the image. This again decreases the pointing accuracy of Star Trackers as well as the resolution of Earth-Observation detectors.
Example of an image acquired in darkness from a CCD after irradiated with protons. The bright pixel streaks are caused by lattice traps and amount to poor Charge Transfer Efficiency
Figure 2. Example of an image acquired in darkness from a CCD after irradiated with protons. The bright pixel streaks are caused by lattice traps and amount to poor Charge Transfer Efficiency

In our next post, we will explore the Single Event Effects (SEEs) and how they affect space electronics. If you want to learn more about DDD, you can check out some of our sources below and don’t forget to subscribe to our newsletter!

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