Total Dose X-Ray Effects in an n-MOS Gate Oxide : Total Dose X-Ray Effects in an n-MOS Gate Oxide

Requires: VictoryDevice/SEE
Minimum Versions: VictoryDevice 1.6.3.R

This examples demonstrates positive charge build up and radiation induced charge de-trapping in an n-MOS gate oxide as a result of 10keV X-Ray irradiation.

A physical effect missing in previous total dose models is radiation induced de-trapping of previously trapped oxide charge, as a result of the trapped charge being released after absorbing energy quanta from the geminate recombination process. Geminate recombination occurs when an electron-hole pair created from absorbtion of radiation, recombine again before an electric field can accelerate the pair in opposite directions. Clearly the probability of geminate recombination increases with reducing electric field. Thus the rate of hole de-trapping in the oxide also increases with a reduction in electric field, as discussed in the reference for this model from Kimpton et al., "A Simple Trap-Detrap Model for Accurate Prediction of Radiation Induced Threshold Voltage Shifts in Radiation Tolerant Oxides, for All Static or Time Variant Oxide Fields" Solid-State Electronics, Vol. 37, No. 1, pp.153-158, 1994

To demonstrate both the radiation induced hole trapping and de-trapping phenomenon, a positive bias is applied to the gate of an n-MOSFET and is irradiated with 10keV X-Rays. Positive oxide charge builds up during irradiaton as is monitored using the probe material=oxide p.conc statement and by comparing the pre and post irradiation threshold voltage. After irradiation to 1MRad, the gate bias is changed to -0.9 volts to minimise the gate oxide field. The device is then re-irradiated with another 1MRad dose and the trapped hole concentration in the gate oxide is monitored.

The greatly reduced oxide field resulting from the change in gate bias increases the geminate recombination process which also greatly increases the rate of hole de-trapping in the oxide, such that it now becomes even greater than the hole trapping rate. It can be seen from this example that rather than increasing the trapped hole concentration with radiation dose, now all the additional energy released from the increased rate of gemnate recombination, now decreases the trapped hole concentration with radiation dose. It should be emphasized here that the hole de-trapping is NOT a result of hole recombination with electrons, but is a direct result of trapped holes absorbing energy quanta from the geminate recombination process.

Finaly, after the second irradiation with low electric field on the gate, the threshold voltage is simulated again, and shows a recovery from the shift observed after the first irtradiation with a high gate oxide field.

To simulate total dose effects in insulators, the oxides are converted to large bandgap semiconductors with the material material=oxide semiconductor statement. Then the concentrations and characteristics of the bulk oxide and interface traps are defined in accordance with the referenced paper above, using the oxidecharging and intoxidecharging statements respectively. Finally the characteristics of the radiation are defined using the radiation doserate=1 XRay statement. Then on any transient solve statement the parameter "radiation" is specified to invoke the time dependent irradiation process.

To load and run this example, select the Load example button in DeckBuild. This will copy the input file and any support files to your current working directory. Select the run button to execute the example.