Threshold Voltage Shift Calculation

sonosex02.in : Threshold Voltage Shift Calculation

Requires: S-Pisces
Minimum Versions: Atlas 5.22.1.R

The example shows :

  • How to set up a SANOS device (variant of a SONOS device)
  • How to obtain the threshold Voltage of a SANOS device
  • How to obtain data for a range of Biases and time frames

The blocking insulator need not be made of the same material as the tunnelling insulator. In this case we have a blocking insulator made of Sapphire (Al203) and a tunnelling insulator made of Silicon dioxide.

This example uses the DBINTERNAL feature of DeckBuild, and the deck sonosex02_aux.in is run for each value of the Gate Bias specified by the parameter sweep of vgate in the sonosex02.in deck.

Each time sonosex02_aux.in is run, it sets up a SANOS structure, very similar to the structure in sonosex01.in , but with the blocking insulator layer made of Sapphire. It is necessary to make this material into a widegap semiconductor and set other parameters on the MATERIAL statement.

The DYNASONOS model is selected on the first INTERFACE statement. Because of the relatively low conduction band offset between the Silicon Nitride and the sapphire, the INTERFACE S.S THERMIONIC statement is used to model any thermionically emitted current leaving the device. This current will not appear in the SONOS outcurrent, but in the gate conduction current.

In the NITRIDECHARGE statement, we set up the relevant parameters and also specify PF.BARRIER. This sets the barrier height for the Poole-Frenkel detrapping model, which is enabled by the PF.NITRIDE flag on the MODELS statement.

The gate bias is ramped and the device charged for 1 second. A structure file is saved out at various preset charging times. The next part of the deck reads in these structure files in turn, and for each one it ramps the drain bias and also the gate bias in order to calculate the threshold voltage using the deckbuild EXTRACT facility. The gate stack layers are treated as insulators in this case, to make the simulation simpler. The charge trapped in the Nitride layer affects the threshold voltage in the same way.

After calculating the threshold voltage for the set of Gate biases and at each of the preset charging times, the threshold data is plotted. The threshold voltage increases with Gate Bias and also with charging time. For a charging time of 1.0 seconds there is a roughly linear relationship between gate bias and threshold voltage (0.65 V of threshold voltage change for every 1.0 V of bias volatge change). The threshold voltage for the uncharged device is also shown, and threshold volatge shifts can be calculated relative to this.

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.