Use of Body Contact to Suppress Kink Effect

soiex10.in : Use of Body Contact to Suppress Kink Effect

Requires: Victory Process , Victory Device
Minimum Versions: Victory Process 7.30.4.R, Victory Device 1.14.1.R

This example shows how a body contact on an SOI NMOSFET can suppress a kink in the IdVd curve that is due to the effects of impact ionization. The effect of lattice heating is also shown.

This example demonstrates:

  • Definition of 3D SOI MOSFET structures using Victory Process
  • The use and effect of a body contact in a 3D MOSFET geometry
  • Calculation of lattice-heating effects
  • Simulation of impact ionization effects

The first Victory Process section in this example defines a basic SOI NMOSFET. Although the geometry of this device is essentially two-dimensional, it is here defined and simulated in 3D so that the results can be directly compared with those from a 3D MOSFET with a body contact. Using a tetrahedral mesh, the first MOSFET structure is saved to the file vdex02_0_0.str, from which it is imported into Victory Device.

In Victory Device, the statement ELECTRODE SUBSTRATE ensures that the substrate of this device is recognized as an electrode, which will be held at ground potential. The succeeding CONTACT statement assigns to the gate the workfunction of n-type polysilicon. The MODELS statement is used to specify the particular models to be used during the simulation. In this example, they include:

  • CVT: A general purpose mobility model including concentration, temperature, parallel field and transverse field dependence.
  • SRH: Shockley-Read-Hall recombination.
  • AUGER: Auger recombination.
  • BGN: Bandgap narrowing in the presence of heavy doping.

In addition to these models, the IMPACT SELB statement specifies that the Selberherr model for impact ionization will be used.

Now some special steps must be taken, because the MOSFET used in this example is not especially stable. That makes it difficult to obtain a consistently steady numerical solution. To facilitate this, we adjust some solver parameters by means of METHOD statements.

Once the contacts, models, and methods have been specified, the simulation begins with a SOLVE INIT to initialize the solution at zero bias. Next a small initial bias is put upon the drain, and then the gate is brought up to its operating bias of 3 V. The bias on the drain is then ramped from 0.2 V up to 4 V, and the IdVd curve saved to the logfile vdex02_0_2.log.

After this, the entire simulation is repeated for this structure, but this time considering lattice heating effects. The CONTACT and MATERIAL statements assign to the gate contact the workfunction, heat capacity, and thermal conductivity of n-type polysilicon. The THERMCONTACT statement here defines a thermal contact at the substrate, and specifies that it will be maintained at 300 K.

Once the gate has been biased, lattice temperature simulation is activated by the statement MODELS LAT.TEMP . The drain is then biased as before, and the IdVd curve is saved to the logfile vdex02_0_4.log.

The second Victory Process section defines a MOSFET with an additional body contact, beside the gate. The difference between this structure and the first one can be seen by comparing vdex02_1_0.str and vdex02_0_0.str in TonyPlot3D .

As with the first structure, simulations are run with and without lattice heating effects. With the addition of the body contact (at 0 V), however, the device is much more stable. The IdVd curves are written to the log files vdex02_1_2.log and vdex02_1_4.log, and all four IdVd curves generated in this example are compared in TonyPlot . This comparison clearly shows that the addition of the body contact suppresses the kink in the curves, which is due to the effects of impact ionization.

In TonyPlot3D , you can examine the structure files vdex02_0_1.str and vdex02_1_1.str to compare the impact ionization patterns for the two devices, and the structure files vdex02_0_2.str and vdex02_1_2.str to compare the heat fluxes as well as other lattice heating effects.

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