8.31 Sspd_chapter 6_part 10_simulation of mosfet  (Page 2/5)

# Ramp the gate

log outf=mos1ex01_1.log master

solve vgate=0 vstep=0.25 vfinal=3.0 name=gate

save outf=mos1ex01_1.str

# plot results

tonyplot mos1ex01_1.log -set mos1ex01_1_log.set

# extract device parameters

extract name="nvt" (xintercept(maxslope(curve(abs(v."gate"),abs(i."drain")))) \

- abs(ave(v."drain"))/2.0)

extract name="nbeta" slope(maxslope(curve(abs(v."gate"),abs(i."drain")))) \

* (1.0/abs(ave(v."drain")))

extract name="ntheta" ((max(abs(v."drain")) * $"nbeta")/max(abs(i."drain"))) \

- (1.0 / (max(abs(v."gate")) - ($"nvt")))

Quit

We have to create the initial structure file.

By Mesh Define Menu, initial rectangular grid is specified. By selecting the VIEW……. Button, we can click the View Grid Window. We can adjust the grid lines according to our requirement. In critical areas we want fine mesh and in passive, non-critical area we require coarse mesh. Accordinfgly we set the grid line and write it into the INPUT file. It comes as shown below:

#

line x loc=0.0 spac=0.1

line x loc=0.2 spac=0.006

line x loc=0.4 spac=0.006

line x loc=0.6 spac=0.01

#

line y loc=0.0 spac=0.002

line y loc=0.2 spac=0.005

line y loc=0.5 spac=0.05

line y loc=0.8 spac=0.15

#

These lines describe the GRID or MESH

Now we define the Initial Substrate. By Mesh Initialize…….Mesh Initialize Menu appears.

Here we have three options for ‘orientation’. For BJT we will select<111>but for MOS structures we will select<100>. This helps minimize the interface impurities and oxide trapped impurities. This in turn helps ensure a stable device.

Background doping is selected by clicking Phosphorous Box. Its concentration is set at 1 by the slider and 14 is written into exponent box.

Dimensionality is chosen at AUTO. This will start in 1-D and automatically switch to 2-D.

Grid Spacing scaling factor is chosen at 2.

By WRITE button the information is written into the input file. It is as shown below.

init orientation=100 c.phos=1e14 space.mul=2

Using these two informations (Grid Information and Grid Initialization) if we RUN Athena we get the INITIAL STRUCTURE as Initial Triangular Grid.

Figure 1. Initial Triangular Grid.

This triangular grid is not drawn according to the specifications given for this example.

The Grid consists of points connected to form a number of triangles. The

physical device is 3D structure but we are showing 2D Grid structure. Hence a point in 2D mesh may correspond to different regions in 3 ^rd dimension. It may belong to Silicon at z = 0µm and to Oxide region at z = 2µm. Hence the same point will have two nodes. A node represents the solution of the coupled equations in the region to which node belongs. Hence in this case a point will have two nodes: one node in Silicon region and another node in Oxide region.

The INIT statement creates<100>Silicon region of 1µm×1µm size which has an uniform N-type doping of Phosphorous of doping concentration of 1×10 ¹⁴ Phos atoms per cc. This simulation structure is ready for any process step e.g. implant, diffusion, reactive ion-etching.

Now structure manipulation statements will follow. These will precede or alternate with PHYSICAL PROCESS STEPS which will be carried out by SUPREME4, ELITE or OPTOLITH modules.