Introduction to Semicoductor Device Simulation

7/30/2019 Introduction to Semicoductor Device Simulation

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ELEC3200 Principles of Semiconductor Devices

Experiment 3 Introduction to Semiconductor Device Simulation

AimTo use an industrial numerical device simulator to simulate an MOSFET and to study

the effect of using different mobility models on the simulated electrical characteristics

Introduction

Numerical simulation of the electrical characteristics of semiconductor devices (i.e.

solving the Poisson Equation and the two carrier continuity equations) is an essential

tool for modern device design and integrated circuit engineering. Provided the

necessary device physics are incorporated in the mathematical models for the device,

numerical simulation produces results that are very close to actual measured device

characteristics. Simulation is also used to study the sensitivity of device

characteristics to changes in individual fabrication processes (such as changing dopantconcentrations). This is a very cost effective approach in view of the high cost

involved in carrying out actual fabrication runs to study process changes.

In this experiment, we will use the two-dimensional finite elements device simulator,

ATLAS, from Silvaco to simulate a short-channel n-MOSFET and to study the

differences between using constant inversion layer mobility (the conmob model) and

a more realistic mobility model (the CVT model) in which the mobility is degraded

both by the transverse field (i.e. field perpendicular to the oxide-silicon interface of

the MOSFET) and through velocity saturation at high longitudinal field (i.e. field in

the source-drain direction parallel to the oxide-silicon interface). See pages 3-30 to 3-

58, Atlas Users Manual (available on:http://WWW.itee.uq.edu.au/~elec3200/laboratory/lab3_2002/atlas_v1.pdf)

It should be noted that a two-dimensional device simulator does not account for the

width of the MOSFET. The current calculated is actually current per unit width of the

transistor, ampere per micron width in the case of the output from ATLAS.

Procedure

Simulation

Atlas runs on an UNIX machine in the School. To access this software you need to

use the X.Win32 on the personal computer in laboratory. The following files are

needed for the laboratory session:

n1.in,n2.in, structure.set, idvd.set, idvg.set

These are kept on the All users desktop under ELEC3200/prac3 of the personal

computer or save each file from the above hyperlink and copy to your raid drive.

To start, login on your account in the student domain , create a new folder on the raid

drive, name it Elec3200 and copy the above files into this folder. For a H: raid drive,

this is H:\Elec3200.

Left click on the X-Win32 icon at the bottom of the computer screen. An X-Window

should appear. Login on the XII server moss or lichen on the X-Window.
http://www.itee.uq.edu.au/~elec3200/laboratory/lab3_2002/atlas_v1.pdfhttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/n1.inhttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/n2.inhttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/n2.inhttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/structure.sethttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/idvd.sethttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/idvg.sethttp://www.itee.uq.edu.au/~elec3200/laboratory/lab3_2002/atlas_v1.pdfhttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/n1.inhttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/n2.inhttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/structure.sethttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/idvd.sethttp://var/www/apps/conversion/releases/20121119213725/tmp/scratch_5/idvg.set


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Change to your favourite folder (i.e. cd Elec3200) where the copied files are kept.

Type silvaco to load the Deckbuild V3.11.5.R Application Window.

Right-click File button of Deckbuild V3.11.5.R Window and follow by Open to

open the Open file menu window. Left click to select the input file n1.in from the

folder in which you have stored it previously. To open the selected file left-click on

the Open button.The content of the input file n1.in will now be displayed on the Deckbuild

V3.11.5.R Window. It describes the structure of an n-channel MOSFET and

the following simulations:

Id-Vg for Vgs = 0 to 3 V at Vds = 0.1V

Id-Vd for Vds = 0 to 3 V at Vgs = 1, 2 and 3V

This file is also shown on the Appendix of this experiment sheets. While it is

not possible to completely digest all the details in this file, you should read the

file before the laboratory session to gain an idea of what this input file

describes. Comments are included in the file below to help you. The first half

of the file describes the device structure, doping distributions and griding

needed for finite elements simulation. The second part of the file describes the

Id-Vg and Id-Vd characteristics of the transistor to be calculated. Note the input

file includes the statement:

models temp=300 print srh conmob

This specifies a concentration dependent mobility (which also implies that it is

independent of the electric fields in the device) to be used in the simulation.

To execute this simulation, left click on Run button. The command line that

is currently being executing is displayed in reverse video. The computer output for the

run is displayed on the bottom half of the Deckbuild V3.11.5.R Window.

When the command line tonyplot n1.str set structure.set is executed

(similarly for subsequent tonyplot commands), a graphics window TonyPlotV2.7..7.R will appear showing the MOSFET structure with its net doping contours.

Other displays and zoom selections for the device structure are available via the

selection buttons. For example, it is possible to measure the metallurgical channel

length of the device if the junction contours are displayed. The graphics display

window can be minimised to follow the execution of the rest of the input file.

However do not close this window. Other TonyPlots will be produced as the

simulation proceeds.

When the command line tonyplot idvg_n1.log set idvg.set is executed

another TonyPlot graphics window showing the Id-Vg characteristics plot (for Vds =

0.1 V, as specified in the input file) will be displayed. Verify this is what you expect

to see in a Id-Vg plot. You need the numerical data from this plot to extract thethreshold voltage subsequently. To collect the data, right click the File button on

this window followed by right click on Export button to display the

TonyPlot:Export menu. Then Right click the data box followed by the Displayed

only box. Type in the file name you wish to store the data (XXX, for example. No

file extension needed, it is defaulted to XXX.out) in the dialog box and left click

Export. This should export the data file XXX.out into the folder you are using.

You will subsequently work on the data in this file using Excel. You might want to

minimise this graphics window to un-clutter the computer screen. However do not

close the window.

Next the Id-Vd simulation is executed. Follow the procedure described for Id-Vgto store the data Id-Vd data in a different .out file. Again you will need to use Excel to


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work on this set of data (for comparison with the next set of Id-Vd data obtained with a

different mobility model, see below). This completes the simulations for

concentration dependent mobility.

Now repeat the whole simulation sequence by going back to the

Deckbuild. Window and load the n2.in input file which uses a field dependent

mobility model, the Lombardi(CVT) mobility model, to simulate the devicecharacteristics for the same device structure. Execute this input file as before to

produce the Id-Vg and Id-Vd data files for this particular mobility model as above.

This completes the simulation runs and the collection of simulation data. You should

now have four data files: Id-Vg, Id-Vd for the conmob mobility model and Id-Vg, Id-Vdfor the Lombardi(CVT) mobility model.

Threshold Voltage Extraction

For each of the Id-Vg data, you are required to extract the threshold voltage, VT, using

the following linear extrapolation procedure:

Plot Id versus Vg

Numerically obtain the differentialdVg

dIdto locate maximum slope, m, and the

location (Vgsm, Idm) at which this occurs.

Formulate and plot the straight line with slope m and passing through the point

(Vgsm, Idm). This line is known as the linear extrapolation line.

Obtain the Vg-axis intercept for the linear extrapolation line. This intercept is

known as Vgsi.

The threshold voltage, VT, is then given by:

VT = Vgsi Vds/2,

(note that Vds was set at 0.1V in the Id-Vg simulation).

This extraction should be done using Excel. Notice that there are other information in

the .out files. You should be able to identify the necessary the Id-Vg data. You should

professionally annotate your Id-Vg plots and show the linear extrapolation line and the

threshold voltage. Repeat for the 2nd set of Id-Vg data. Comment on the two values of

threshold voltage.

Comparison of Id-Vd characteristics for the two mobility models used

Load both set of Id-Vd data (from the two different mobility model runs) into a single

Excel spreadsheet. Plot the two Id versus Vds on a single plot. Identify clearly the

constant Vgs lines. Comment on the similarities and differences between the two sets

of data and explain the differences in terms of what you know about the two mobilitymodels.

Report

Your report should include a very brief description of the simulation runs completed,

professionally produced plots and a comparison of the two sets of simulation results.


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Appendix

Input file, n1.in for simulation with conmob mobility model. Comment lines start with

#.

go atlas

# MESH DEFINITION FOR DEVICE STRUCTURE

meshx.m loc=0.0 spacing=0.1x.m loc=0.5 spacing=0.1x.m loc=0.65 spacing=0.025x.m loc=0.75 spacing=0.025x.m loc=0.9 spacing=0.005x.m loc=1.2 spacing=0.1x.m loc=1.5 spacing=0.1

y.m loc=-0.02 spacing=0.005

y.m loc=0.0 spacing=0.001y.m loc=0.1 spacing=0.025y.m loc=0.2 spacing=0.05y.m loc=0.4 spacing=0.05y.m loc=1.0 spacing=0.2y.m loc=2.0 spacing=0.5

# REGIONSregion num=1 y.min=0 siliconregion num=2 y.max=0.0 oxide# ELECTRODESelect num=1 name=gate x.min=0.5 length=0.5 y.min=-0.02 y.max=-0.02elect num=2 name=source left length=0.2 y.min=0.0 y.max=0.0elect num=3 name=drain right length=0.2 y.min=0.0 y.max=0.0elect num=4 name=substrate substrate# DEVICE DOPING USING GAUSSIAN PROFILEdoping uniform p.type conc=2.e16doping gauss p.type conc=1.e17 char=0.1doping gauss n.type conc=1.e20 x.right=0.5 junc=0.2 ratio=0.6doping gauss n.type conc=1.e20 x.left=1.0 junc=0.2 ratio=0.6

save outfile=n1.str

go atlas# IMPORT THE MESHmesh inf=n1.str master.in# MATERIAL CONTACT INTERFACE AND MODELS#USING n+ polysilicon gate material.contact num=1 n.polysiliconinterf qf=5.e10# SPECIFY the conmob (concentration dependent mobility model to beused)models temp=300 print srh conmob

# SOLVING INITIAL SOLUTIONSsolve initsave outf=n1.str# DISPLAY MOSFET STRUCTUREtonyplot n1.str -set structure.set

# USING 2-CARRIERS (ELECTRON AND HOLE) TO SOLVE CONTINUITY EQUATION


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method newton trap carr=2#SIMULATING IDVG CHARACTERISTICSsolve prevsolve vdrain=0.1log outf=idvg_n1.log mastersolve vgate=0 vstep=0.1 name=gate vfinal=3

save outf=idvg_n1.str# DISPLAY IDVG CHARACTERISITCStonyplot idvg_n1.log -set idvg.set

# SIMULATING IDVD CHARACTERISTICS# set gate biases with Vds=0.0 initiallysolve initsolve vgate=1.0 outf=solve_tmp1solve vgate=2.0 outf=solve_tmp2solve vgate=3.0 outf=solve_tmp3

#load in temporary files and ramp Vdsload infile=solve_tmp1

log outf=idvdn1_1.logsolve name=drain vdrain=0 vfinal=3.0 vstep=0.1

load infile=solve_tmp2log outf=idvdn1_2.logsolve name=drain vdrain=0 vfinal=3.0 vstep=0.1load infile=solve_tmp3log outf=idvdn1_3.logsolve name=drain vdrain=0 vfinal=3.0 vstep=0.1

# DISPLAY IDVD CHARACTERISTICStonyplot -overlay -st idvdn1_1.log idvdn1_2.log idvdn1_3.log -set

idvd.set

quit

Introduction to Semicoductor Device Simulation

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