8.32 Sspd_chapter 6_part 10_simulation of mosfet_conclusion

SSPD_Chapter 6_Part 10_MOSFET Simulation2

In MOSFET Simulation1 we used Process Simulator(Athena) to generate the basic (E)NMOS device in P-Well formed in lightly doped N-body. Now we will use ATLAS for device testing and parameterization

go atlas

# set material models

models cvt srh print

MODEL MOS PRINT

The above statement invokes all the relevant models in context of parameterization of MOS device namely:

CVT , SRH, FERMIDIRAC.

SRH is Shockley-Hall-Reed model deals with indirect recombination through traps and defects.

FERMIDIRAC is fermidirac statistics

CVT is a stand-alone model which accounts for the 2D nature of the inversion layer.

Mobility degradation in 2D inversion layer, which forms the channel, is handled by three different methods:

1. SURFMOB- mobility degradation due to interface scattering (the interface between silicon substrate and gate oxide);
2. SHIRAHATA – transverse electric field model. A transverse electric field exists along the channel;
3. CVT, YAMAGUCHI, TASCH- all these three are standalone model for accounting the mobility degradation in the inversion layer. CVT accounts for transverse electric field effect, doping dependence and temperature dependence.

contact name=gate n.poly

interface qf=3e10

In the above two statements we are naming the Gate Contact and giving the interface statements. The interface statements enable define interface fixed charges, surface recombination velocity and thermionic emission. Here we are enabling a fixed charge of 3×10 ¹⁰ at the interface of Si and Oxide.

method newton

solve init

This states that we re using coupled Newton Method of numeric calculations. Newton Method is used where we have strongly coupled system with quadratic convergence. But this requires a more accurate initial guess to the problem to obtain convergence.

# Bias the drain

solve vdrain=0.1

We are solving for drain current with drain voltage at 0.1V and gate voltage at 0V

# 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

In the above commands we are solving the drain current for Gate Voltage being given a Ramp Voltage starting from 0V and terminating at 3V incrementing in 0.25V steps.

The result is saved as a string.

# plot results

tonyplot mos1ex01_1.log -set mos1ex01_1_log.set

Now we plot the transfer characteristics at a constant drain voltage of 0.1V.

# 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

The above statements extract the Threshold Voltage of MOS Transistor, Beta and Theta.

Threshold voltage is extracted by calculating the maximum slope of Id/Vgs characteristics, extending the tangent drawn at the maximum slope point to make an intercept on Vgs axis. The X-intercept minus the half of drain voltage is Threshold Voltage.

In this extract statement, nbeta is the intercept on X-axis.

Similarly ntheta is the final value of Threshold Voltage.

Figure 16. Plot of I D -V GS at constant V DS and extraction of V Th .

The Tony Plot of the transfer characteristics of the simulated device is given in Figure 16.The whole program has been deciphered and explained in the simplest terms. Following the same set of statements similar simulations can be carried out.