**TCAD TFT AMLCD Pixel Simulation**

**1. Introduction**

The main drawbacks of circuit level simulation are the many
assumptions made of the device model. For example the a-Si:H TFT model assumes
that the channel
is uniform and ignores interface trap effects. For more accurate circuit
level simulation, a device numerical modeling approach is attractive and
predictive. Silvaco’s ** ATLAS/MixedMode** module enables users to predict
device performance and also the circuit level behavior of transient switching
characteristics in AMLCD pixel simulation. Figure 1 shows conventional equivalent
circuit diagram of the unit pixel.

Figure 1. This figures shows AMLCD unit pixel of TFT AMLCD unit pixel. |

**2. Liquid Crystal Capacitance Model**

In order to simulate transient behavior
of the unit pixel in ** MixedMode**, a time and voltage dependent liquid crystal
capacitance model is to be used.

The
total
amount of LC capacitance(CLC) is calculated from above _{ps} and the geometry
of the LC cell as follows:

here, L and W are total area of the LC cell which is connected to each TFT and D is the thickness of the LC cell(cell gap).

The parameters used in the simulation are listed in Table 1.

Table 1. LC parameters in MixedMode simulation. |

**3. MixedMode Circuit Description**

In order to simulate liquid crystal capacitance
with ** MixedMode**, a user-defined two terminal function with C-Interpreter is
necessary.

Bxxx n+ n- infile=”filename”

function=”function_name”

Bxxx is a user-defined name and infile=”filename” is the source file which includes the function name.

An example C-Interpreter source file is listed below.

#include <math.h>

#include <stdio.h>

double my_lc_rc(double v, double temp, double ktq, double time, double *curr, double *didv, double *cap, double *charge)

{

double eps,e0;

double epl,clc;

double theta,gamma;

double Dtime;

double vc;

double L,W,D;

L=152;

W=148;

Dtime=100e-3;

theta=51.0; /* sec */

gamma=51.2e-3; /* sec */

epl=3.1;

vc=1.887;

D=10.02;

e0 = 8.854e-12;

if(v > vc)

eps = epl + theta*gamma*exp(Dtime)*sqrt(v/vc - 1.0);

else if( v <= vc)

eps =epl;

clc= e0*eps*L*W*1e-6/D; /* F */

*curr=v/10e9;

*didv=1/10e9;

*cap=clc;

*charge=*cap*v;

printf(“clc = %e(F)\n”, clc);

printf(“charge = %e\n”, *charge);

return(0);

}

In the calculation above, a user-defined two terminal current is defined by the following formula:

I=F1(V,t) + F2(V,t)*dF/dV

The 1st term is the DC current and the 2nd term is the capacitive current.

MixedMode performs capacitance and total charge calculation based on the user-defined C-Interpreter function. A typical voltage driven response of unit pixel is shown Figure 2.

Figure 2. A typical TFT AMLCD unit pixel voltage driven response. |

Before transient simulation in MixedMode, the DC characteristics of the a-Si:H TFT is simulated to reproduce the experimental transfer curve and output curve.

Interface traps are specified for the bulk and front and back channel using continuous DEFECT and INTDEFECT statements. Interface fixed charge is also included.

In a TFT-LCD pixel simulation, the following a-Si:H TFT model and circuit behavior should be considered:

- the charging state which is driven by the on-current of an a-Si:H TFT
- the holding state which is affected by the off-current of an a-Si:H TFT
- the voltage drop characteristics of an a-Si:H TFT and LC capacitance

The ** MixedMode** circuit description input deck is
listed below:

.begin

vcom 6 0 5

vg 1 0 0 pulse 0 20 0 1e-6 1e-6 40us 180us

vd 3 0 0 pulse 0 10 0 1e-6 1e-6 160us 320us

atft 2=source 1=gate 3=drain infile=a-Si-TFT.str width=41

re 2 4 1.28k

co 4 5 317f

rlc 5 6 10g

cst 2 6 1.06p

#clc 4 0 125f

bLC 5 6 infile=lc_cap.lib function=my_lc_rc

.numeric vchange=0.5 dtmin=1e-9 imaxtr=50

.options print

.load infile=tft_dc

.log outfile=tft

.tran 0.1us 320us

.end

In Figure 3, the AMLCD pixel dynamics are correctly reproduced, accordingly the source voltage shape shows pixel charging, holding, and voltage drop.

Figure 3. TFT AMLCD pixel voltage. |

**5.
Conclusion**

* ATLAS/TFT/MixedMode* is a useful tool for TFT AMLCD
unit pixel simulation and predicts transient pixel characteristics with trap
density of a-Si:H TFT
and
liquid crystal modeling through a user-defined two terminal device.

TCAD approach to pixel design and combined device level capacitance characteristics is necessary for both circuit and device performance.

**References**:

- “Dynamic Characterization of a-Si TFT-LCD pixels”, Hitoshi Akoi,ULSI Research Laboratory, HP Labs, Hewlett-Packard Company, 3500 Deer Creek Rd., Palo Alto, CA 94304
- “
**ATLAS***User’s Manual”*, Silvaco, Santa Clara, California, USA.