In this thesis, we investigate the device variation issue in the LTPS TFTs digital circuit. A propagation delay is one of the most important performances, so that it is necessary to analyze the variability of the propagation delay. Firstly we aim at the reason causes the shift register circuit to fail with Monte Carlo simulations. By analyzing the variance of transition characteristic with respect to device variation, we determine the equations of propagation delay corresponds to clock operation frequency. At the same time, we derive the equation derivation of Tdelay in detail based on the RPI model of HSPICE.
Next we examine the distributions of threshold voltage and mobility variations from different glasses by adopting statistical analysis method with the histogram, the Q-Q plot and the detrend Q-Q plot. It is observe that the distributions of threshold voltage and mobility are the normal and random distributions, respectively. Thus, the Gaussian function of Vth and random function of mobility parameters in the models will be simulated by SPICE. For discussing the sencitivity of Vth and mobility variations on delay compared with Monte Carlo simulation. We start form classical sensitivity function to derive the sensitivity equations of threshold voltage and mobility variations on Tdelay. The results of the dependences of Vth and mobility variations on average and deviation delay for Monte Carlo simulations are in agreement with the sensitivity equations. Moreover, we also observe the results make it clear that the Vth variations cause the variances of digital circuit are larger than mobility.
For predicting circuit performance, we proposed a simulation skill to save computational time with Monte Carlo simulation. It is founded that the operation frequency of an n-stage shift register can be obtained through simplifying propagation delay from an n-stage one to an 1-stage one. The power dissipation of an n-stage shift register is also estimated in the same way. For the frequencies with enough good cases, the errors for the average and deviation are as low as 3% and 8%, respectively.
From the viewpoints of circuit performance, the variation of device behavior will lead to extra difficulties in prediction. From the scope of statistics, the database of variability can be constructed with different device distance so that one can predict the fluctuation range of device parameter as the device distance is known. In our work, we have classified and quantitatively distinguished macro and micro variation.
This would be helpful for designers in predicting the circuit performance and device reliability. We also, furthermore, have to investigate low voltage digital circuit correspond to low field effect before LTPS TFTs can be widely adopted in flat panel display.
References
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Chapter 2
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Fig. 2-1-1 Probability density function for (a) a Gaussian and (b) a uniform random variable.
Fig. 2-3-1 System Block Diagram
Fig. 2-3-2 Block diagram of Timing Controller
Fig. 2-3-3 Block diagram of Data driver
Fig. 2-3-3 The master-slave D flip-flop (version 1)
Fig. 2-3-4 The original low-power D flip-flop
Q Q
Q Q
Fig. 2-3-5 CMOS implementation of the D-latch (version 2)
Fig. 2-3-6 Time diagram of shift register
Fig. 2-4-1 Output simulation waveforms of supply voltage 5V, at 15MHz
Fig. 2-4-2 Output simulation waveforms of supply voltage 5V, at 19MHz
Fig. 2-4-3 Monte Carlo simulation results of n-stage shift register, at 15MHz
Fig. 2-4-4 Monte Carlo simulation results of n+1-stage shift register, at 15MHz
Fig. 2-4-5 Tdealy consistsof TC2
MOS and TInv
Fig. 3-1-1 The mean value and deviation of threshold voltage of different site.
Fig. 3-1-2 The mean value and deviation of mobility of different sites
Fig. 3-1-3 The relative position of the eight sites
Fig. 3-1-4 The histogram of Vth of horizontal crosstie devices.
Fig. 3-1-5 The Q-Q plot of Vth of horizontal crosstie devices.
Fig. 3-1-6 The detrended Q-Q plot of Vth of horizontal crosstie devices.
Fig. 3-1-7 The histogram of mobility of horizontal crosstie devices.
Fig. 3-1-8 The Q-Q plot of mobility of horizontal crosstie devices.
Fig. 3-1-9 The detrended Q-Q plot of mobility of horizontal crosstie devices.