SPICE Models for Thin-Film Transistor Devices

The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) serves as the foundation for most thin-film transistor technologies used in lcd panel applications. MOSFETs operate by controlling the flow of current between the source and drain terminals through an electric field generated by the gate electrode, separated from the channel by a thin insulating oxide layer.

The core SPICE models for MOSFETs, such as the Level 1 to Level 49 models in SPICE and BSIM (Berkeley Short-Channel IGFET Model) series, provide increasingly accurate representations of device behavior. These models account for various physical phenomena including channel length modulation, mobility degradation, velocity saturation, and leakage currents.

For lcd panel applications, MOSFET models must accurately predict threshold voltage, on-current, off-current, and switching characteristics under different operating conditions. The drain current in saturation region is typically modeled as:

I_D = (μ_nC_ox/2)(W/L)(V_GS - V_T)²(1 + λV_DS)

where μ_n is the electron mobility, C_ox is the oxide capacitance per unit area, W/L is the width-to-length ratio, V_GS is the gate-source voltage, V_T is the threshold voltage, and λ is the channel length modulation parameter.

Modern MOSFET models for lcd panel integration include advanced effects such as short-channel effects, hot-carrier degradation, and temperature dependence, ensuring accurate simulation of circuit behavior across the entire operating range of display devices.

Low-Temperature Poly-Silicon (LTPS) TFTs represent a significant advancement in lcd panel technology, offering higher electron mobility and improved performance compared to a-Si:H TFTs. Fabricated using laser annealing processes that convert amorphous silicon into polycrystalline silicon at temperatures compatible with glass substrates, LTPS TFTs enable higher resolution and more compact pixel designs.

The SPICE models for LTPS TFTs must account for the polycrystalline nature of the channel material, including grain boundaries, trap states at grain interfaces, and variations in crystallinity. These factors contribute to phenomena such as threshold voltage variability, kink effects, and anomalous leakage currents.

In lcd panel applications, LTPS TFT models require accurate representation of both n-channel and p-channel devices, as LTPS technology enables complementary circuits that reduce power consumption and improve noise immunity. The drain current model typically includes terms for:

• Grain boundary scattering effects on mobility
• Trap-limited conduction in the channel
• Band-to-band tunneling in the drain junction
• Self-heating effects in small-pixel applications

The higher mobility of LTPS TFTs (100-300 cm²/Vs) enables faster switching speeds, making them ideal for high-refresh-rate lcd panel applications and integration of driver circuits directly on the display substrate, reducing bezel size and system cost.

Advanced LTPS models incorporate bias temperature instability (BTI) and hot carrier injection (HCI) effects to predict long-term reliability, essential for automotive and industrial lcd panel applications requiring extended operational lifetimes.

Stress and aging effects in thin-film transistors significantly impact the long-term performance and reliability of lcd panel devices. These effects manifest as shifts in key device parameters, particularly threshold voltage (V_T), which can lead to image sticking, brightness non-uniformity, and ultimately display failure if not properly accounted for in device models.

The primary stress mechanisms affecting TFTs in lcd panel applications include:

• Bias Temperature Stress (BTS): Threshold voltage shifts caused by prolonged application of gate voltage at elevated temperatures
• Light Illumination Stress: Photo-induced changes in device characteristics, particularly significant in a-Si:H TFTs
• Humidity Stress: Moisture-induced degradation of device performance, especially at the interfaces
• Mechanical Stress: Effects from thermal expansion mismatch and bending in flexible displays

The time dependence of threshold voltage shift under constant stress is often modeled using a stretched exponential function:

ΔV_T(t) = ΔV_T0 [1 - exp(-(t/τ)^β)]

where ΔV_T0 is the saturated threshold voltage shift, τ is the characteristic time constant, and β is the stretching exponent (0 < β < 1).

For lcd panel reliability modeling, these stress effects must be incorporated into SPICE models to predict long-term performance. Advanced models include temperature acceleration factors, enabling extrapolation of device lifetime from accelerated stress tests.

Understanding and modeling stress-induced degradation is critical for developing lcd panel technologies with extended lifetimes, particularly for automotive, avionic, and industrial applications where displays must operate reliably for 10+ years under harsh conditions. Compensation circuits based on these models can actively adjust for threshold voltage shifts, maintaining consistent display performance over the product lifetime.