Device models

Explain the following: Device models and device characteristics.

1. SPICE (Simulation Program with Integrated Circuit Emphasis) provides a wide variety of MOS transistor models with various trade-offs between complexity and accuracy.
2. Level 1 and Level 3 models were important, but they are no longer adequate to accurately model very small modern transistors.
3. BSIM models are more accurate and are presently the most widely used.

i. Level 1 model:

• The SPICE Level 1, or Shichman-Hodges Model is closely related to the Shockley model, enhanced with channel length modulation and the body effect.
• The basic current model is:

•  The parameters from the SPICE model are given in ALL CAPS.
•  β is written instead as KP (Weff /Leff ), where KP is a model parameter. Weff and Leff are the effective width and length.
•  The LAMBDA term (LAMBDA =1/VA) models channel length modulation.
•  The threshold voltage is modulated by the source-to-body voltage Vsb through the body effect.
•  For non negative Vsb , the threshold voltage, Vt is

Where, VTO is the “zero-bias” threshold voltage Vt0,

GAMMA is the body effect coefficient , and PHI is the surface potential.

•  Level 1 model is easy to correlate with hand analysis, but it is too simplistic for modern design.

ii. Level 2 and 3 models

•  The SPICE Level 2 and 3 models add effects of velocity saturation, mobility degradation, subthreshold conduction and drain-induced barrier lowering.
•  The Level 2 model is based on the Grove-Frohman equations.
•  Level 3 model is based on empirical equations that provide similar accuracy, faster simulation times and better convergence.
•  These models are not efficient models for measuring I-V characteristics of modern transistors.

iii. BSIM models

•  The Berkeley Short-Channel IGFET Model (BSIM) is a very elaborate model that is now widely used in circuit simulation.
•  The models are derived from the fundamental device physics but uses many number of parameters to fit the behavior of modern transistors.
•  BSIM versions 1, 2, 3v3, and 4 are implemented as SPICE levels 13, 39, 49, and 54, respectively.
•  BSIM is quite good for digital circuit simulation. 

Features of the model are

•  Continuous and differentiable I-V characteristics across subthreshold, linear and saturation regions for good convergence.
•  Sensitivity of parameters such as Vt to transistor length and width.
•  Detailed threshold voltage model including body effect and drain-induced barrier Lowering (DIBL).
•  Velocity saturation, mobility degradation and other short-channel effects.
•  Multiple gate capacitance models.
•  Diffusion capacitance and resistance models.
•  Gate leakage models.