Subject :- VLSI Design

Topic :- Technology Trend

Difficulty - Medium

• SPICE (Simulation Program with Integrated Circuit Emphasis) is a general-purpose circuit simulator which is used very widely both in the microelectronics industry and in educational institutions as an essential computer-aided design (CAD) tool for circuit design.
• SPICE has four built-in MOSFET models: LEVEL 1 (MOS1) is described by a square-law current-voltage characteristic, LEVEL 2 (MOS2) is a detailed analytical MOSFET model, LEVEL 3 (MOS3) is a semi-empirical model, and BSIM is the Berkley Short Channel model for IGFET.
• The level (type) of the MOSFET model to be used in a particular simulation task is declared on the MODEL statement.
• Both MOS2 and MOS3 include second-order effects such as the short-channel threshold voltage, sub-threshold conduction, scattering-limited velocity saturation, and charge-controlled capacitances.

LEVEL 1

• The equivalent circuit structure of the NMOS LEVEL 1 model, which is the default MOSFET model in SPICE, is shown in figure below

  

• The LEVEL 1 model is the simplest current-voltage description of the MOSFET

• The equations used for the LEVEL 1 n-channel MOSFET model in SPICE are as follows.

  

Five electrical parameters completely characterize this model: k’, VT0, λ, γ and |2ФF|

• LEVEL 1 model is not very precise due to gradual channel approximation.
• It is used in the derivation of model equation is too approximate and the number of fitting parameters is too small.
• It is useful for quick and rough estimate of circuit performance without much accuracy.

LEVEL 2

• To obtain a more accurate model for the drain current, it is necessary to eliminate some of the simplifying assumptions made in the original GCA analysis. Specifically, the bulk depletion charge must be calculated by taking into account its dependence on the channel voltage.

• Solving the drain current equation using the voltage-dependent bulk charge term, the following current-voltage characteristics can be obtained:

  

• Here, VFB denotes the flat-band voltage of the MOSFET. The saturation condition is reached when the channel (inversion) charge at the drain end becomes equal to zero. From this definition, the saturation voltage VDSAT can be calculated as

  

• The zero-bias threshold voltage VTO corresponding to the LEVEL 2 model can be calculated as follows:

  

• LEVEL 2 supports additional effects as listed above

• If all model parameters in model 2 are specified by the user then greatest level of complexity will be obtained.
• In such cases it requires large amount of CPU time for calculations.

LEVEL 3

• In this level we are getting the same approximate as level 2 but CPU time is significantly less and number of iterations are also fewer.
• However disadvantage is that it leads to complexity while calculating sum of its model parameters.

LEVEL 4 – BSIM

• This level is analytically simple.
• It is based on small number of parameters which are normally extracted from experimental data.
• One of the most popular models used in micro electronic industry due to its accuracy & efficiency.
• It is widely used to accurately model the electrical behavior of sub-micron MOSFETs that are manufactured with various sub-micron CMOS fabrication processes.