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A top-down design approach in IC industry comprises of three levels which includes:
% F# |8 w7 V2 i8 KIC design (circuit-level), model / device(device-level), IC process technology(fabrication-level).! t0 Q' M" f N* u4 N+ Q
On the circuit-level, ' s7 p8 c% a6 o F; o& C
a compact model provides the external terminal electrical characteristics ) [5 J* ~) H: U& u3 _4 |
resulted from the mathematic expressions of an electronic device.$ _* V. W' A7 X* H; ~. U: h+ ?
The external terminal characteristics (Pin Characteristics) includes terminal voltages, currents or charges,
5 L c+ ?6 F0 C+ n. i ^2 w: _2 xare featured as the input and output ports values." H; A$ y% Q) l D5 M9 p. h
The unknown ports values of a device are solved by a simulator when performing circuit analysis.% y% r- n" X) K7 V: y! L
After the structure and behavior of the individual compact model is specified, the description(structure and behavior) are 6 m" ~" `- X$ G
submit to the simulator. The simulator employees KCL and KVL to create a set of nonlinear equations.
# j5 b1 D2 K! FThe nonlinear differential equations are not solved directly, but with approximation and iterative methods. Under certain
/ z e8 _! r/ y2 P- happroximation, the equations are solved with the Newton-Raphson method. The solutions are equilibrium points of nodal analysis.: E4 P/ _2 ]( r
IC design engineers work on a higher abstraction level than the device(transistor) level.0 j' u$ ?& }+ u; G6 i
In other words, transistors are the primitive components in the eye of IC designer.6 N8 j, W% e' x
A virtual symbol is the representive of a real device(component)./ `( H: B2 _( Y
For instance, transistor's compact model is seen as a 4 pins symbol. - l/ k4 f$ R: O
In Advanced Design System(ADS), three design types are allowed: schematic, symbol, and layout.9 {1 X6 r1 l* y' M, c- U' e
Those designs can all be stored in a small containner names "cell" and a big containner names "library". $ I. `6 |& ?; M; t- @
IC designer works with the connection of some symbols in a schematic." D3 w! k3 L# D' }7 z
Each symbol represents an electronic device (component). ! E; G$ ~: p8 h7 ?+ O) r! n
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+ q% A4 |3 L# u. a: X" {. wLittle knowledge of a device's internal structures and behaviours are required for IC designers. Because a device works as a funtional block. In stead, a device's external structures (connection) and behaviours are of concerns. ; X0 U0 e: x- k; I5 E( ?
On the fabrication-level,
/ d' o+ A( D! b/ D3 na compact model has the internal description of the device characteristics by means of a set of physics-based expressions with . x: [, E& l! Y0 v+ W& R1 O% H
technology dependent model parameters. The physic-based model parameters values accounts for the actual behavior and properties 1 C6 A% _* B1 J$ ~
of a device are defined by its process variables such as: geometrical dimensions and doping profiles.7 J. g) v+ m" B+ R7 q3 V9 l) R/ O# [9 E% P
The true parameters values need to be carefully measured by the experimental setup of device characterization.
& e+ u8 {2 b0 S( P& G/ [Accordingly,
8 ~/ v* B6 v; Z9 hthe verified compact models are expected to be implemented in simulators.! B" A) q' @+ W( @0 e4 [
Thus the modelling accuracy and computational efficiency that a simulator can provide to integrate circuits' analysis 9 v0 h+ c" Z& K
is the same as its implemented compact model. Meanwhile, a compact model is the most crucial process design kit, which plays as the interface between circuit designers and device developers. * Z% w9 W1 j0 ~) H
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