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A top-down design approach in IC industry comprises of three levels which includes: & O5 J/ a2 A+ B0 e1 l* U
IC design (circuit-level), model / device(device-level), IC process technology(fabrication-level).
. `; K& R; L1 @9 _" wOn the circuit-level,
+ k% e/ r* Y/ H% I( N3 ba compact model provides the external terminal electrical characteristics
) B6 p& S& \7 Aresulted from the mathematic expressions of an electronic device.
8 `" o7 [! E- C: A; s' U0 i5 EThe external terminal characteristics (Pin Characteristics) includes terminal voltages, currents or charges,
2 a g" ]- y/ O$ T! _are featured as the input and output ports values./ L0 M% n9 W' p! y( q# K
The unknown ports values of a device are solved by a simulator when performing circuit analysis.: t+ a( _7 ~7 c' Y7 `5 V6 b" H
After the structure and behavior of the individual compact model is specified, the description(structure and behavior) are ( i5 @. ~4 A9 w, V; _1 u0 [
submit to the simulator. The simulator employees KCL and KVL to create a set of nonlinear equations. / ?! M, D) [% B; a- U4 M# o8 r
The nonlinear differential equations are not solved directly, but with approximation and iterative methods. Under certain ' f T3 T/ J. b$ A0 l, V
approximation, the equations are solved with the Newton-Raphson method. The solutions are equilibrium points of nodal analysis." P2 m6 Z/ X) W( N) I
IC design engineers work on a higher abstraction level than the device(transistor) level.) ~. _. X: b" [% a
In other words, transistors are the primitive components in the eye of IC designer.
, [5 V+ Z) P8 `, _" L* f& nA virtual symbol is the representive of a real device(component).& o4 T0 X' }( S4 Y# {9 s) m9 G g
For instance, transistor's compact model is seen as a 4 pins symbol. & Y# X: ?0 }$ a* s7 m
In Advanced Design System(ADS), three design types are allowed: schematic, symbol, and layout.( f- E' C5 L) ?- @
Those designs can all be stored in a small containner names "cell" and a big containner names "library". 5 _# S' P/ }& w- E2 a
IC designer works with the connection of some symbols in a schematic.
* W: y* |1 u# h$ UEach symbol represents an electronic device (component).
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: A& `% G/ O& T. t& }Little 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. ( A1 |' \, L! b6 t: P
On the fabrication-level,
- n* j, K+ z# f( n4 o" oa compact model has the internal description of the device characteristics by means of a set of physics-based expressions with - P1 H% o3 R! I1 d8 }
technology dependent model parameters. The physic-based model parameters values accounts for the actual behavior and properties
+ N2 g* K' A5 q6 G5 bof a device are defined by its process variables such as: geometrical dimensions and doping profiles.
& {2 S3 ~8 c" tThe true parameters values need to be carefully measured by the experimental setup of device characterization.
- c3 E# {- z/ _6 W( p) P x8 k% zAccordingly,
! l! J* h5 R0 r5 ^' Nthe verified compact models are expected to be implemented in simulators.
5 A1 i; R- L3 |4 C5 kThus the modelling accuracy and computational efficiency that a simulator can provide to integrate circuits' analysis
# e2 B# X. d7 l' V" o1 K) R) c/ pis 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. ' k9 v i0 ^9 c& t# l
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