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A top-down design approach in IC industry comprises of three levels which includes: ; ?- Q2 u$ n# h$ T: O; {4 B" t
IC design (circuit-level), model / device(device-level), IC process technology(fabrication-level).
, m, x3 n, F0 W/ N' l0 KOn the circuit-level, ; M( W$ P$ a o0 U% ?, l. V. N
a compact model provides the external terminal electrical characteristics 6 A( f+ n( [3 o% A: i: _
resulted from the mathematic expressions of an electronic device.
/ r5 z6 S0 N3 v1 X! ^The external terminal characteristics (Pin Characteristics) includes terminal voltages, currents or charges,
* G- Q$ F0 W/ ]1 Y8 k/ iare featured as the input and output ports values.6 y- _8 R# N& L
The unknown ports values of a device are solved by a simulator when performing circuit analysis.+ y5 I1 y7 G: m3 q
After the structure and behavior of the individual compact model is specified, the description(structure and behavior) are 8 O9 E3 O/ x9 j4 F1 N+ S8 a
submit to the simulator. The simulator employees KCL and KVL to create a set of nonlinear equations. ' V8 t1 ^5 l, t' E
The nonlinear differential equations are not solved directly, but with approximation and iterative methods. Under certain 2 q9 q$ u$ U( \' k9 E# C
approximation, the equations are solved with the Newton-Raphson method. The solutions are equilibrium points of nodal analysis.( i. o% ~+ P' T% }+ o5 H1 d
IC design engineers work on a higher abstraction level than the device(transistor) level.
' F( R* N3 [5 f% Z. U3 f3 Y+ j- E oIn other words, transistors are the primitive components in the eye of IC designer.
2 @; Y2 c0 ^ @' a3 lA virtual symbol is the representive of a real device(component).) j. N& Y9 v c! {6 k
For instance, transistor's compact model is seen as a 4 pins symbol. 6 F; B7 g+ C, u1 b
In Advanced Design System(ADS), three design types are allowed: schematic, symbol, and layout., ^- A$ |& R) ^2 D$ Z, R {9 Y9 l
Those designs can all be stored in a small containner names "cell" and a big containner names "library".
0 l& z! Z; {& Y( }7 VIC designer works with the connection of some symbols in a schematic.
" Y* i. P+ y" ZEach symbol represents an electronic device (component).
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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. 5 |1 t- B# h A! d) ]
On the fabrication-level,
h2 W/ h; T( X- q1 \$ c# V' p( {! \& Pa compact model has the internal description of the device characteristics by means of a set of physics-based expressions with
! @7 U, M) S2 L3 N6 stechnology dependent model parameters. The physic-based model parameters values accounts for the actual behavior and properties
0 r5 a& V3 W. q) jof a device are defined by its process variables such as: geometrical dimensions and doping profiles.
$ u( L& [, G/ CThe true parameters values need to be carefully measured by the experimental setup of device characterization. 0 l# |9 a2 ^! W0 H% a: a4 G
Accordingly,
: b' k1 L2 \- E0 @9 nthe verified compact models are expected to be implemented in simulators.
( s4 ? m, N& |4 rThus the modelling accuracy and computational efficiency that a simulator can provide to integrate circuits' analysis + r: W8 w- t: E: }& Y7 I/ E
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.
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