Difference between revisions of "Passive CMOS Devices"

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An illustrative cross-section of resistors using these layers are shown in Fig. 1.
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An illustrative cross-section of resistors using these layers are shown in Figs. 1 and 2.
  
 
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| [[File:Si resistors.png|thumb|500px|Figure 1: Resistors built using semiconductor layers<ref name="Allen2016">Phillip Allen's [https://aicdesign.org/wp-content/uploads/2018/08/lecture03-151116.pdf slides] on Deep Submicron CMOS Technologies</ref>.]]
 
| [[File:Si resistors.png|thumb|500px|Figure 1: Resistors built using semiconductor layers<ref name="Allen2016">Phillip Allen's [https://aicdesign.org/wp-content/uploads/2018/08/lecture03-151116.pdf slides] on Deep Submicron CMOS Technologies</ref>.]]
| [[File:Metal resistor.png|thumb|500px|Figure 2: Resistors built using metal layers<ref name="Allen2016"/>.]]
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| [[File:Metal resistor.png|thumb|450px|Figure 2: Resistors built using metal layers<ref name="Allen2016"/>.]]
 
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Revision as of 20:52, 20 September 2020

Passive devices such as resistors, capacitors, and inductors, are commonly used in biasing circuits, feedback networks, and signal or energy storage blocks. However, these passive devices, when built on fabrication processes that are optimized for transistors, may have characteristics different from their ideal or discrete counterparts. In this module, we examine the behavior of passive devices built alongside CMOS transistors.

Resistors

In standard digital CMOS processes, there is usually no provision for high resistance layers, since resistances are typically deemed bad for digital circuits. But in analog design, we often need well-controlled resistors, with relatively large resistance values. In order to evaluate if a particular layer could be used as to build a resistor, we look to their sheet resistance. Recall that for a resistor:

 

 

 

 

(1)

Where is the sheet resistance of the layer, is the resistivity of the material, is the length along the direction of the current, and , is the cross sectional area normal to the current flow, which is equal to the product of the layer thickness, and is the width of the layer perpendicular to the current flow. The table below shows indicative values of the common conductive layers in a CMOS process:

Layer Sheet Resistance
Metal
Polysilicon
or Diffusion
-well

An illustrative cross-section of resistors using these layers are shown in Figs. 1 and 2.

Figure 1: Resistors built using semiconductor layers[1].
Figure 2: Resistors built using metal layers[1].

Capacitors

Inductors

References

  1. 1.0 1.1 Phillip Allen's slides on Deep Submicron CMOS Technologies