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resistor [2012/07/01 18:07]
mcmaster
resistor [2013/10/20 10:59]
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-====== Depletion load ====== 
- 
-These are arranged with the transistor tied to be always on.  A MOSFET is classified into 3 areas of operation: 
-  * NMOS 
-    * Cutoff (VGS < Vth): little conduction 
-    * Triode (VGS > Vth and VDS < (VGS - Vth)): resistive 
-    * Saturation (VGS > Vth and VDS > (VGS - Vth)): conductive 
-  * PMOS 
-    * Cutoff (VGS > Vth): little conduction 
-    * Triode (VGS < Vth and VDS > (VGS - Vth)): resistive 
-    * Saturation (VGS < Vth and VDS < (VGS - Vth)): conductive 
- 
-In essence a MOSFET is off if you don't apply any voltage between gate and source. ​ Its fully on if you apply lots of voltage between drain and source and a little between the gate and source (ie switching a high voltage load). ​ And then there are the cases in between which is what we want. 
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-Pulldown PMOS resistor on Intel 4004 (IC images courtesy of Flylogic, mask from http://​www.4004.com):​ 
- 
-{{:​flylogic:​intel_400:​pullup_metal_flylogic.png}} 
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-{{:​flylogic:​intel_4004:​pullup_delayered.png}} 
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-{{:​intel_4004:​pullup_mask_cif.png?​150}} 
- 
-Which translates to the following schematic: 
- 
-{{:​mcmaster:​resistor:​pmos_pulldown_sch.png}} 
- 
-(FIXME: compare with analysis at [NMOS logic design] The metal is VDD (-10V). For PMOS, cutoff occurs when VGS > Vth and assume Vth = -3V or something like that.  If VGS is 0 the resistor does nothing as there is no potential difference. ​ Now note that since VG = VD and so VGS = VDS.  We are in the triode region if VDS > VGS - Vth and so if 0 > 0 - -3V or 0 > 3V) 
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- 
-====== Active area ====== 
- 
-One common way to form resistors is to lay out one type of active area (ex: p) inside of another (ex: n) in a thin strip. ​ Then reverse bias the junction to increase the resistance by shrinking the conductive area within the inner region. ​ Example: (ST 24C02): 
- 
-{{:​mcmaster:​resistor:​st_24c02_sec17_hf1.jpg?​200|}} 
- 
-{{:​mcmaster:​resistor:​st_24c02_sec17_hf2.jpg?​200|}} 
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-{{:​mcmaster:​resistor:​st_24c02_sec17_stripped.jpg?​200|}} 
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-Which translates into: 
- 
-{{:​mcmaster:​resistor:​st_24c02_sec17_layout.png?​300}} 
- 
-I've grayed out the irrelevant parts and added labels for power and the two resistor terminals (T1/​T2). ​ The heavily doped green P+ areas form a connection to the P-well around the resistor while the resistor is N doped. ​ VSS keeps a negative bias on the P doped region pulling charge away and thus making the region less conductive. ​ VDD is applied to this essentially huge gate N channel transistor to keep it always on.  (TODO; what are those reddish marks between squiggles?) 
- 
-Unfortunately,​ because the size of the depletion region is a function of the voltage applied to it the actual resistance is non-trivial to calculate. ​ For example, if you know the total resistance is 10k and it was tapped in the center you would not get two 5k resistors. ​ The higher the voltage the larger the gap so in the above example if T1 was connected to ground, T2 to center tap would have a larger resistance than center tap to T1.  Fortunately,​ in many applications we only need a single pullup or pulldown resistor and such things aren't an issue. ​ However, above is a clock generator where one might actually want to know the clock frequency. ​ The best solution is probably to simply measure it. 
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-===== Diffused ===== 
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-===== Ion-implanted ===== 
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-====== Thin film ====== ​ 
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-Thin film resistors "are fabricated by vacuum evaporation or sputtering of thin films of resistive materials directly on top of the oxide layer of the substrate"​ [Resistor Fabrication on Semiconductor Wafers]. ​ Additionally,​ these can be laser trimmed by blasting away the metal. ​ Example laser trimmed resistor (AD534): 
- 
-{{:​mcmaster:​resistor:​ad534_laser_trimmed.jpg?​300|}} 
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-====== Polysilicon ====== ​ 
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-Polysilicon is a so-so conductor and so simply making a long strip of it will create a significant resistance. 
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-====== References ====== ​ 
-  - Resistor Fabrication on Semiconductor Wafers: http://​www.siliconfareast.com/​resistor-fab.htm 
-  - http://​uvicrec.blogspot.com/​2011/​09/​understanding-intel-4004.html 
-  - NMOS logic design: http://​www.ittc.ku.edu/​~jstiles/​312/​handouts/​section_10_4_NMOS_Logic_Design_package.pdf 
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resistor.txt ยท Last modified: 2013/10/20 10:59 (external edit)
 
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