Differences

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--- resistor [2012/07/01 21:54] – mcmaster
+++ resistor [2012/07/01 22:06] – mcmaster
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-====== Always on transistor ======
+====== Depletion load transistor ======
 These are arranged with the transistor tied to be always on.  A MOSFET is classified into 3 areas of operation:
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     * 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.
 Pulldown PMOS resistor on Intel 4004 (IC images courtesy of Flylogic, mask from http://www.4004.com):
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 {{:mcmaster:resistor:pmos_pulldown_sch.png}}
-(FIXME: look into analysis. 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)
+(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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   - 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