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Complementary Metal Oxide Semiconductor

The Metal Oxide Semiconductor Field Effect Transistor MOS-FET was invented in the late 50ies, say 10 years after the Bipolar Junction Transistor (BJT)

You may look at a BJT as a:  Current generator controlled by a current - meaning the current which lead into the base will result in a larger collector current.   Ic = hFE * Ib

The MOS-FET or just MOS is controlled by an electric field - the base or gate acts as one plate of a capacitor and when charged (positive or negative depending on the MOS type) will the MOS be able to lead a current.
You may look as MOS as a: Current generator controlled by a voltage - or as a - Resistance controlled by a voltage - which the case depends of the voltage ranges (gate-source and drain-source).

CMOS - means Complementary Metal Oxide Semiconductor and formed by a p-type and a n-type MOS transistor. The CMOS inverter will only use energy when the input changes state.  xx% of the energy by at shift goes to charge/ un-charge the capacitor (formed by the gate) - the rest the energy lost at the stage where where the gates got a voltage between Vcc and Gnd. At this stage will both MOS -transistors have a resistance in the range of say 1 kOhm, which could be regarded as a Short-circuit condition.

The most critical situation will occur if the gates are floating - hence will you find build protection inside FPGA in form of Pull-up / Pull-down or keeper devices in the IOB.


Power consumption of CMOS logic = Constant * Frequency * Voltage

  •  Constant which depends mostly on the physics of the chip (die) - The capacity of the gates - switch times, internal resistances etc.

  •  Frequency  which not the crystal frequency of the system (Bits of a binary counter doesn't change with the same frequency)

  •  Voltage a real "killer" and hence lots of effort made to reduce this (to say 1.5 Volts)



CMOS inverter with input low



CMOS inverter with input floating (danger)


CMOS inverter with input high

  CMOS only demands voltage to keep a state 0/1
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