How Does CMOS Switching Happen ?

In this article we will discuss about CMOS switching with respect to MOSFET I-V curve. This the one among most asked question in interview. Read the article to clear your concept.

Complementary Metal–Oxide–Semiconductor (CMOS) is a  type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFETs for implementation of logic functions.  Important characteristics of CMOS devices are high noise immunity and low static power consumption.  

Above circuit is basic CMOS cuircuit and it is used as the building block in any circuit design. Here PMOS is connected to power and NMOS is connected to ground. Since either one transistor of the MOSFET-Pair is always off, the series combination draws significant power only momentarily during switching operation.  Hence CMOS devices are less power hungry as compared to its counterparts such as NMOS logic or TTL).  Consequently CMOS is used to integrate a high density of logic functions on a VLSI chip.

Inverter Input Output Behavior:

The CMOS inverter uses an NMOS and a PMOS transistor in a complementary push/pull configuration.

For a Logic "1" output, the PMOS=ON and the NMOS=OFF

For a Logic "0" output, the PMOS=OFF and the NMOS=ON

Inverter Logic Levels:

We need to define boundaries when the signal is considered HIGH (=1) or LOW (=0). These are called the "Logic Levels".There are ranges of voltages rather than absolute voltage numbers.  Any voltage in between the two bands are unacceptable to any of the logic levels. During switching the voltage obviously cross this no-mans-land.

Real CMOS Switching Curves:

In a real inverter the output doesn't switch instantaneously. There are two critical points on the real curve which occur when the slope is -1 

VIL is the input low voltage which corresponds to an output high voltage with a slope of -1. VIL is Maximum Input Voltage guaranteed to be recognized as a LOW.

VIH is the input high voltage which corresponds to an output low voltage with a slope of -1. VIH is Minimum input voltage guaranteed to be recognized as a HIGH.

VOH is the output voltage when the output level is logic "1".

VOH is Minimum output voltage guaranteed when driving a

HIGH.

VOL is the output voltage when the output level is logic "0". VOL is Maximum output voltage guaranteed when driving a LOW.

Vth is the point at which Vout=Vin

MOSFET IV Curve :

Ohmic or Linear Region is a region where in the current IDS increases with an increase in the value of VDS. In Saturation Region, the MOSFETs have their IDS constant inspite of an increase in VDS. Saturation sets in once VDS exceeds the value of pinch-off voltage VP.  Under this condition, the device will act like a closed switch through which a saturated value of IDS flows.  As a result, this operating region is chosen whenever MOSFETs are required to perform switching operations. Cut-off Region is a region in which the MOSFET will be OFF as there will be no current flow through it. In this region, MOSFET behaves like an open switch and is thus used when they are required to function as electronic switches.

CMOS & IV Curve Relation : 

Watch the video lecture here: 

Courtesy : www.pngegg.com