MOS Transistor nMOS and pMOS transistor

Explain the basic concept of nMOS and pMOS transistor with relevant symbol.

•  A Metal-Oxide-Semiconductor (MOS) structure is created by superimposing layers of conducting and insulating materials.
•  CMOS technology provides two types of transistors. They are n-type transistor (nMOS) and ptype transistor (pMOS).
•  As transistor operation is controlled by electric fields, the devices are also called Metal Oxide Semiconductor Field Effect Transistors (MOSFETs).
•  The transistor consists of a stack of the conducting gate, an insulating layer of silicon dioxide (SiO2) and the silicon wafer, also called as substrate, body or bulk.
•  A pMOS transistor consists of p-type source and drain region with an n-type body.
•  An nMOS transistor consists of n-type source and drain region with a p-type body.

nMOS Transistor:

1.  In an nMOS transistor, the body is grounded and the p–n junction of the source and drain to body are reverse-biased.
2.  As the gate is grounded, no current flows through junction. Hence, the transistor is OFF.
3.  If the gate voltage is raised, it creates an electric field, that start to attract free electrons to the underside of the Si–SiO2 interface.
4.  If the voltage is raised more, a thin region under the gate called the channel is inverted.
5.  Since a conducting path of electron carriers is formed from source to drain, current starts to flow. So, the transistor is said to be ON.

pMOS Transistor:

•  For a pMOS transistor, the body is held at a positive voltage.
•  When the gate terminal has a positive voltage, the source and drain junctions are reversebiased and no current flows. So, the transistor is said to OFF.
•  When the gate voltage is lowered, positive charges are attracted to the underside of the Si–SiO2 interface.
•  When a sufficient low gate voltage is applied, the channel inverts and a conducting path of positive carriers is formed from source to drain, which makes the transistor ON. 

NOTE:

 The symbol for the pMOS transistor has a bubble on the gate, indicating that the transisto behavior is opposite to nMOS.

 When the gate of an nMOS transistor is 1, the transistor is ON. When the gate is 0, the nMOS transistor is OFF.

 A pMOS transistor is ON when the gate is low(0) and OFF when the gate is high(1).