Difference Between NMOS and PMOS

Main Difference –NMOS vs. PMOS

NMOS and PMOS are two different types of MOSFETs. The main difference between NMOS and PMOS is that, in NMOS, the source and the drain terminals are made of n-type semiconductors whereas, in PMOS, the source and the drain are made of p-type semiconductors.

What is MOSFET

A MOSFET is a type of unipolar transistor used in electronics. MOSFET stands for “Metal Oxide Semiconductor Field Effect Transistor“. Essentially, in a MOSFET, the current flow from one terminal to the other (the source and the drain) is determined by the voltage applied to a “gate” terminal. The current flows within the region called the “bulk” region. How a MOSFET works is explained in this article. Depending on which type of semiconductor makes up the different terminals, MOSFETS are classified into NMOS and PMOS.

What is NMOS

In NMOS devices, the source and the drain are made from n-type semiconductors while the bulk is made of p-type semiconductors. When the gate is given a positive voltage, holes between the two n-type regions get repelled and allow for electrons to flow between the source and the drain. The diagram below shows the structure of a MOSFET.

The structure of an NMOS MOSFET

The majority carriers in NMOS devices are electrons, and they can flow much faster than holes. As a result, NMOS transistors are smaller than corresponding PMOS devices. Consequently, NMOS are cheaper to produce than PMOS as well. Since electrons are faster than holes, NMOSs are also more useful in fast-switching applications. For instance, NMOS had been used for logic gates, although nowadays they have been largely replaced by “CMOSs” which contain a combination of NMOS and PMOS.

Most of the contaminants in MOSFETs are positively-charged. This gives a disadvantage to NMOSs because the accumulation of these contaminants around the gate could turn an NMOS device on when it is supposed to be off.

What is PMOS

In PMOS devices, the source and the drain are made of p-type material while the bulk is made of n–type semiconductors. When a negative voltage is applied to the gate, the electrons get repelled and so holes are able to form a channel and travel between the source and the drain. In PMOS, the majority carriers are holes. Holes flow much more slowly compared to electrons, therefore it is much easier to control the current.

Difference Between NMOS and PMOS

Fabrication:

In NMOS, the source and the drain are made of n-type semiconductors while the bulk is made of a p-type semiconductor.

In PMOS, the source and the drain are made of p-type semiconductors while the bulk is made of an n-type semiconductor.

Majority Carriers:

In NMOS, the majority carriers are electrons.

In PMOS, the majority carriers are holes.

Size:

NMOS devices are comparatively smaller compared with PMOS devices with complimentary conducting properties.

Operating Speed:

NMOS devices can be switched faster compared to PMOS devices.

Image Courtesy:

“MOSFET structure” by Brews ohare (Own work) [CC BY-SA 3.0], via Wikimedia Commons

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