NMOS Transistors and PMOS Transistors Explained

Summary: nMOS and pMOS transistors are the two main types of MOS devices used in microprocessors. nMOS transistors turn on with a high gate voltage, while pMOS transistors turn on with a low gate voltage. Together, they form complementary circuits like cMOS logic gates.

Microprocessors are built from transistors, most commonly metal-oxide semiconductor (MOS) transistors.

The two types of MOS transistors are n-type MOS (nMOS) and p-type MOS (pMOS), which differ in structure, behavior and their charge carriers. Both devices have three terminals: gate, source and drain.

What are NMOS Transistors and PMOS Transistors?

NMOS Transistor: Turns on when the gate voltage is high relative to the source, creating a conductive path between drain and source.
PMOS Transistor: Turns on when the gate voltage is low relative to the source, also creating a conductive path between drain and source.

To understand how a pMOS and an nMOS operate, you also need to know a couple key terms:

Closed circuit: Electricity is flowing between the source and the drain, enabling the circuit to conduct.
Open circuit: Electricity is not flowing between the source and drain, as the path is blocked by transistor.

A tutorial on the basics of nMOS, pMOS and cMOS transistors. | Video: Electric Videos

How Does an NMOS Transistor Work?

When the gate-to-source voltage of an nMOS transistor exceeds its threshold voltage, a conductive channel forms between the drain and source, allowing current to flow. This means the transistor is in a closed-circuit state, or on.

If gate-to-source voltage is below the threshold (e.g., near 0 volts), the channel does not form and no current flows between the drain and source. This means the transistor is in an open-circuit state, or off.

nmos transistor diagram — Example of an nMOS transistor. | Image: Brendan Massey

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How Does a PMOS Transistor Work?

A pMOS transistor works in the opposite manner of an nMOS transistor. It turns on when the gate voltage is low relative to the source.

On circuit diagrams (like the one below), the pMOS symbol includes a small circle (or “bubble”) at the gate terminal. This bubble signifies that the transistor is active-low — it turns on when receiving a low logic level. As a note, the bubble represents logical inversion, not a literal voltage inversion; it simply indicates the gate responds to opposite logic compared to the nMOS.

nmos transistor example of a pmos transistor — Example of a pMOS transistor. | Image: Brendan Massey

Constructing NMOS and PMOS Transistor Gates

Because pMOS and nMOS transistors operate in complementary ways, they can be combined into a complementary metal-oxide semiconductor (cMOS) circuit, and be used to build logic gates.

‘NOT’ Gate With NMOS and PMOS Transistors

Pairing a pMOS connected to the source or supply voltage with an nMOS connected to ground allows us to build logic structures, starting with the simplest example: a NOT gate.

nmos transistor NOT Gate diagram — Example of a NOT gate. | Image: Brendan Massey

This cMOS transistor acts in a similar manner to the NOT logical function.

A NOT gate can be built using one pMOS transistor at the top and one nMOS transistor at the bottom:

Input = 0: The pMOS is on, connecting the output to the supply voltage. The nMOS is off, so there is no path to ground. Output = 1.
Input = 1: The pMOS is off, disconnecting the supply. The nMOS is on, pulling the output down to ground. Output = 0.

This matches the truth table of a NOT gate: the output is always the opposite of the input.

nmos transistor in and out value table — An IN value of 0 produces an OUT value of 1. | Image: Brendan Massey

nmos transistor in and out table — An IN value of 1 produces an OUT value of 0. | Image: Brendan Massey

Putting the two sets of input/output together yields:

nmos transistor truth table — The truth table for a NOT gate. | Image: Brendan Massey

It’s quite easy to see that this truth table is the exact same as that which the logical function NOT produces:

nmos transistor NOT truth table — NOT truth table. | Image Brendan Massey

As a result, this is known as a NOT gate.

Can we use these two simple transistors to make more complicated structures? Absolutely.

‘NOT-OR’ Gate With NMOS and PMOS Transistors

Next, we will build a NOT-OR (NOR) gate.

nmos transistor NOT OR gate illustration — An example of a NOR gate. | Image: Brendan Massey

This circuit uses two pMOS transistors at the top and two nMOS transistors at the bottom.

A = 0, B = 0: Both pMOS conduct, pulling the output high. Both nMOS are off, so no path to ground. Output = 1.
A = 0, B = 1: The pMOS connected to A is on, but the pMOS connected to B is off. At least one pMOS is off, so the output is not pulled high. The nMOS for B conducts, pulling the output low. Output = 0.
A = 1, B = 0: Similar to the previous case, one pMOS is off and the corresponding nMOS is on, pulling the output low. Output = 0.
A = 1, B = 1: Both pMOS are off, so the output isn’t pulled high. Both nMOS are on, pulling the output firmly low. Output = 0.

The above gate’s truth table is as follows:

nmos transistor gate truth table — The output of the NOR gate. | Image: Brendan Massey

This matches the truth table of a NOR gate: the output is high only when both inputs are 0, and low in all other cases.

nmos transistor NOR logical function table — The output of the NOR logical function. | Image: Brendan Massey

Thus, we have confirmed that this gate is a NOR gate because it shares its truth table with the NOR logical function.

‘OR’ Gate With NMOS and PMOS Transistors

Now, we will put both of the gates that we have created together in order to produce an OR gate. Remember, NOR stands for NOT OR. So, if we invert an already inverted gate, we will get the original back.

Let’s put this to the test to see it in action.

nmos transistor OR Gate Illustration — Example of an OR gate. | Image: Brendan Massey

The OR gate is constructed by taking the NOR gate and adding an inverter (a NOT gate) to its output.

A = 0, B = 0: The NOR gate produces 1, but the inverter flips it to 0. Output = 0.
A = 0, B = 1: The NOR gate produces 0, the inverter flips it to 1. Output = 1.
A = 1, B = 0: The NOR gate produces 0, the inverter flips it to 1. Output = 1.
A = 1, B = 1: The NOR gate produces 0, the inverter flips it to 1. Output = 1.

Thus, the truth table will be as follows:

nmos transistor OR gate truth table — Truth table of a NOR gate and an OR gate. | Image: Brendan Massey

This matches the truth table of an OR gate: the output is high when at least one input is high.

‘NOT-AND’ Gate With NMOS and PMOS Transistors

nmos transistor NOT AND gate illustration — Example of a NAND gate. | Image: Brendan Massey

I claim this is a NOT AND (NAND) gate, but let’s test this gate’s truth table to determine if it really is a NAND gate.

This circuit combines pMOS transistors at the top and nMOS transistors at the bottom.

A = 0, B = 0: Both pMOS conduct, pulling the output high. Both nMOS are off, so no path to ground. Output = 1.
A = 0, B = 1: The pMOS for A conducts, pulling the output high. The nMOS for B is on, but since A’s nMOS is off, there’s no full path to ground. Output = 1.
A = 1, B = 0: The pMOS for B conducts, pulling the output high. The nMOS for A is on, but B’s nMOS is off, so no path to ground. Output = 1.
A = 1, B = 1: Both pMOS are off, so no pull-up. Both nMOS are on, creating a path to ground. Output = 0.

The above gate’s truth table is as follows:

This matches the truth table of a NAND gate: the output is high for all input combinations except when both inputs are 1.

nmos transistor NAND truth table — NAND truth table. | Image: Brendan Massey

Thus, we have verified that this is, indeed, a NAND gate.

‘AND’ Gate With NMOS and PMOS Transistors

Now we will build an AND gate the exact same way we built an OR gate from a NOR gate. We will attach an inverter.

nmos transistor AND gate illustration — Example of an AND gate. | Image: Brendan Massey

Since all we have done is applied a NOT function to a NAND gate’s output, the truth table will look like this:

nmos transistor AND NAND truth table — Complete truth table of an AND and NAND gate. | Image: Brendan Massey

Again, please verify to make sure that what I am telling you is true.

Thus we’ve covered what pMOS and nMOS transistors are, as well as how to use them to build more complex structures.

Frequently Asked Questions

What is a NMOS transistor?

An NMOS (n-type MOS) transistor is a transistor with a metal-oxide semiconductor (MOS) structure, where n-type material is used for the source and the drain terminals and p-type material is used for the substrate.

An NMOS transistor turns on (current flow between source and drain) when the gate-to-source voltage is high enough to exceed its threshold voltage, and turns off (no current) when it receives a voltage around 0 volts or below the threshold.

What is a PMOS transistor?

A PMOS (p-type MOS) transistor is a transistor with a metal-oxide semiconductor (MOS) structure, where p-type material is used for the source and drain terminals and n-type material is used for the substrate.

A PMOS transistor turns on (current flow between source and drain) when the gate-to-source voltage is low relative to the source, and turns off (no current) when the gate voltage is high relative to the source.

What is the difference between NMOS and PMOS transistors?

NMOS transistors have n-type (electron-based) source and drain terminals and a p-type substrate, while PMOS transistors have p-type (hole-based) source and drain terminals and an n-type substrate. NMOS transistors turn on with a high gate voltage, and PMOS transistors turn on with a low gate voltage.