# Details regarding PMOS and NMOS transistors used as gates

My lab instructor explained very briefly what transistors are and started by naming them NPN and PNP then switched to PMOS and NMOS and I (as well as my classmates) am very confused, I need someone to explain to me these points because I am not finding clear explanations anywhere:

1. is PMOS the same as PNP? (and NMOS same as NPN)?
2. How do they actually work as "gates"? how does it relate to the input given and the actual properties of these transistors?
3. What do Logical 0 and 1 really represent? (as voltages)

PS: Sorry if these questions are wrong or badly formulated I really am very confused about everything.

• Did you miss the jump from bipolar transistors to FETs, or did he?
– user16324
Commented Sep 8, 2020 at 16:39

is PMOS the same as PNP? (and NMOS same as NPN)?

No. PMOS and NMOS are types of field-effect transistors (often called FETs, the most common type of which is MOSFET). PNP and NPN are types of bipolar junction transistors (often shortened to BJT). An NMOS transistor is analogous to an NPN, but they are by no means the same.

How do they actually work as "gates"? how does it relate to the input given and the actual properties of these transistors?

"Gate" is a poor choice of wording, because the control terminal on a FET is called the "gate".

Transistors work as switches because when you put a low-powered signal on their control terminal (gate for FETs, base for bipolars) the transistor will switch from off to on, and can control orders of magnitude more power at its active terminal than you put into its control terminal.

(Sorry for all the abstract language -- I'm trying to cover both FETs and BJTs here).

You should look this up -- Wikipedia is a good place to start -- there is a lot of information out there.

What do Logical 0 and 1 really represent? (as voltages).

Any two distinct voltages can be labeled as being 0 and 1 (or false and true, or 'apple' and 'orange'). Typically, in modern usage, a voltage close to the positive supply is considered a 1 and a voltage close to the negative supply is considered a 0.

Whether to map high voltage to 1 or 0 and low voltage to the other is up to the circuit designer -- and, these days, to the mob of young engineers with torches and pitchforks who will assemble if you choose anything other than high = 1 and low = 0.

• Thank you so much for the detailed explanation. I have one more question in mind: is it correct to say that PMOS transistors output "1" when given "0" and vice versa? And NMOS transistors output "1" when given "1" and "0" when given "0"? The instructor reallllllly confused me. Commented Sep 8, 2020 at 16:36
• @RalphAouad, that is part of your question 2, and requires quite a bit more explanation than can be provided here. But to try to answer your specific question, the simplest gate you can make is an inverter (0 output for 1 input and vice versa) and you can make it with either an NPN or a PNP plus one resistor. Commented Sep 8, 2020 at 16:43
• @ThePhoton or a NMOS or PMOS and one resistor. Or an NMOS and a PMOS together -- etc.. Commented Sep 8, 2020 at 18:30
• @TimWescott, the important thing to answer OP's question is that whether you use an NMOS or PMOS, or NPN or BJT, you get an inverter, not a non-inverting buffer. Commented Sep 8, 2020 at 18:32
• @RalphAouad: this general subject is heavily covered in books and on the Internet. I think the biggest problem is that your instructor (or the class) is heavily over-simplifying things, to the point where all sense is lost. Or worse, your instructor doesn't understand the subject. An internet search on "logic circuits" or "make logic from a transistor" may help. Commented Sep 8, 2020 at 18:35

is PMOS the same as PNP? (and NMOS same as NPN)?

PNP and NPN are two kinds of bipolar junction transistors (BJTs).

PMOS and NMOS (short for p-channel and n-channel MOS) are two kinds of metal-oxide-semiconductor field effect transistors (MOSFETs).

They are not the same, but there is some analogy between the behavior of PNP BJTs and p-channel MOSFETs, and between NPN BJTs and n-channel MOSFETs. Many circuit topologies using PNP, for example, can be adapted with minimal changes to work with PMOS.

How do they actually work as "gates"? how does it relate to the input given and the actual properties of these transistors?

This is a topic for a complete textbook chapter, not a few short paragraphs.

What do Logical 0 and 1 really represent? (as voltages)

They can represent whatever you want:

• Whether to display an 'a' or a 'b' on the screen.

• Whether a water valve is closed or open

• Whether Mario is running or jumping

• etc.

Or, of course, they can be combined with other 1's and 0's to form larger numbers, distinguish between wider arrays of choices, etc.

The voltages involved are how you choose to represent the logic levels, not what the levels represent. You could pick whatever choice of voltages you want, although of course it will be more convenient to make a choice that someone has already designed chips to work with. Some commonly used choices:

• 5 V TTL: Any voltage below 0.8 V is '0', any voltage above 2.4 V is '1'

• 3 V CMOS: Any voltage above 2.1 V is '1', any voltage below 1.5 V is '0'

• 5 V ECL: Any voltage between -1.13 and -0.8 V is '1', any voltage between -1.95 and -1.48 V is '0'.

Let me simplify some points for you here

• Transistors have various types. We have Bipolar Junction Transistors (BJT) that are current controlled in nature. We also have Field effect Transistors (MOSFETs) that are voltage controlled in nature

• In BJT there are two types, PNP and NPN. These signify the configurations in which a transistor is made using P and N type elements

• In MOSFET there are two types, PMOS and NMOS. These also signify the configurations.

• PNP & PMOS, NPN & NMOS are not the same. They are different components in general.

• Regarding your gates question, A Transistor has 3 terminals. Lets call them Input, Output, Gate (These names are just to make it easy to understand in general).

• In PMOS, if your Gate voltage is 0V, then input=output and in NMOS if your Gate voltage is 5V, then input=output. This is now acting as a switch and the main application involves building logic devices.

• Logic 1 represents 5V and Logic 0 represents 0V in Digital system design. It can also be described in Boolean Equations as True (Logic 1) or False (Logic 0)

• I suggest you take time and learn more about the Digital System Designing to understand in depth how the transistors play a major role today.

Hope this helped.

Regarding the terminology "gate", it has two uses that need to be distinguished. Most posters here have focussed on the gate terminal of the MOS transistors, but that does not appear to be the use your instructor intended. In that case he appeared to be referring to logic gates which are made up of transistors.

It is unfortunate that the same word is used for two purposes when in this context you need both of them to describe how transistors with gate inputs can be used to make logic gates. Both uses are however standard.

I suggest the short description of the NAND gate here https://en.wikipedia.org/wiki/NAND_gate The final section gives schematics of a NAND gate in different transistor technologies. The CMOS one is the most commonly used today. The others may be considered more of historical interest. There is a longer article on CMOS https://en.wikipedia.org/wiki/CMOS which goes into this in far more depth.

How 0 and 1 translates into measurable voltages is complicated by differing logic implementations and evolution of technology over time. A few examples have been given by @The Photon, but there are many more and they are by no means all compatible. In practice you would need to check the component data sheets for details and to ensure compatibility with other components. For smaller circuits it is preferable to keep within a single logic family, but it is not always possible.