# How do I simulate a p-n diode using NMOS, PMOS or CMOS in LTSpice?

#### lp203

Joined Dec 16, 2022
8
I googled and kept running into circuits that have a diode between the Drain and Source. Isn't there a way to simulate an ordinary diode without using any diode?

#### BobTPH

Joined Jun 5, 2013
6,304
Tell us what you are trying to accomplish.

Are you trying to use a MOSFET as an active rectifier? These are normally used to overcome the limitations of a junction diode. You would not want it to simulate a junction diode for that purpose.

#### lp203

Joined Dec 16, 2022
8
Tell us what you are trying to accomplish.

Are you trying to use a MOSFET as an active rectifier? These are normally used to overcome the limitations of a junction diode. You would not want it to simulate a junction diode for that purpose.
My lab instructor told me to design a diode using MOSFET and I wasn't given anything specific to work with. Really sorry for that

#### BobTPH

Joined Jun 5, 2013
6,304
Did he say “diode” or “rectifier?”. The implications are quite different. On the other hand, he might have been imprecise in his language. Do you understand the difference? Can you ask for clarification?

#### lp203

Joined Dec 16, 2022
8
Did he say “diode” or “rectifier?”. The implications are quite different. On the other hand, he might have been imprecise in his language. Do you understand the difference? Can you ask for clarification?
It was definitely a diode, To simulate the characteristics of an ordinary diode

#### BobTPH

Joined Jun 5, 2013
6,304
Okay, a much harder problem. You are not likely to find a circuit for that, because no one would do that.

What are the characteristics of a PN diode as you understand it?

#### BobTPH

Joined Jun 5, 2013
6,304
Or perhaps it is a trick question, and all he wants is for you to tie the gate to the source. Your MOSFET now acts like an ordinary diode.

#### lp203

Joined Dec 16, 2022
8
Okay, a much harder problem. You are not likely to find a circuit for that, because no one would do that.

What are the characteristics of a PN diode as you understand it?
In forward bias current increases exponentially with voltage.
In reverse bias current produces negligible change in voltage till it reaches breakdown voltage and then there is a steep decrease.

#### lp203

Joined Dec 16, 2022
8
Or perhaps it is a trick question, and all he wants is for you to tie the gate to the source. Your MOSFET now acts like an ordinary diode.
I suppose that lab's difficulty would require you do to something like that. That makes sense. I'll try that

#### BobTPH

Joined Jun 5, 2013
6,304
I suppose that lab's difficulty would require you do to something like that. That makes sense. I'll try that
I would get clarification from the prof. He might not be amused.

#### lp203

Joined Dec 16, 2022
8
Or perhaps it is a trick question, and all he wants is for you to tie the gate to the source. Your MOSFET now acts like an ordinary diode.
I would get clarification from the prof. He might not be amused.
I got the characteristic curve without the breakdown region. I'll ask the prof if that's enough. Thanks for the help.

#### crutschow

Joined Mar 14, 2008
31,535
Or perhaps it is a trick question, and all he wants is for you to tie the gate to the source. Your MOSFET now acts like an ordinary diode.
You are referring to the intrinsic source-drain substrate diode I presume.

#### dl324

Joined Mar 30, 2015
15,511
Isn't there a way to simulate an ordinary diode without using any diode?
You need a diode, even if it's a parasitic diode.

Since this is school work, you should post the complete text for the problem. I'll flag your post to be moved to Homework Help.

#### WBahn

Joined Mar 31, 2012
28,182
Look up diode-connected transistor.

#### lp203

Joined Dec 16, 2022
8
Look up diode-connected transistor.
Got it. This is what @BobTPH told me. I got the characteristic curve without the breakdown. Appreciate the help.

#### lp203

Joined Dec 16, 2022
8
You need a diode, even if it's a parasitic diode.

Since this is school work, you should post the complete text for the problem. I'll flag your post to be moved to Homework Help.
Thank you.
Fair enough. I'll take care of the forum next time

#### BobTPH

Joined Jun 5, 2013
6,304
You are referring to the intrinsic source-drain substrate diode I presume.
Yes.