# What is a PN Junction Diode?

**URL:** <https://forumautomation.com/t/what-is-a-pn-junction-diode/13612>\
**Category:** Electrical\
**Tags:** basic\
**Created:** [October 8, 2025, 12:08pm UTC](https://forumautomation.com/t/what-is-a-pn-junction-diode/13612 "2025-10-08T12:08:50Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Rabert](https://forumautomation.com/user_avatar/forumautomation.com/rabert/32/6784_2.png) [@Rabert](https://forumautomation.com/u/Rabert)\
**Post date:** [October 8, 2025, 12:08pm UTC](https://forumautomation.com/t/what-is-a-pn-junction-diode/13612/1 "2025-10-08T12:08:50Z")

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# What is a PN Junction Diode?

A PN junction **[diode](https://forumelectrical.com/explain-diode-and-its-types/)** is the most basic semiconductor device made of

- P-type materials and
- N-type materials.

It only enables current to flow in one way which makes it the electrical barrier.

## V-I characteristics of a PN diode (Germanium-Ge)

## Forward Bias

When the diode’s positive terminal is connected to positive power supply, electricity can flow.

Germanium diodes have a knee voltage of approximately 0.3 V.

## Reverse Bias

When polarity is reversed, no current should flow (save for a small leakage current).

At some time, Reverse Breakdown Voltage (VRB) develops causing a significant increase in current.

## Key Parameters

Knee Voltage (Vb): 0.3 V for Ge (0.7 V for Si)

Reverse Saturation Current (Is): Minimal leakage during reverse bias.

Thermal Voltage (VT): 26 mV at 300 K

Reverse Breakdown Voltage (VRB): Sudden current increase in reverse.

 ![image](https://cdn.forumautomation.com/original/2X/2/26d17fbfb36d99af05b8ff28a216d923b2fb8290.jpeg)

## Equation

ID = IS (exp(eV/ηkBT) - 1)

 
## Why Does It Matter?

Basis of rectifiers (AC-DC conversion)

Protects circuits from the reverse polarity.

Utilized in signal processing, switching & logic circuits.

Foundation for LEDs, Solar cells, Zener diodes and more.

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