# IEC backward diode: backward (unitunnel) diode

Canonical URL: https://eloragrid.ai/symbols/iec-backward-diode-backward-unitunnel-diode
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: BACKWARD
Drawing use: Schematic
Reference families: IEC, Manufacturer

## Symbol convention

Convention: IEC
Form: Backward (Unitunnel) Diode
Autodesk IEC-60617 Symbol Preview: Semiconductor Components, HDI1_S00648 / VDI1_S00648. This identifies the publisher's library context; exact IEC graphics and database identities are not asserted.

Backward diode form: Backward (Unitunnel) Diode.

## In plain English

### What does IEC backward diode: backward (unitunnel) diode mean?

A backward diode uses tunnelling behaviour to provide useful conduction at a small reverse bias. Its operating region is different from the forward-conduction region of an ordinary rectifier. This entry identifies the backward (unitunnel) diode form in the publisher's IEC-context family.

### A simple example

A small-signal detector can exploit the low-bias response. Trace the bias and signal polarity in the complete circuit rather than treating the device as a reverse-polarity protection rectifier. 

### What the drawings help you do

A backward diode is related to tunnel-device physics; it is not simply an ordinary diode drawn in the opposite direction.

### This particular reference

Backward diode form: Backward (Unitunnel) Diode. This entry adds the backward (unitunnel) diode detail to the broader backward diode family. A backward diode is related to tunnel-device physics; it is not simply an ordinary diode drawn in the opposite direction.

### Terms

- Signal: Electrical information passed between devices, rather than necessarily the main power supply.
- Rating: A value stating the conditions for which equipment is specified.
- Schematic: A drawing showing function and connections rather than physical appearance.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [AutoCAD Electrical: Semiconductor components](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-AC954072-7314-4682-88AF-38160D61E9A2.htm)


## What it represents

A backward diode uses tunnelling behaviour to provide useful conduction at a small reverse bias. Its operating region is different from the forward-conduction region of an ordinary rectifier. This entry identifies the backward (unitunnel) diode form in the publisher's IEC-context family.
A small-signal detector can exploit the low-bias response. Trace the bias and signal polarity in the complete circuit rather than treating the device as a reverse-polarity protection rectifier. 
A backward diode is related to tunnel-device physics; it is not simply an ordinary diode drawn in the opposite direction.

## How to read it

- A small-signal detector can exploit the low-bias response. Trace the bias and signal polarity in the complete circuit rather than treating the device as a reverse-polarity protection rectifier.
- A backward diode is related to tunnel-device physics; it is not simply an ordinary diode drawn in the opposite direction.

## Information to show or reference

- Identify terminal polarity and the required bias region.
- Reference the selected component characteristics and package pinout.
- Show the surrounding bias and signal-conditioning network.

## Important distinction

This entry adds the backward (unitunnel) diode detail to the broader backward diode family. A backward diode is related to tunnel-device physics; it is not simply an ordinary diode drawn in the opposite direction.

## Facts

- Drawing form: Backward (Unitunnel) Diode
- Public vendor block names: HDI1_S00648 / VDI1_S00648. Autodesk identifiers, not standards database IDs.
- Drawing purpose: backward diode
- Graphic scope: Independently authored educational illustration; not certified standard geometry, a terminal pinout or a product footprint.

## Public references

- [AutoCAD Electrical: Semiconductor components](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-AC954072-7314-4682-88AF-38160D61E9A2.htm) — Autodesk. Public Semiconductor Components table explicitly lists HDI1_S00648 / VDI1_S00648 with this operating or connection form. Vendor graphics are linked, not copied.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.