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Lesson 1: Diode

# Diode

Introducing the diode as a circuit element. A diode conducts current in one direction but not the other. Created by Willy McAllister.

## Want to join the conversation?

• what would a diode look like on a circuit board?
• If you have a circuit board and you are trying to figure out which parts are which, you can't always tell for sure, but you can make an educated guess. If the board is made with "surface-mount" components, (search google for "surface mount [your favorite component type here]" to see examples of what they look like. Some things that give a diode away: A diode has only two terminals, never more. It might have a single white or black stripe near one end, indicating the cathode. And look at the white lettering on the circuit board (the "silkscreen" layer) to see if there is a small diode symbol, or if the component is labeled "D1" or "D2" or something like that. That will be a diode.
• Where can I find a video on transistors as amplifiers in Common -Emitter configuration and the explanation of Input current/voltage and Output current/voltage? Thank you.
• Hi Rahul,

I'm not familiar with videos but there is a good textbook here:

I'd like to give you a challenge - do you have access to electronic parts and a soldering iron?

Regards,

APD
• So a diode conducts in some circumstances, and in other circumstances it doesn't. Is it true than, as I understand, that the resistance of a diode is dependant of the direction and amount of voltage on the diode.
• You are correct. The "resistance" of a diode depends on the magnitude and sign of the voltage across it. I put "resistance" in quotes for a diode because it is not a constant value like it is for a real resistor. For any voltage applied to the diode, there will be a current flowing. The ratio of v/i at that point can be called the "resistance" of the diode. This idea of v/i = r at a specific voltage is how we create a so-called "small-signal model" of a diode or transistor.
• what is the direction of current in a circuit (outside of a DC battery)? Negative to positive? Electron flow vs proton flow? Your answer would help me develop voltage polarities in circuits. OBEARCC
• Why don't the graphs at beyond the moment that there stops being zero current? ie what should the graph look like for higher voltages? Does the current just go to infinity because there is no resistance or something?
• What was the reason behind the drastic increase of current after 0.7
• Great question. Unfortunately the answer for why a silicon diode does that is really complicated. It involves quantum mechanics at the atomic level. You treat an electron as a cloud of probability.
• What is transistor?
• A transistor is a semiconductor device with three terminals. Two of the terminals are the main current path. The third terminal controls (modifies) the current in the main path.

There are two families of transistors, Bipolar Junction Transistors (BJT) and Field-Effect Transistors (FET).

The main current path for a BJT is from its Collector terminal to its Emitter. The current is controlled by the Base electrode.

The main current path in an FET is from its Drain terminal to its Source terminal. The Gate terminal controls the main current.
• So what is difference between Diode and semi conductor
• Semiconductor is the general name for a type of material. Silicon, for example, is a semiconductor. A diode is a specific kind of electronic component made from a semiconductor material.
• We are learning about Silicon Diodes. But the question is, how they work like that? what's the science behind them? how silicon-based component work? Can someone please answer these question?
• at it is said that: "and what is left is IS times one".
My question is, why it is not IS times negative one ?
Thank you !
• Hello Majka,

"Negative one" appears appropriate because we get back what we started with. Here I_s is the reverse diode current that we originally measured.

Wikipedia has a helpful summary here https://en.wikipedia.org/wiki/Shockley_diode_equation

Regards,

APD
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