Semiconductor diode tutorial

Semiconductor Diode – Definition, Characteristic and Applications

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Semiconductor diode – type of diode, which contains “p-n junction” made from differently doped semiconductor materials. It is double-ended, nonlinear electronic component, where the end attached to the “p” layer (+) is called anode and “n” layer () cathode. This electronic component is mainly used because of it’s ability of making electric current flow only in one direction (from anode to cathode) after forward-biasing the aforementioned “p-n junction” with the positive electric voltage.

semiconductor diode symbol

Semiconductor diode symbol

However, in the opposite direction (reverse bias of the “p-n” junction with the negative electric voltage) we can say that in ideal semiconductor diode electric current won’t flow. This is why semiconductor diode is often described as the “electric valve”, which can pass or block the flow of the electric current.

Semiconductor diode consists of two, differently doped semiconductor crystals – “p” and “n” types. Together, they form so-called “p-n junction”, where the “n” layer (with electron donor dopants) has an excess amount of electrons, which are the majority carriers there (we have more electrons (-) than electron holes (+)). However, in the “p” layer (electron acceptor dopants) the majority carriers are electron holes (+) rather than electrons (-), so we have more holes “to fill”, than electrons available. The electron hole is a vacancy created by the electron “travelling” from its initial place to some other location in that crystal. In reality, there is no such thing as “a hole”, but that lack of electron kind of makes it a positively charged particle, which attracts negative electrons to form a pair again (holes can move too).

After they combine, a proportional distribution of electrons begins. Electrons, which previously lacked in “p” layer are transferred there from “n” layer, where were too many of them. So, “n” layer is a good friend for “p” layer, right? 🙂 And this is where so-called depletion region is formed, which prevents the flow of the electric current (thermodynamic equilibrium).

PN junction in state of thermodynamic equilibrium

PN junction in state of thermodynamic equilibrium

To allow the flow of the electric current through the “p-n junction” (electric valve on), external positive electric voltage must be applied to “push” and help large group of electrons and holes to meet together (forward bias of the diode). After they are “pushed” through the depletion region with enough force (Vf = 0,7V) diode starts conducting current, so it starts to flow through it.

PN junction after forward bias

PN junction forward-biased (electric valve on)

To make sure, that the electric current won’t flow (electric valve off), it is needed to apply external negative voltage to the semiconductor diode (reverse bias) to make depletion region even larger (illustration below).

PN junction after reverse bias

PN junction reverse-biased (electric valve off)

With passing time, technological requirements were increasing what resulted in development of new types of diodes. When a semiconductor is combined with the corresponding metal, we acquire MS junction (metal-semiconductor), which also possesses rectifying properties (current conduction in one direction) – it is used for example in fast Schottky diodes.

semiconductor diode

MS junctions can have one of two current-voltage characteristics:

  • Unsymmetrical non-linear
  • Symmetrical, linear

MS junction properties depend mainly on the surface state of semiconductor and on the output work difference of electrons from metal and semiconductor itself. Schottky diode is mainly used in systems that require fast switching time (small junction capacitance Cj of the diode has a decisive impact) with frequencies up to several tens of GHz.

Semiconductor diode characteristic

The graph below shows the current-voltage characteristic of the semiconductor diode. This is a typical characteristic for semiconductor diodes used in electronics (Vf = 0,7V). The semiconductor diode starts conducting current after exceeding the threshold of the forward voltage value specified by the manufacturer in the data sheet. Semi-thermal diodes are mainly used to protect other electronic components.

semiconductor diode characteristics

Current-Voltage characteristic of the semiconductor diode

How to determine where is the anode and where is the cathode?

Simple multimeter can be used to determine the polarity of a diode. There are at least three ways to do this but I will show here two most popular ways that can be done even with the cheapest multimeters (Get Basetech BT-11 Multimeter):

a) Using ohmmeter (2kΩ range):

semiconductor diode ohmmeter forward

Forward-bias: Ohmmeter will indicate the approximate forward voltage of the diode (near 0,7V)


semiconductor diode ohmmeter reverse

Reverse-bias: Ohmmeter indicates “1” what means very high resistance (electric valve off)

You can also use “diode check” function (diode symbol on the multimeter) but the result will be the same as the above with using ohmmeter.

b) Using VDC measurement function:

semiconductor diode voltmeter forward

Forward-bias: Multimeter should indicate voltage drop of approximately 0,7V for silicon diodes


semiconductor diode voltmeter reverse

Reverse-bias: Multimeter will indicate the approximate full voltage of the supply (Note: Here diode is inserted in opposite way compared to the example above. In reality, I would change the polarity of the Power Supply, because you can’t unmount “with your hands” once soldered component, unless you desolder it. Of course, we don’t want to do that to the good operating component. I just wanted to show you an example, that you should also pay attention to correct component placement at your PCB or Breadboard)

Experiment for self-execution

This experiment will allow you to visualize the principle of operation of a semiconductor diode, whether current conducts or not. Because you will do it by yourself, you will better remember this lesson.

Items needed:

We will be using two circuit schematics you’ve seen earlier:

semiconductor diode LED1

In this case, LED should conduct current and you should see it lighting


semiconductor diode LED2

Here, LED shouldn’t be lighting – diode is not conducting current (Note: Here diode is inserted in opposite way compared to the example above. In reality, I would change the polarity of the Power Supply, because you can’t unmount “with your hands” once soldered component, unless you desolder it. Of course, we don’t want to do that to the good operating component. I just wanted to show you an example, that you should also pay attention to correct component placement at your PCB or Breadboard)

Below you can see pictures showing the circuit mounted on the breadboard and visualization of two opposite positions of the LED diode (reverse polarity).

semiconductor diode LED3

Circuit „transferred” to the breadboard (diode is conducting current)

 

semiconductor diode LED4

In this case, as you can see diode isn’t conducting current (inserted in opposite way)  (Note: Here diode is inserted in opposite way compared to the example above. In reality, I would change the polarity of the Power Supply, because you can’t unmount “with your hands” once soldered component, unless you desolder it. Of course, we don’t want to do that to the good operating component. I just wanted to show you an example, that you should also pay attention to correct component placement at your PCB or Breadboard)

On the first picture, the LED was put into conducting state. Voltage potential at the anode was higher (+) than at the cathode (-), so the flow of the current was possible. In our experiment we used the 9V battery, so the current flowing through the diode will be about 9mA (calculated from the Ohm’s law).

On the second picture, the diode was inserted in the opposite way (the voltage potential at the cathode was higher (+) than at the anode (-)), so the diode was behaving like the closed electric valve, which prevented the flow of the current – LED isn’t lit.

Common types of semiconductor diodes (and their main functions):

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