Diodes, you may not know it, but you use them every day. They can be found as full or half wave rectifiers to make AC current into DC for a ton of electronics. (e.g. cellphone chargers, DC motors in mixers etc.)
For us to understand what diodes really are and how to use them we need to start from the very rock bottom of electronics which is, you guessed it (probably not if you are new to this) chemistry. But do not worry it is not that hard or complicated, we only have to go over basic types of semiconductor elements. That being said let's begin :
Semiconductor - is a type of material that has both electrical conductivity of a conductor (e.g. copper) and a insulator(e.g. rubber)
In these examples we are going to use Silicon ( Si ), and in order to determine P and N type of silicone we need to look at its crystal structure of a pure/intrinsic silicon
- At intrinsic silicon we can see that each silicon atom has 4 valence electrons (electrons on the most outer shell) which are connected via covalent bond with their neighboured atom
Adding impurities (other elements) we can alter the type :
- N - Type - We get N-Type silicon when we add Phosphorus, because it has 5 valence electrons which makes 1 of them an extra electron. Because of that extra electron electricity can be conducted trough the crystalline structure via electrons as they move throughout the structure. We say that major carriers of electricity are electrons.The N-type name came to be because electrons represent negative charge. (N stands for negative)
- P - Type - We get P-Type silicon when we add Boron, because it has 3 valence electrons which makes a hole which breaks one of the covalent bonds . Because of that hole electricity can be conducted trough the crystalline structure via holes as electrons from neighboured atoms replace the hole and make a hole in an that atom. We say that major carriers of electricity are holes.The P-type name came to be because holes represent positive charge. (P stands for positive)
Now that we know this we can start talking about diodes.
Diodes essentially are two-terminal components with a P-N junction. Now you can see why we needed to learn this. Essentially that P-N junction makes two fields; one with minimal resistance(ideally zero) and one with high resistance(ideally infinite).
Now we are going to take a look at how this P-N junction is made :
In this picture you can see that there is a metallurgical limit, which means that P and N type are not mixing, but why is that?
Well, negative charge and positive charge will start to mix and equal out (concentration of boron and phosphorus isn't equal), as they are doing that they are crossing the metallurgical limit. The consequence of that is forming of space charge region. Space charge region around the metallurgical limit is called the P-N Barrier, because it is not allowing major carriers of electricity to pass trough it.
In order to pass trough the P-N barrier major carriers of electricity need to overcome the contact potential (lets call it Vc) of the barrier.
This contact potential is actually the voltage you need to get to the diode so it can conduct electricity.
Now we can talk about polarization of diodes. We have 3 base polarizations :
- No voltage
- Normal polarization - positive end of a battery is connected to the P type and the negative side is connected to the N type
- Reverse polarization - positive end of a battery is connected to the N type and the negative side is connected to the P type
This essentially shows that if you connect the diode in wrong order you will get a some kind of a wall that doesn't let electricity to flow trough.
A symbol for a diode is this :
Positive end of a diode (P-type in P-N junction) is called an Anode and a negative end of a diode (N-type in P-N junction) is called an Cathode
So if you connect positive end of a battery/power to the anode and a negative/ground to the cathode electricity will flow trough it just like trough a normal conductor (with an additional voltage drop), but if we reverse polarize it so that the anode is connected to the ground and the cathode to the positive end, electricity would not flow trough because the contact potential would be enormous (much bigger than the voltage). In datasheets that is called maximum reverse voltage commonly called VR.
So that is all we need to know about diodes in terms of how are they made and how do they work, if you liked this post like it and come back every week or two where i will be posting a lot more posts like these.
Let me know in the comments if you liked it, if not why, and next i would like to do Rectifiers (sequel to diodes) or a post about transistors.
Until then bye,
Matija Martinec
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