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Magic Crystals and Magic Potions …. no just a real diode.

This post has been corrected – Somehow I managed to calculate the resistance of the diode and moved the decimal point.

The Symbol for a Diode

The Symbol for a Diode

In last night’s post I got us to the point where a PN junction was formed on a single pure silicon crystal by doping one part of it with an N type element and doping another part with a P type element. Another set of names for these two types of doping is donor elements for the N type and acceptor elements for the P type. The amount of doping and the areas doped is precisely controlled and this is how the properties of the final device is determined.

Finally, we are now at the strange properties; the magic.  The excess electrons are free to drift though the material.  This is called diffusion.   Since the P side of the junction wants some electrons some drift to that side of the junction.   The same thing happens with the holes and they drift to the N side of the junction.   However, they cannot drift very far to the other side of the junction because each side wants to remain neutral electrically. This situation of being pulled in both directions at the same time causes charges to build up very near the junction and creates something called the barrier potential.  This creates two unique properties.

Now you need to look at a datasheet for an actual diode.   The kind of diode we are going to look at is a very common power diode called the 1N4001 through the 1N4007.  The datasheet can be found at the Vishay Intertechnology site.  We will be referring to this in a few more posts as well.   In the days of electron tubes a tube with only two elements was referred to as a diode tube.   One with 3 was called a triode and one with 5 was called a pentode. The only name that as carried over to semiconductors is the diode, but both types of diodes did a very similar function.

First, I need to describe the marking on the package of the diode.  There is a band on the package, this is the P side of the junction and is also called the anode.  The unbanded side connects to the N side of junction and is called the cathode.  When a positive voltage is connected to the P side the diode is said to be forward biased.  Fig 4 on page 3 of the datasheet shows the current relationship with the a forward bias.   Notice very little current flows until this bias reaches a little over 0.6 volts.  The number usually used for a silicon PN junction is 0.65 V and is called the built in voltage, Vbi.  This number will be very important when we get to transistors.model of the diode1

To complete our model of the forward biased diode we need to determine how much resistance the diode has while conducting.  I took several points off the curve to try to calculate the resistance.  I finally ended up using the numbers from page 1 to calculate  R.  For 1 Amp IF, the VF was given to be 1.1 V. So RF = (1.1-0.65)/1.0 = 4.5Ω  0.45Ω.  Other points on the curve were all lower so I chose 4Ω.  As you can tell from looking at the curve as well as the way I am somewhat “winging” it on choosing the numbers electronics involves lots of approximations.

The second thing about the barrier potential is the charge clouds on either side of the PN junction form conductive areas.  In-between these charge clouds is an area that is completely depleted of charges.   If you think back to the explanation of a capacitor in “Energy Storage – Capacitors – Equalization of Pressure“, we have just defined a capacitor.  If you look at Fig 6 on page 3 of the data sheet you will notice the capacitance goes down as the reverse bias is increased.  This is because the charge clouds are pulled farther apart and leave a larger depletion area between the clouds.  There are special diodes that make use of this property to act as a variable capacitor for tuning radios and TV’s for example.

Some of the other numbers we need to be concerned about is the Peak Reverse Voltage.   As the reverse bias is increase more current “leaks” as is shown in Fig 5 of the datasheet.  This is because impurities in the production process causes some of the wrong kind of carriers to be present in the crystal.  For example the P material may contain some N type material and these are called “minority” carriers.  Increased reverse bias pulls more and more of these loose.   Temperature also knocks some of these loose and that is why there are 3 curves on Fig. 5, one for each of 3 temperatures.  At some voltage the moving minority carriers start knocking other minority carriers loose.  This is called the avalanche voltage and the diode breaks down.  If you look at the Maximum Ratings chart on Page 1 of the datasheet, the Peak Reverse Voltage rating is the difference between the various models of this diode.  We will deal specifically with this in a future post.

Because there is at least a 0.65 voltage drop across the diode and even more as the current gets higher, there is a power loss across the diode. (V x I).  This must be dissipated if the diode is to remain cool and functional.  The datasheet contains a lot of thermal information because of this.  Fig. 1 shows how the Average Forward Current must be derated if the diode is operated in a hot environment.  Fig. 2 is an interesting piece of information that we will be dealing with very soon.   Although the diode can only handle 1 A of average forward current, it can handle much higher for short durations.

To give you a hint of future attractions, very often a diode feeds a circuit containing a large capacitor.  Remember a capacitor acts as a short circuit until it builds up charge.  A short circuit means lots of current.  The internal resistance of the diode may be a very good thing to limit this current.

In the last two posts we have been on an adventure with magic crystals that get power from potions and then block some things but allow others to pass.  I think we are ready to wire this thing up and start slaying dragons… or something.

Gary


 

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