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Introduction to Bipolar Junction Transistors.

Bipolar Junction Transistors

Bipolar Junction Transistors

It has always been my goal on this site to present information to help to understand every project I attempt. I am currently working on the Desulfator. Although I am primarily taking the design of someone else and building a printed circuit for it, both you and I need to know how the circuit operates. (Yes you heard that right, I don’t fully know how it operates … Yet, but WE will.) The circuit is built around two active components. One is an integrated circuit and the other is a transistor called a MOSFET. MOSFET is an abbreviation for “Metal Oxide Semicondutor Field Effect Transistor”, quite a mouthful isn’t it. We will be getting into the workings of a MOSFET eventually, but first the real heart of the circuit is the integrated circuit called a NE555 or LM555. To understand that circuit at even a functional level we will need to understand the fundamentals of some devices called Bipolar Junction Transistors. That is the goal of this post.

Bipolar Junction Transistors come in two basic types: a PNP and an NPN.  Most often the full name is not used and they are simply called a PNP transistor or an NPN transistor.   Both types are used, but the NPN is the most common.   These transistors have two PN junctions inside them as shown ub the top two blocks in the diagram.  The fact that there are two junctions is where the word Bipolar comes from.   This builds upon knowledge presented in previous posts and the two that are mandatory to understand are:  “The Chemistry of Semiconductors” and “Magic Crystals and Magic Potions … no just a real diode“.  If you have not followed all the posts on basic electricity leading up to those a list of significant posts is near the end of this post.

In those two posts, I described how a diode conducts electricity if it is “forward biased” and above a voltage called the cut-off voltage of approximately 0.65 volts.  Below that voltage next to no current flows in the diode.    If you look at the upper diagram of the NPN transistor, it is basically two diodes stuck together on one silicon crystal.  Three leads come out of this crystal, one connected to each of the different types of doped crystal.   The  material shared by both of the junctions, the P material in the NPN transistor, is called the Base.  The one at the bottom is called the Emitter and the one on the top is called the Collector.   The Emitter in the schematic symbol at the bottom of the picture is the one with the arrow on it.   The memory device to remember which type of transistor you are seeing in the schematic is:  Pointed -iN-Pointed for PNP and Not Pointed iN for the NPN.  Often in schematic diagrams the transistors are turned sideways or even upside down so the Emitter is not always drawn on the bottom, but it is always the one with the arrow.  The line the other two connections join into is always the Base.

 

A testing biasing circuit for an NPN Transistor.

A testing biasing circuit for an NPN Transistor.

I have several things being shown in the second diagram. The Red letters are the names for the voltage at each of the three leads. In the case of the circuit shown in this diagram the two voltage sources are attached to the emitter and that would be our reference point. So Ve = 0. The unusual thing in this circuit is the IEEE convention that all three currents, Ie, Ib, & Ic are shown flowing into the transistor. Obviously that cannot be so at least one of these currents will actually have a negative value and we will be seeing that very soon. Now comes the really confusing part… please put up with me, because there really is no way to avoid it.

Way back there when we first started talking about electricity, I said current through wires is really the movement of electrons and the electrons move from negative to positive.  However, because of definitions written a long time ago “conventional current” is assumed to move from positive to negative.  Outside of the transistor we will be using the conventional current definition when deciding if our three currents are positive or negative values.   Inside the transistor we will define our current flowing from the Emitter toward that Base and we will use electron flow if the Emitter is N type material for an NPN transistor.   If we are using a PNP transistor will will use “hole” flow because the Emitter is P type material.    (Don’t worry if you have to read that several times to get all the conventions straight in your head. It is confusing but actually simplifies your life in the long run.)

Our diagram is a NPN transistor, and excess electrons are hanging around in the Emitter.   If V1 is greater than +0.65 Volts then the PN junction is forward biased and electron current will flow from the Emitter to the Base.   Rb is installed to limit that current.   For example, say V1 is at +1.0 Volts then Rb will have a voltage drop across  maintaining Vb very close to 0.65 Volts.  As a matter of fact, if a transistor is operating above cut off and the transistor is a silicon transistor, the voltage between Vb and Ve will always be near 0.65 Volts.   This is a way of troubleshooting circuits. Now is where the real “magic” occurs in transistors.  The Base area of the transistor is very thin in comparison to the Emitter and Collector.  If V2 is significantly larger than the Voltage of V1, the fast moving electrons from the Emitter will blow right on through the Base and be collected by the Collector.   Most of the electrons will flow from the Emitter through the Base and into the Collector.  My little mental picture of this is the electrons from the Emitter missed the turn to the Base and were captured by the Collector.

The really significant thing is the current flowing in the Collector is much more affected by the Base current than the Voltage Vc.  Bipolar Transistors are said to be current controlled devices because of this fact.

Now we have just a few more things to wade through before we call this quits.   Since we started talking about electron current flow from the Emitter inside the NPN transistor this means electrons have to flow into the Emitter from the outside word and this means Ie will be a negative number since we are using conventional current.  (If you are following this well I will add a joke here… if you are not following along skip this joke.  The worlds very first case of two positives made a negative!)  Electrons are flowing into both the Base and the Collector on the inside which means in both cases electrons are flowing out the leads.   Conventional current is the opposite of electron current so both Ib and Ic will be positive.

Although our theory is not complete yet we can write two formulas.

The amount of Emitter Current = Ic+Ib.    To correct this to the IEEE conventional current directions this becomes -Ie = Ib + Ic.   (There is another term we have not talked about yet, so this formula will grow slightly.)

The second formula is a gain function.   Beta (β) = Ic/Ib.   Again this will slightly change.

The next post will talk about the same thing with a PNP transistor and it will be much shorter since it is really the same story except we will be talking about hole currents instead of electron currents..   In that post I will introduce the extra term which is also minor factor.

This was a fairly complex set of theory, do not feel bad if it takes a while to absorb it.  However, it will be helpful to you in electronics if you “make it your own”.
For those of you who have missed the electrical theory posts building up to this one the important ones for this theory in addition to the two listed earlier are:

 

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Introduction to Bipolar Junction Transistors.” by Create-and-Make.com is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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