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Transistors: In Depth Explanation by Dante

Hi,

Sorry again if I seem arrogant about this, I'm not. It's just that the explanation of how tubes and transistors work is so far off it needs to be addressed if anyone is interested in how they really work.

In the first place, it's a basic mistake to compare transistors to tubes. I did this in the beginning and it took me years to really understand the transistor. If you don't understand it, and treat it as a tube, it confuses troubleshooting dramatically. It's true that FET's and MOSFET's have an action similar to tubes, but that's where it ends.

The basic BJT transistor (Bipolar Junction Transistor) has little in common with a tube. And electron flow is only part of the equation. There is also a phenomenon called hole flow, which does not apply to tubes or circuit conductors. This translates to minority carriers and majority carriers, depending on the type of silicon is use. In N-type silicon, the majority carrier is the electron, whereas it is the hole in P-type. Many people get confused about these properties of semiconductors like silicon and try to compare the action to electron flow in copper. They are not the same.

If you take a piece of pure silicon, you can run an electron current through it in either direction. The FET works basically on this principle. But there are two kinds of FETs, one using P-type silicon and one using N- type. The same goes for MOSFETs. If you're using a P-type FET, there's no point talking about electron flow, because the majority carrier is the hole.

Electron vs. hole flow isn't all that complicated. If you take a basic silicon atom, it has 4 valence (free) electrons available for current flow. If you think of a chunk of silicon setup so that electrons are forced along it, the electrons jump from atom to atom. That's not actually correct either. This is a very primitive explanation of what actually happens, but it will do. As an electron leaves an atomic orbit, it leaves a 'hole' behind. This hole can be thought of as a positive charge. As another electron jumps into the vacated hole, the hole moves in the other direction. So, you have holes moving one way and electrons in the other.

The difference between N-type and P-type silicon is in the doping. Impurities (donor atoms) are purposely introduced into the silicon. If I remember correctly, the doping atoms have only three free electons in P- type and 5 free electrons in N-type. The P-type doping atoms introduce an abundance of positive charges, or holes. In this manner, there will always be more holes than electrons, so this type of doped silicon is refered to as P-type. The opposite is true for N-type, where electrons are more abundant.The BJT transistor is made up of a combination of both types.

In an NPN BJT, there is a P-type base region between N-type emmitter and collector regions. Where these regions join, there is a natural action between the silicon types whereby electrons bleed into the P-type and holes bleed into the N-type. This sets up a 'potential hill' which must be overcome for current to flow. This hill accounts for the typical 0.7 volt base voltage in a BJT.

Without getting too far into the theory of the BJT, while trying to compare it to the triode, it's important to understand the differences. In a BJT circuit, the emmitter-base circuit is equivalent to the cathode-grid circuit in a triode. The emmitter-collector BJT circuit could be compared to the triode cathode-plate circuit, but the action is quite different. It really doesn't matter how that action is different, it's important to understand only the differences.

For one, in a triode, a small voltage 'at' the grid controls a much higher current through the tube. In a BJT, a small current 'through' the emitter- base junction controls a much higher current through the emitter-collector region. In a FET, a small voltage 'at' the gate controls a much higher current through the source-drain region. In this way, FET's are similar to triodes. It's a big mistake, however, to take the comparison much beyond that.

Because there are two types of BJT's, the circuit voltages are applied differently. This is one major difference between BJT's and triodes. The thing to remember is that the emitter-collector junction is always reverse biased while the emitter-base junction is always forward biased. With an NPN BJT, The positive supply, Vcc, goes to the N-type collector. The positive supply also goes via a bias network to the P-type base (Vbb). The opposite is true for a PNP BJT.

The reasoning for this in an NPN, is to provide a relatively large positive field on the collector to pull electrons past the emitter-base region. In an NPN, you always think of electrons flow through it from emitter to base. On a schematic, the NPN has the arrow pointing out. If you think of electrons as flowing 'into' the arrow, you can't go wrong. The arrow was designed for positive current flow, which is the basic unit in physics. With the PNP BJT, think of electron flow from collector to emitter. Of course, if you're talented, with good visual imagery, you can switch over and think in conventional current flow, where current flows from positive to negative. It's easier for me to think only in terms of electron flow. On a schematic, a PNP has the arrow pointing in. Again, thinking of electrons as flowing into the arrow will help you analyze the circuit in terms of electron flow. Also, to remember arrow directions, think 'Not Pointing iN' for NPN's.

The FET works on the principle of pinching off a silicon channel with a voltage impressed on a gate region. The silicon channel can, again, be either P-type or N-type. Think of the channel as a long stick of silicon with a gate region surrounding it, like a doughnut. The drain is at one end and equivalent to a triode plate. The source is at the other end and equivalent to a triode cathode. The douhgnut region is the gate.

If the channel is N-type, the power supply is attached so the drain is positive (Vdd) and the source is negative (Vss). The gate voltage will be negative too (Vgg). The majority carrier will be the electron and the analogy to the triode holds true. If the channel is P-type, however, the analogy fails. This is because the majority carrier is the hole (positive charge), and current can only flow one way in a triode. There is no such thing as hole flow in a tube. Also, if you think in tube analogies, you will get into a lot of trouble with MOSFET's and CMOS devices. Again, that's because there is no analog to current flowing backwards through a triode.

I don't want to go any farther with this. Hope this has been helpful.

Dante

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