Transistors (BJT)

A transistor is a semiconductor device that can be used as a switch or to amplify a current. They can come in a variety of packages as can be seen in Figure 1. There are a number of transistor types such as: bipolar junction transistors (BJT), field-effect transistors (FET), etc... This discussion focuses on BJTs. Wikipedia has excellent articles discussing BJTs as well as the other transistor types; the reader is referred to this article: Wikipedia: transistors.

Figure 1: Various transistor packages.

To understand how BJTs work, one should first understand how p-n junction diodes work. For that, the reader is referred to the p-n junction diode tutorial. Quickly recapping, when a p-type semiconductor material is joined to an n-type semiconductor material, a depleted zone forms between the two which prevents current flow. By applying a positive charge to the n-type material, the depleted zone size is reduced. Alternatively, when a positive charge is applied to the p-type material, the depleted zone size increases. The existance of the depletion zone prevents current from flowing. When a positive voltage differential applied from the n-type material to the p-type material that is greater than the p-n junction's forward bias voltage, the depleted zone is reduced to zero and current is allowed to flow.

A BJT is a lot like a p-n junction diode in that they are also made of n-type and p-type semiconductor materials. There are two types of BJT transistors: NPN and PNP types. A PNP transistor consists of an n-type semi-conductor material layer sandwiched between two p-type semi-conductor material layers while an NPN transistor consists of a p-type semi-conductor material layer sandwiched by two n-type semi-conductor material layers as shown in Figure 2. The BJT has 3 pins, one attached to each layer. The base pin is attached to the middle layer, with the collector and emitter pins attached to the other layers. The differences between the pins will be explained later.

Figure 2: PNP and NPN BJT transistors showing the p-type and n-type semi-conductor material layer configurations.

Like with the p-n junction diode, when a p-type semi-conductor material is joined to an n-type semiconductor material, a depleted zone is created at the junction. As shown in Figure 3, a BJT has two such junctions and two such depleted zones. This is where the BJT, bipolar junction transistor, gets its name from.

Figure 3: The PNP and NPN BJT showing their depleted zones while at rest.

When a voltage differential is applied across the collector and emitter pins as shown in Figure 4, the electrons become attracted to the positively charged pin, the collector pin. This shrinks the size of one of the depleted zones but grows it for the other. In this configuration, no current can flow across the collector and emitter pins.

Figure 4: The shrinking and growing depleted zones for BJT without forward biasing.

If a positive charge is also applied to the base pin, more electrons will be attracted to the p-type semi-conductor filling some of its holes. In fact, when the voltage at the base pin reaches the forward biasing voltage, the p-type semi-conductor will have attracted enough electrons to essentially become an n-type semi-conductor and the depleted region will disappear and current will be allowed to flow as shown in Figure 5.

Figure 5: The disappearing depleted region as the base pin voltage reaches near forward biasing voltage.

When performing circuit analysis, we often think of current flowing from positive to negative like in Figure 6, like the current is flow of positive charge. In reality, the current flows with the electrons, from negative to positive as shown in Figure 5. Don't let this confused you. The schematic symbols for NPN and PNP transistors in Figure 7 show the flow of current as we would assume when performing a circuit analysis from base to emitter for NPN and from emitter to base for PNP.

Figure 6: The forward biased transistor's depletion zones have disappeared and electrons are allowed to flow.

PNP transistors behave much the same way as NPN transistors. Applying a positive voltage across the emitter and collector causes the reduction of one depletion zone and the growth of the other. If a voltage lower than at the emitter by an amount less than the forward bias voltage is applied to the base, the depletion zone will disappear and allow current to flow from the emitter to the collector.

Figure 7: NPN and PNP type Bipolar Junction Transistors (BJT) schematic diagram symbols. Shows the current flow from base to emitter for NPN type and 'current flow' from emitter to base for PNP types.

As mentioned earlier, transistors don't only act like switches allowing current to flow when forward biased; they also act like current amplifiers. That is, the transistors will allow current to flow across collector/emitter by an amount equal to the base pin current times the transistor's gain. For example, a 2N2222 can have gains of around 150 times the base current. This means that allowing a current of 1mA to pass through the base pin will allow a current of 150 mA across the collector/emitter pins. The gain can of course be limited by current limiting resistance.

One important thing to note is that most BJTs don't usually operate in a symmetrical fashion. That is, the collector and emitter pins can generally be interchanged however the current gain for current flow from the collector to the emitter is not identical to the current gain for flow from the emitter to the collector. This asymmetric behaviour occurs because the collector/emitter semi-conductor materials are each doped with different doping ratios. This is done to allow for a large collector-base reverse bias voltage that must be overcome before the collector-base junction breaks down.


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