Bipolar Junction Transistor (BJT) Image Image

A Bipolar Junction Transistor (a.k.a. a BJT or Bipolar Transistor) is an active semiconductor device formed by two P-N junctions whose function is amplification of an electric current.

Bipolar transistors are made from 3 sections of semiconductor material (alternating P-type and N-type), with 2 resulting P-N junctions. Schematically, a bipolar transistor can be thought of in this fashion:

BJT layers

One P-N junction is between the emitter and the base; the other P-N junction is between the collector and the base. Note that the emitter and collector are usually doped somewhat differently, so they are rarely electrically interchangeable. While the terms "collector" and "emitter" go back to vacuum tube days, the base derives its name from the first point-contact transistors -- here the center connection also formed the mechanical base for the structure. In modern practice, the base region is made as thin as possible to achieve reasonable levels of current gain; it is often only about one millionth of a meter thick.

Bipolar transistors are classified as either NPN or PNP according to the arrangement of their N-type and P-type materials. Their basic construction and chemical treatment is implied by their names. So an NPN transistors is formed by introducing a thin region of P-type material between two regions of N-type material.

NPN layers

On the other hand, a PNP transistor is formed by introducing a thin region of N-type material between two regions of P-type material.

PNP layers

Since the majority and minority current carriers are different for N-type and P-type materials, it stands to reason that the internal operation of the NPN and PNP transistors will also be different. These two basic types of transistors along with their circuit symbols are shown here:

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NPN
PNP

Note that the two symbols are subtly different. The vertical line represents the base (B), the angular line with the arrow on it represents the emitter (E), and the other angular line represents the collector (C). The direction of the arrow on the emitter distinguishes (graphically) the NPN from the PNP transistor. If the arrow points in, (Points iN) the transistor is a PNP. On the other hand if the arrow points out, the transistor is an NPN (Not Pointing iN).

Bear in mind that the arrow always points in the direction of hole flow (current), or from the P-type to N-type sections, no matter whether the P-type section is the emitter or base. On the other hand, electron flow is always "against" the arrow, just like in the junction diode.

As a result, a PNP transistor is "triggered" when its base is pulled low; an NPN transistor is "triggered" when its base is brought high.

Note that the bipolar transistor is a current-amplifying device, unlike the vacuum tube and the field-effect transistor (FET), both of which depend upon voltage changes to operate. It is the amount of current flowing in the base circuit that controls the amount of current flowing in the collector circuit.

Wilf Rigter has graciously contributed the following explanation of bipolar transistor behavior in circuits:


You have to think in terms of circuit configurations and the voltage and current in each lead when discussing how transistors behave. There are 3 configurations -- the emitter follower which is a current amplifier but has no voltage gain, the common emitter amplifier which has current and voltage gain, and the common base amplifier which has voltage gain but no current gain.

  1. In an emitter follower circuit with the collector connected to +V and a load connected between the emitter and ground, the voltage applied to the base minus the base emitter forward voltage drop (~0.6 V) will appear across the load (i.e., 5 V base = 4.4 V emitter). The only caveat is that the voltage source at the base must be able to supply about 5% of the load current without appreciable voltage drop. This is a non-inverting voltage follower circuit.

  2. In a common emitter circuit with the emitter connected to ground and the load connected between the collector and +V, a voltage connected to the base which exceeds the base emitter forward voltage (0.6V) will rapidly turn on the transistor in proportion to the voltage rise as the base emitter current rapidly increases for a small increase in base voltage. The base voltage source must be able to supply about 5% of the load current into the base emitter diode (i.e., short circuit) for the circuit to develop a large voltage across the load. This is an inverting voltage amplifier circuit.

  3. In a common base circuit with the base grounded (or at a reference voltage) and the load connected between the collector and +V, a control voltage connected to the emitter which is more negative than the base emitter forward voltage (~0.6V) causes the transistor to rapidly turn on. The control voltage source must be able to supply about 105% of the load current to develop the full voltage across the load. This is a non-inverting voltage amplifier circuit.

Note that operation of a bipolar transistor depends on the migration of both electrons and holes, in contrast to field-effect transistors, where only one polarity carrier predominates.

For bipolar transistor selection and comparison information, see the bipolar transistor section of the BEAM Reference Library's BEAM Pieces collection.


Legalities  Image 

Page author: Eric Seale  
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