3.2 The Bipolar Junction Transistor

Structure

The Bipolar Junction Transistor, abbreviated by BJT, is the oldest type of transistor. It consists of three parts: the base, the emitter and the collector.  Each part is made of doped Silicon.  Emitter and collector are always of the same type (p- or n-), the base is of the opposite type: one speaks of npn-transistors, like in the figure below, or of pnp-transistors.  It is important for the proper operation of BJTs that the base is relatively small and lightly doped.  If only the structure is considered, one can say that the npn-transistor exists of two backwards coupled diodes (pn-junctions).  In many circumstances the base-emitter diode will indeed behave like a diode, but this is certainly not the case for the base-collector junction (see further).

BJT_structureFigure 3.4 Structure of an npn-transistor, resulting in two pn-junctions.

Without applied voltages, two space charge layers will emerge around each pn-junction, just like in a diode.  In case of an npn-transistor the free electrons in the emitter and the collector will diffuse to the base under the influence of concentration differences, and they will recombine with the holes in the base.  This process ends when the force of the emerging electric field has the same magnitude (but opposite polarity) as the diffusion force.

BJT_operation_0Figure 3.5 In an unconnected transistor, two space charge layers will emerge around the pn-junctions.

Active Mode

The slideshow below illustrates the operation of the transistor in a very common configuration.

In sum:

  1. If the knee voltage is applied between base and emitter, then the emitter emits electrons to the base, and
  2. If a sufficiently high voltage is applied between collector and emitter, then most electrons continue to the collector that collects the electrons, enabled by the electric field. Only a relative small amount of electrons leave the transistor at the base.
  3. Moreover, under these conditions the ratio of collector current over base current is approximately constant over a broad range of current value.

This operating mode is called the linear mode or the active mode. It is called the linear mode because there is a linear relationship between the collector current and the base current, often denoted by:

IC = β.IB

in which β is the current gain of the transistor, which depends on the transistor’s dimensions, materials and doping levels. The name active mode refers to the fact that the transistor is dissipating power, since there is a non-zero current through the transistor, and a non-zero voltage across the transistor. This power is converted into heat. The active mode is typically used in transistor amplifiers that are e.g. intended to convert small voltages into large ones.

Saturation Mode

Now suppose the collector-emitter voltage drops to low values, like a few tenths of a Volt. In that case the base-collector junction will become forward biased (but the voltage being still below the knee voltage), resulting in a very small space charge layer, and thus a very small electric field. The transistor no longer works in the active or linear mode.

BJT_operation_saturation

Figure 3.6 When the collector-emitter voltage drops to a few tenths of a Volt, the transistor operates no longer in the active area. The ratio of collector current over base current now becomes much smaller.

Due to the diminished force of the electric field, less electrons will reach the collector, and more electrons will leave the transistor at the base, compared with the linear area, and as a result:

IC < β.IB

The transistor is said to be in saturation mode. The collector current seems not to be able to “follow” the base current. At first sight, it might seem that the transistor performs worse in some way. However, this mode is very interesting because the collector current can still be relatively large (though smaller than β.IB) while the collector-emitter voltage is almost 0 V. This corresponds to the behavior of a switch: a non-zero current and no voltage. When using the transistor as a switch, the saturation mode is used.

In sum:

  1. If the knee voltage is applied between base and emitter, then the emitter emits electrons to the base, and
  2. If the voltage between collector and emitter is close to 0 V, then the transistor is in saturation mode.
  3. Under these conditions the ratio of collector current over base current is lower than in the active mode, but the collector current can still be relatively large (and still much larger than the base current).

Cut-off mode

Perhaps the easiest mode to understand is the cut-off mode:

  1. If the voltage between base and emitter is below the knee voltage, then the emitter does not emit electrons to the base.
  2. As a result, there are also no electrons to be collected by the collector, whatever the voltage between collector and emitter. So, all currents are 0 A.
  3. The transistor is now in cut-off mode.

BJT_operation_cutoff

Figure 3.7 When the base-emitter voltage drops below the knee voltage, the transistor is in cut-off mode. Whatever the voltage between collector and emitter, all currents will be 0 A.

Also this behavior is interesting, since it corresponds to the behavior of an open switch: no current and a non-zero voltage. The figure below summarizes the three operating modes.

BJT_operation_modes

Figure 3.8 The three operating modes of the BJT transistor. The arrows indicate the electron flow.

 Go to 3.3 BJT Characteristics

 

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