Bipolar Junction Transistor Circuits:Nonideal Effects and Ohmic Effects.

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Nonideal Effects

In addition to the desired effects that lead to amplification, there are some inherent undesired effects that detract from the amplification of the BJT stage. Some of the sources of these undesired effects are ohmic effects, the Early effect, and reactive effects. To accurately analyze the behavior of the BJT, these effects must be included.

Ohmic Effects

The metal connections to the semiconductor regions exhibit some ohmic resistance. The emitter contact resistance and collector contact resistance is often in the ohm range and does not affect the BJT operation in most applications. The base region is very narrow and offers little area for a metal contact. Furthermore, because this region is narrow and only lightly doped compared with the emitter, the ohmic resistance of the base region itself is rather high. The total resistance between the contact and the intrinsic base region may be 20–150 W. This resistance can become significant in determining the behavior of the BJT, especially at higher frequencies.

Base-Width Modulation (Early Effect)

The widths of the depletion regions are functions of the applied voltages. The collector voltage generally exhibits the largest voltage change and as this voltage changes, so also does the collector–base depletion region width. As the depletion layer extends further into the base region, the slope of the electron

Bipolar Junction Transistor Circuits-0009

distribution in the base region becomes greater since the width of the base region is decreased. A slightly steeper slope leads to a slightly more collector current. As reverse bias decreases, the base width increases and the current decreases. This effect is called base-width modulation and can be expressed in terms of the Early voltage [2], VA, by the expression

Bipolar Junction Transistor Circuits-0010

where CJ0 is the junction capacitance at zero bias, f the built-in junction barrier voltage, Vapp the applied junction voltage, and m a constant. For modern BJTs m is close to 0.33. The applied junction voltage has a positive sign for a forward bias and a negative sign for a reverse bias.

An increase in forward base–emitter voltage results in a higher density of electrons injected into the base region. The charge distribution in the base region changes with this voltage change and this leads to a capacitance called the diffusion capacitance. The diffusion capacitance is a function of the emitter current and can be written as where k2 is a constant for a given device.

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