Early effect

Early voltage in the output characteristic field of a bipolar transistor with base width modulation ( Early effect )

Scheme of an npn bipolar transistor

pnp transistor
U _CE  (above) <U _CE  (below)

The Early effect , also called base width modulation , named after its discoverer James M. Early , describes in semiconductor technology the change in the effective base width W of a bipolar transistor due to the base-collector voltage U _BC , i.e. H. Expansion of the space charge zone of the base-collector diode: W  =  f ( U _BC ).

A similar effect can also be observed in the characteristics of field effect transistors . There this property is described as the so-called channel length modulation .

root cause

If the collector-emitter voltage U _{CE is} increased, the space charge zone (RLZ) of the collector-base pn junction widens and the width of the base is reduced.

${\displaystyle U_{CB}=U_{CE}-U_{BE}}$

impact

With the bipolar transistor, the Early effect causes the collector current to depend on the collector-emitter voltage U _CE , so the transistor is not an ideal current source . The characteristic curve runs linearly in the typical working range, as in the output characteristic curve field shown on the right, and its extrapolated point of intersection with the U _CE axis is referred to as the Early voltage U _EA . It is an important parameter for characterizing a bipolar transistor and for simulation, e.g. B. with SPICE , and the amount is between 15 V and 150 V depending on the transistor type.

The effects of the Early effect increase as the base width decreases, as the relative change in the space charge zones increases.

calculation

The calculation of the Early voltage can be derived from the straight line equation. Two points from the linear area are required. These are used in the formula.

${\displaystyle U_{EA}={\frac {U_{1}I_{2}-U_{2}I_{1}}{I_{2}-I_{1}}}}$

literature

• Electrical engineering. Electronics I. Structure of matter - semiconductor conduction mechanism - PN transition , Berlin, 1979, Beuth Verlag, ISBN 3-4103-8478-2
• Hartmut Schrenk, Bipolar Transistors , Heidelberg, 1994, Springer Verlag, ISBN 3-4862-5561-4
• Wolfgang Steimle, The bipolar transistor in linear circuits I. Basics, equivalent images, programs. , Munich 1984, Oldenbourg-Verlag, ISBN 3-4862-5561-4
• Donald A. Neamen, Semiconductor Physics & Devices, McGraw-Hill, ISBN 0-07-232107-5