SCR ,R E
The current amplificationB or h
can beexpressedas the relationshipof I
The symbol B is used to state the large signal current amplification, while for small
signal current amplification, i.e. differential current amplification assuming a constant
, the symbol
Due to the interaction of majority andminority carriers, the component is described as a
bipolar (junction) transistor.
1.4.2 Field effect transistors (FETs)
Unlike bipolar (junction) transistors, in which two types of charge carriers contribute to
transporting the current, in a field effect transistor
only one type of charge carrier is
present. This type of transistor is therefore calleda unipolar transistor.
22.214.171.124 Junction fieldeffect transistors (JFETs)
Junction fieldeffect transistors are the easiest field effect transistor to construct.
A JFET is a normally-on component. If there is a positive voltageU
between the drain
and the source in an n-channel JFET and the gate terminal is open, current will flow
through the n-region between the drain and the source. In this case, the JFET behaves
likeanohmic resistance. For the resistance valueR
l: Length of conducting channel [cm]
A: Channel cross-section [cm
When the voltage U
between the drain and the source is positive, and the gate
terminal is joined to the source terminal, a space-charge region forms at the pn-junction
that pinches off the available current channel with the surface A along the length of the
n-zone. The current increases as the voltage U
increases until a point is reached at
which the available channel surface A can no longer carry any more current. In this
case, further increasing the voltage U
does not increase the current. The transistor is
in the pinch-off region, which is also the normal operating point of a JFET. The level of
the pinch-off voltage depends on the doping concentration and the physical width of the
Instead of connecting the gate terminal directly with the source contact, a negative
voltage can be applied between the gate and the source. This creates the opportunity of
actively modulating the channel width. The greater the setting of the negative voltage
The principle of the field effect transistor (FET) was discovered in 1926 by the Austro-Hungarian physicist J.E.
Lilienfeld (1881 - 1963), which makes the principle of FETs older than that of bipolar junction transistors.
However, it was almost four decades before the first transistor basedon the field effect could beused inpractice.