Physics of semiconductor devices
Andreas Dörfler
Students project
Bipolar transistors
Exam question:
How does a bipolar transistor works?
This exam question is separated in two parts:
a) How does a bipolar transistor works?
b) What is the Early effect in a bipolar transistor?
Solution:
a) How does a bipolar transistor works?
Basically bipolar transistors consist out of two pn- junctions and have three contacts (base, emitter
and collector).
Figure 1: npn- Transistor
First pn- junction is located between E emitter and B the base. This pn- junction is forward biased and
had a thin depletion width. The second pn- junction between base B and collector C is reverse biased
and have a larger depletion width. The area outside the depletion regions are field free this means
particularly for the region in the middle of the two pn- junctions, the base. The polarization of a
bipolar transistor is called active mode. In the figure 2 the band structure of an npn- transistor is
shown.
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Physics of semiconductor devices
Andreas Dörfler
Students project
Figure 2: band- structure of a npn- bipolar transistor
Electrons flow from the emitter through the left pn- junction in the base. The most of these electrons
can flow directly to the base-collector-junction. In this junction the electric field is strong because the
pn-junction is reverse biased. The collector with his positive polarization absorbs the electrons from
the emitter. For this reason the emitter must transmit electrons constantly. Also a charge current
(npn: holes) must flow in the base.
The main part of the electron current flows from the emitter to the collector but a small part is
needed in the base for the recombination with the holes from the base connection. This part is
necessary for the function of the transistor. The more electrons are available, the greater is the
probability that a hole meets an electron and recombined. The (bigger) electron current in the
emitter is directly proportional to the (small) hole current of the base.
b) What is the Early- effect in a bipolar transistor?
Early effect or base width modulation:
The early effect is the variation in the width of the base in a bipolar transistor due to a variation in
the applied base-to-collector voltage. For example a greater reverse bias across the collector- base
junction increases the collector-base depletion width. If VCE increases VCB increases too.
E
n
p
n
C
Weff
+
-
B
VBE
-
+
VCE1
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Depletion width
Physics of semiconductor devices
E
n
Andreas Dörfler
p
n
Students project
C
Weff
+
-
B
VBE
-
+
Depletion width
VCE2
VBE < VCE1 << VCE2
The emitter-base junction is unchanged because the voltage Vbe is the same.
Base narrowing has two consequences that affect the current:
-
There is a lesser chance for recombination within the “smaller” base region.
The charge gradient is increased across the base, and consequently, the current of minority
carriers injected across the emitter junction increases.
To countervail the Early- effect is a lightly doping of the collector region and a heavy doping of the
emitter region.
Early- voltage:
-VA
Figure 3: Early- voltage
Tangents to the characteristics at large voltages extrapolate backward to intercept the voltage axis at
a voltage called EARLY- voltage VA.
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Physics of semiconductor devices
Andreas Dörfler
Students project
Source:
Figure 1: Frank Thuselt; Physik der Halbleiterbauelemente; Kapitel 5.
Figure 2: Frank Thuselt; Physik der Halbleiterbauelemente; Kapitel 5.
Figure 3: Wikipedia; http://de.wikipedia.org/wiki/Early-Effekt
How does a transistor or works? : Frank Thuselt; Physik der Halbleiterbauelemente; Kapitel 5.
Early Effect: http://en.wikipedia.org/wiki/Early_effect
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