ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 2
f. NPN Transistor Operation. Just as is the case of the PN
junction diode, the N material, comprising two end sections of
the NPN transistor, contains a number of free electrons, while
the center P section contains an excess number of holes. The
action at each junction between these sections is the same as
that previously described for the diode; that is, depletion
regions develop and the junction barrier appears. In order to
use the transistor as an amplifier, each of these junctions must
be modified by some external bias voltage. For the transistor
to function in this capacity, the first PN junction (emitter
base junction) is biased in the forward, or lowresistance,
direction. At the same time, the second PN junction (base
collector junction) is biased in the reverse, or high
resistance, direction.
(1) NPN Forwardbraised Junction. An important point to bring
out at this time, one which was not mentioned during the
explanation of the diode, is that the N material on one side of
the forwardbiased junction is more heavily doped than the P
material. This results in more current being carried across the
junction by the majority carrier electrons from the N material
than the majority carrier from the material. Therefore,
conduction through the forwardbiased junction, as shown in
figure 89 on the following page, is mainly by majority carrier
electrons from the N material (emitter).
With the emittertobase junction in the figure biased in the
forward direction, electrons leave the negative terminal of the
battery and enter the N material, they pass easily through the
emitter, cross over the junction, and combine with holes in the
P material (base). For each electron that fills a hole in the P
material, another electron will leave the P material (creating a
new hole) and the positive terminal of the battery.
(2) NPN ReverseBiased Junction. The second PN junction
(basetocollector), or reversebiased junction as it is called
(Figure 90 on page 126), blocks the majority (: current carriers
from crossing the junction. However, there is a very small
current that does pass through this junction. This current is
called minority current or everse urren . This current is
r
c
t
produced by the electron
124
Previous Page
