Elements of Modern Physics
286
only into the conduction band on the p-side, but also tunnel into some of the
vacant levels on the p-side. This again gives rise to a large current. On the other
hand, if the forward bias is quite large, there will no longer be any overlap of
the valence band on the p-side and the conduction band on the n-side. The
resulting current which is due to electrons moving across the potential barrier
from the conduction band on the n-side to the conduction band on the p-side,
actually shows a decrease [Fig. 8.11(c)]. For still higher potentials, the current
will begin to increase again, as in the case of the ordinary diode. The main
characteristic of these tunnel or Esaki diodes is the negative-resistance section,
which is used in high-frequency oscillator circuits in the microwave region.
Transistor
An important application of semiconductor junctions is the transistor. It consists
of two semiconductor junctions close together, which serve as an amplifier of
current or voltage.
To be specific, consider a pnp transistor (similarly one can have an npn
transistor) which consists of three regions (Fig. 8.12). The first region is the
emitter, the small, narrow, middle region is the base, and the third region is the
collector. The equilibrium potential consists of a potential barrier in the n-region.
If now the base is connected to a small negative potential V b (the common
emitter may be taken to be at V e = 0), and the collector to a fairly large negative
potential V c , the potential across the two junctions is modified [Fig. 8.12(b)].
There is a forward bias across the emitter-base junction for the holes, and the
current across the junction is (Eq. (8.74)]
I = I 0 [exp (eV b /kT) – 1]
(8.75)
Since the base is very narrow (less than 10
–3
cm in width), most of the holes
that enter the base from the emitter roll down into the collectotor, though a few
of them will be annihilated by the electrons in the base. Thus, a major part of
the emitter current flows through the collector while only a small fraction of it
flows out from the base. In a general way, the collector current is controlled by
the changes in the barrier introduced by V b , and the changes in the base current
I b are amplified into the changes in the collector current I c . The amplification in
the current is estimated by
β = I c /I b
=
hole lifetime
base transit time
(8.76)
(holes annihilated in the base contribute to I b ) which in practice has a value of
about 100.
286
only into the conduction band on the p-side, but also tunnel into some of the
vacant levels on the p-side. This again gives rise to a large current. On the other
hand, if the forward bias is quite large, there will no longer be any overlap of
the valence band on the p-side and the conduction band on the n-side. The
resulting current which is due to electrons moving across the potential barrier
from the conduction band on the n-side to the conduction band on the p-side,
actually shows a decrease [Fig. 8.11(c)]. For still higher potentials, the current
will begin to increase again, as in the case of the ordinary diode. The main
characteristic of these tunnel or Esaki diodes is the negative-resistance section,
which is used in high-frequency oscillator circuits in the microwave region.
Transistor
An important application of semiconductor junctions is the transistor. It consists
of two semiconductor junctions close together, which serve as an amplifier of
current or voltage.
To be specific, consider a pnp transistor (similarly one can have an npn
transistor) which consists of three regions (Fig. 8.12). The first region is the
emitter, the small, narrow, middle region is the base, and the third region is the
collector. The equilibrium potential consists of a potential barrier in the n-region.
If now the base is connected to a small negative potential V b (the common
emitter may be taken to be at V e = 0), and the collector to a fairly large negative
potential V c , the potential across the two junctions is modified [Fig. 8.12(b)].
There is a forward bias across the emitter-base junction for the holes, and the
current across the junction is (Eq. (8.74)]
I = I 0 [exp (eV b /kT) – 1]
(8.75)
Since the base is very narrow (less than 10
–3
cm in width), most of the holes
that enter the base from the emitter roll down into the collectotor, though a few
of them will be annihilated by the electrons in the base. Thus, a major part of
the emitter current flows through the collector while only a small fraction of it
flows out from the base. In a general way, the collector current is controlled by
the changes in the barrier introduced by V b , and the changes in the base current
I b are amplified into the changes in the collector current I c . The amplification in
the current is estimated by
β = I c /I b
=
hole lifetime
base transit time
(8.76)
(holes annihilated in the base contribute to I b ) which in practice has a value of
about 100.