Chapter 15
Semiconductors
Semiconductors are crystalline materials that are
characterized by specific energy bands for electrons.
Energy
Between the bands are gaps;
these gaps represent energies
Conduction band
that electrons cannot posses.
The last energy band is the
conduction band, where electrons
are mobile.
The next to the last band is the
valence band, which is the energy
level associated with electrons
involved in bonding.
Energy gap
Valence band
Energy gap
Second band
Energy gap
First band
Nucleus
Electron and hole current
At room temperature, some electrons have enough
energy to jump into the conduction band.
After jumping the gap, these electrons are free to drift throughout
the material and form electron current when a voltage is applied.
Electronhole pair
Energy
For every electron
in the conduction
band, a hole is left
behind in the
valence band.
Conduction band
Energy gap
Valence band
Heat
energy
Electron and hole current
The electrons in the conduction band and the holes in
the valence band are the charge carriers. In other words,
current in the conduction band is by electrons; current
in the valence band is by holes.
When an electron jumps to the conduction band, valence
electrons move from hole-to-hole in the valence band,
effectively creating “hole current” shown by gray arrows.
Si
Si
Si
Free
electron
Impurities
By adding certain impurities to pure (intrinsic) silicon,
more holes or more electrons can be produced within
the crystal.
To increase the number of conduction
band electrons, pentavalent impurities
are added, forming an n-type
semiconductor. These are elements to
the right of Si on the Periodic Table.
To increase the number of holes, trivalent
impurities are added, forming a p-type
semiconductor. These are elements to the
left of Si on the Periodic Table.
III IV V
B C N
Al Si P
Ga Ge As
In Sn Sb
The pn junction diode
When a pn junction is formed, electrons in the n-material
diffuse across the junction and recombine with holes in
the p-material. This action continues until the voltage of
the barrier repels further diffusion. Further diffusion
across the barrier requires the application of a voltage.
The pn junction is basically a diode,
which is a device that allows current
in only one direction. A few typical
diodes are shown.
Forward bias
When a pn junction is forward-biased, current is permitted.
The bias voltage pushes conduction-band electrons in the
n-region and holes in the p-region toward the junction
where they combine.
p-region n-region
The barrier potential in the depletion
region must be overcome in order
for the external source to cause
current. For a silicon diode, this is
about 0.7 V.
p
n
R
+
−
VBIAS
The forward-bias causes the depletion region to be narrow.
Reverse bias
When a pn junction is reverse-biased, the bias voltage
moves conduction-band electrons and holes away from the
junction, so current is prevented.
p-region n-region
The diode effectively acts as an
insulator. A relatively few electrons
manage to diffuse across the junction,
creating only a tiny reverse current.
p
n
R
+
−
VBIAS
The reverse-bias causes the depletion region to widen.
Diode characteristics
The forward and reverse characteristics are shown on
a V-I characteristic curve.
In the forward bias region, current
increases dramatically after the
barrier potential (0.7 V for Si) is
reached. The voltage across the
diode remains approximately
equal to the barrier potential.
The reverse-biased diode
effectively acts as an insulator
until breakdown is reached.
IF
VBR (breakdown)
Forward
bias
VR
0.7 V
Reverse
bias
Barrier
potential
IR
VF
Diode models
The characteristic curve for a diode can be approximated
by various models of diode behavior. The model you will
IF
use depends on your requirements.
The ideal model assumes the diode is
either an open or closed switch.
The practical model includes the VR
barrier voltage in the approximation.
Forward
bias
0.7 V
Reverse
bias
The complete model includes the
forward resistance of the diode.
IR
VF
Half-wave Rectifier
Rectifiers are circuits that convert ac to dc. Special
diodes, called rectifier diodes, are designed to handle the
higher current requirements in these circuits.
The half-wave rectifier
converts ac to pulsating
dc by acting as a closed
switch during the
positive alteration.
The diode acts as an
open switch during the
negative alteration.
+
D
−
RL
D
− +
RL
Full-wave Rectifier
The full-wave rectifier allows unidirectional current on
both alterations of the input. The center-tapped full-wave
rectifier uses two diodes and a center-tapped transformer.
The ac on each side of the center-tap is ½ of the total secondary
voltage. Only one diode will be biased on at a time.
D1
F
Vsec
2
Vsec
2
D2
RL
Bridge Rectifier
The bridge rectifier is a type of full-wave circuit that uses
four diodes. The bridge rectifier does not require a
center-tapped transformer.
At any instant, two of the diodes are conducting and two are off.
F
D3
D2
D1
D4
RL
Peak inverse voltage
Diodes must be able to withstand a reverse voltage when
they are reverse biased. This is called the peak inverse
voltage (PIV). The PIV depends on the type of rectifier
circuit and the maximum secondary voltage.
For example, in a full-wave circuit, if one diode is conducting
(assuming 0 V drop), the other diode has the secondary voltage
across it as you can see from applying KVL around the green path.
Notice that Vp(sec) = 2Vp(out) for
the full-wave circuit because
the output is referenced to the
center tap.
0V
Vsec
Peak inverse voltage
For the bridge rectifier, KVL can be applied to a loop that
includes two of the diodes. Assume the top diode is
conducting (ideally, 0 V) and the lower diode is off. The
secondary voltage will appear across the non-conducting
diode in the loop.
Notice that Vp(sec) = Vp(out) for the bridge because the output is
across the entire secondary.
0V
Vsec
Power supplies
By adding a filter and regulator to the basic rectifier, a
basic power supply is formed.
Typically, a large electrolytic capacitor is used as a filter before the
regulator, with a smaller one following the regulator to complete
filtering action.
IC regulator
F
D3
D1
7805
D2
D4
C1
1000 µF
C2
1 µF
Special-purpose diodes
Special purpose diodes include
Zener diodes – used for establishing a reference voltage
Varactor diodes – used as variable capacitors
Light-emitting diodes – used in displays
Photodiodes – used as light sensors