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                    Semiconductors and Semiconductor Devices

The crystalline materials are classified by their conductance to metals, insulators and semiconductors.

In metals, the valence electrons are shared by all metal ions, the electrons can move freely in
the crystal and drifted by the effect of an electric field. The high concentration of free
electrons makes the metal a good conductor.

In insulators, the electrons are localized to their parent atoms, molecules or ions. The electric
field can not remove the electrons from the bonds, they can not take part in conduction.

The electrons of a semiconductor also belong to particular bonds but they can be relatively
easily removed by thermal excitation even at room temperature and move freely in the
semiconductor crystal. When an electron gets free from a bond, sooner or later its empty
place will be filled with an electron from a neighbouring bond. This empty place will
migrate in the crystal, like a bubble. This missing electron is called a free hole, and it is
considered a free carrier, like the free electron, only with positive charge.

The number of free carriers in a semiconductor is very low (~1014 particles/cm3) and their
number increases with increasing temperature. Therefore the conductivity of semiconductors
increases with temperature in contrast with the metals.
There are elementary semiconductors like silicon and germanium, and compound
semiconductors, like GaAs, InSb and other.
A pure semiconductor contains equal number of free electrons and holes, produced by thermal
excitation. Such a crystal is called intrinsic semiconductor.
Each atom of elementary semiconductors like silicon and germanium is connected to its four
neighbour by covalent bonds. The number of free carriers in silicon or germanium can be
increased by adding group III (boron for example) or group V impurity (arsenic or
phosphorus) atoms to the crystal. These impurities substitute for the silicon or germanium
atoms.




          Donor and acceptor atoms and free electrons and holes in a silicon crystal
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A five-valence atom is connected by four bonds to its neighbours but its fifth valence electron
is superficial, not needed to the bonds. It is only loosely bound to the parent atom and easily
removed, contributing to the free electrons. Such a group -V impurity atom is called donor,
and the semiconductor containing donor atoms is called n-type because the majority of the
free carriers are electrons.
Substituting an atom by a three-valence atom like boron, one electron is missing to the four
bounds. This place of the missing electron is filled by an electron, the empty place moves
around in the crystal as a free hole. Such group-III impurity atoms are called acceptors, and
the crystal having free holes as majority carriers is called p-type.

The p-n junction

If one side of the silicon (germanium) crystal is p-type and the other is n-type, the boundary
between these region is called p-n junction.




The free carriers at each side behave as gas, they diffuse to the other side, and their they
recombine with the other type of carriers. While the bulk of both sides is electrically neutral,
having equal number of negative acceptor ions and positive holes in the P sides, and equal
number of positive donor atoms and free electrons in the N side, there are no free carriers in
the vicinity of the junction. These region is called depleted region, and its contains localized
negative acceptor ions at the P side and positive donor ions at the N side. This charge
distribution produces an electric field near the junction which prevents other free carriers to
cross the junction.

                                   The p-n junction diode
If we connect leads to both sides of the semiconductor piece we get a p-n junction diode.




                                         Picture of some diodes: The n part is shown by a strip.
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Rectifying property of the p-n junction diode



                                                 Forward bias: Connecting the positive
                                                 terminal of the battery to the p side of diode,
                                                 the accumulated charges at the junction are
                                                 neutralized by the charges supplied from the
                                                 battery. The depleted region shrinks, current
                                                 can flow.




Reverse bias: Connecting the positive pole of the
battery to the n side of the diode, the battery sucks
away more free carriers from the vicinity of the
pn junction. The depleted region becomes wider.
The electric field built up by the space charge at
the junction prevents the free carriers to cross the
junction. No. or very low current, due to minority carriers, can flow.


Voltage-current characteristic of a p-n junction diode




The I-V characteristic of a semiconductor diode can be approximated by two regions of
operation. Below a certain forward voltage, the depletion layer has significant width, and the
diode is closed. As the voltage is increased, at some stage the diode will become conductive.
                                                                                                 4

In a normal silicon diode at rated currents, the voltage drop across a conducting diode is
approximately 0.6 to 0.7 volts.
In the reverse bias region for a normal P-N rectifier diode, the current through the device is
very low (in the µA range) for all reverse voltages up to a point called the peak-inverse-
voltage (PIV). Beyond this point a process called breakdown occurs which causes the device
to be damaged by the high current. Special purpose diodes like the avalanche or zener diodes
have a deliberate breakdown at a certain voltage (called zener voltage). They are used to
stabilize voltage.

                                Types of semiconductor diode




            Diode              Light-Emitting             Zener Diode           Schottky Diode
                               Diode


                                        Diode symbols

Normal (p-n) diodes which operate as described above. Usually made of doped silicon or,
more rarely, germanium.
Zener diodes can be made to conduct backwards. This effect, called Zener breakdown,
occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage
reference.
Avalanche diodes conduct in the reverse direction when the reverse bias voltage exceeds the
breakdown voltage. These are electrically very similar to Zener diodes, and are often
mistakenly called Zener diodes, but break down by a different mechanism, the Avalanche
Effect. This occurs when the reverse electric field across the p-n junction causes a wave of
ionization, reminiscent of an avalanche, leading to a large current. Avalanche diodes are
designed to break down at a well-defined reverse voltage without being destroyed.
Photodiodes: these have wide, transparent junctions. Photons can push electrons over the
junction, causing a current to flow. Photo diodes can be used as solar cells, and in photometry.
If a photon doesn't have enough energy, it will not overcome the band gap, and will pass
through the junction.
Light-emitting diodes (LEDs) In a diode formed from an direct band-gap semiconductor,
such as gallium arsenide, carriers that cross the junction emit photons when they recombine
with the majority carrier on the other side. Depending on the material, wavelengths (or colors)
from the infrared to the near ultraviolet may be produced. The forward potential of these
diodes depends on the wavelength of the emitted photons: 1.2 V corresponds to red, 2.4 to
violet. The first LED's were red and yellow, and higher-frequency diodes have been
developed over time. All LEDs are monochromatic; 'white' LED's are actually combinations
of three LED's of a different color, or a blue LED with a yellow scintillator coating.
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Laser diodes When an LED-like structure is contained in a resonant cavity formed by
polishing the parallel end faces, a laser can be formed. Laser diodes are commonly used in
optical storage devices and for high speed optical communication.
Point Contact Diode This works the same as the junction semiconductor diodes described
above, but its construction is simpler. A block of n-type semiconductor is built, and a
conducting sharp-point contact made with some group-3 metal is placed in contact with the
semiconductor. Some metal migrates into the semiconductor to make a small region of p-type
semiconductor near the contact.
Varicap or varactor diodes These are used as voltage-controlled capacitors.

                                   Applications of diodes
Power conversion from AC to DC

A half wave rectifier can be constructed from a single diode where it is used to convert
alternating current electricity into direct current, by removing either the negative or positive
portion of the AC input waveform as shown in the oscilloscope picture. The output voltage can
be smoothed by connecting a large enough capacitor.




A special arrangement of four diodes (Graetz Bridge) which uses both the positive and
negative excursions of the AC input voltage is a full wave rectifier.




A full-wave rectifier. If A is positive and B negative, the current flows in the direction
ADFECB. If B is positive and A is negative the current direction is BDFECA. VFE  0
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Full-wave rectification                              Full wave rectification with smoothing

Radio demodulation
The first use for the diode was the demodulation of amplitude modulated (AM) radio
broadcasts. The history of this discovery is treated in depth in the radio article. In summary,
an AM signal consists of alternating positive and negative peaks of voltage, whose amplitude
or 'envelope' is proportional to the original audio signal, but whose average value is zero. The
diode rectifies the AM signal (i.e. it eliminates peaks of one polarity), leaving a signal whose
average amplitude is the desired audio signal. The average value is extracted using a simple
filter and fed into a loudspeaker which generates sound.

Logic gates. Diodes can be combined with other components to construct AND and OR logic
gates.


                                   The Bipolar Transistor
The transistors are three-terminal devices. There are two pn junctions in a bipolar transistor.
We can imagine it as two diodes, joined back-to-back. The transistor consist of a piece of
semiconductor with one highly doped collector region, a very thin (~10–6 m ) oppositely
doped base layer and a heavily doped collector region with the same type doping as the
collector.




Normally the EB diode is forward biased and the BC diode reverse biased. If it is a pnp
transistor, the base is positive with respect to the collector (~0,6 V) and the emitter is positive
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with respect to the base.(several volts ) The current of free holes can flow easily from the
emitter into the base, and as the base is very thin, most of the holes arrive to the BC junction.
Only a fraction flows out at the base lead. For the holes, the BC bias is attracting so they pass
into the collector and from the collector they flow across the external circuit back into the
emitter. The npn transistors have opposite doping and the base is positively biased with
respect to the emitter and the collector is positive with respect to the base. The symbol for
both types of transistor:


                                         Symbols for bipolar transistors




         High-power npn silicon transistors            Low-power high-frequency transistor

The sum of the base current and the collector current is equal to the emitter current, and the
collector current is proportional to the emitter current.
                                                          
I E  IC  I B           IC    I E  IC                  IC    I B
                                                         1
Typical values are ~100 for  and ~ 0.99 for .

Transistor characteristics

The transistor is a current amplifier as small variations of the base current will produce large
variations in the collector current. Moreover, the collector current is practically independent
on the CE voltage if it is higher than ~1 volt so the transistor behaves as a current source. The
transistor is also applied as a voltage amplifier: Slight variations of VBE will produce
considerable changes in the IB which in turn, produces high variation in IE. Connecting the
collector across a resistor to the battery, we can get voltage variation across the resistor, much
larger than the variation of VBE.
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As the BE characteristic of the transistor is like that of a diode, the input resistance of a the
simple transistor amplifier is very low. That allows only low voltage amplification with a
single transistor possible.

Example

 = 100 of the npn transistor in the picture, and the B-E voltage is approximately 0.6 V. The
sliding contact of the potentiometer is set so as the voltmeter at the input reads 1.7 V.
    a. How much is the output voltage?
    b. How much would the output voltage change if the input voltage is varied by 0.1 V?


                                            a. Vin = IB22 k+VBE =1.7 V  IB = 0.05 mA,
                                            IC = IB = 5 mA. The voltage across the 1,2 k
                                            resistor is 1.2 k IC = 5 V. The potential drops
                                            from 12 V by 6 V, so the output voltage is
                                            Vout = 6 V.
                                            b. In the same way, we get the output voltage
                                            when the input voltage is 1.8 or 1.6 V.
                                            If Vin = 1.6 V Vout = 7.45 V. If Vin = 1.8 V Vout
                                            = 6.55 V. So the output voltage changes between
                                            6.55 and 7.45 V. If the the peak-to peak value of
                                            an input AC signal is 0.2 V, that of the output
                                            signal is 0.9 V. The transistor works as a voltage
amplifier, and the input and output signals are of opposite phase.


                             The field-effect transistor (FET)

In application, the bipolar junction transistor has the disadvantage of a low input impedance
because the base of the transistor is the signal input and the base-emitter diode is forward
biased. Another device with the input diode junction reversed biased, is called a "field effect
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transistor" or a "junction field effect transistor", JFET. With the reverse biased input junction,
it has a very high input impedance. Having a high input impedance minimizes the interference
with loading the signal source.

                                                   The field-effect transistor (FET) is a
                                                   transistor that relies on an electric field to
                                                   control the shape and hence the conductivity
                                                   of a 'channel' in a semiconductor material. A
                                                   FET has three terminals, which are known as
                                                   the Gate (G), Drain (D) and Source (S).
                                                   The voltage applied between the gate and
                                                   source terminals modulates the current
                                                   between them.




Structure of a JFET transistor                    symbol of n-channel and p-channel JFET

In case of an n-channel JFET shown in the figure, electrons flow from S to the D if VDS > 0.
Making the potential of G negative with respect to S, the depletion layer widens and the
resistance of the channel between Source and Drain increases – less current can flow through
the channel.