A
two-terminal semiconductor (rectifying) device, that exhibits a nonlinear current-voltage characteristics. The function of a diode is to allow current in
one direction and to block current in the opposite
direction. The terminals of a diode arecalled
the anode and cathode. There are two kinds of semiconductor diodes:
a P-N junction diode,which forms
an electrical barrier at the interface between N- and P-type semiconductor layers,
and a Schottky
diode, whose barrier is formed between metal
and semiconductor regions. But this discussion
really ought to start with a bit semiconductors as materials. Semiconductors are
crystals that, in their pure state, are resistive (that is, their electrical
properties lie between those of conductors and insulators) -- but when the
proper impurities are added (this process is called doping) in
trace amounts (often measured in parts per billion), display interesting and
useful properties.
History of Diode :
The oldest ancestor of semiconductor devices was
the crystal detector, used in early wireless radios. This device (patented by a
German scientist, Ferdinand Braun, in 1899) was made of a single metal wire
(fondly called a "cat's whisker") touching against
a semiconductor crystal. The result was a
"rectifying diode" (so called because it has two terminals),
which lets current through easily one way, but hinders flow the other
way. By 1930, though, vacuum-tube diodes had all but replaced the
smaller but much quirkier crystal detector. The crystal and "cat's
whisker" were left to languish as a kids' toy in the form of "crystal
radios."
The development of radar
during World War II did much to revive the fortunes of crystal detectors (and,
as a result, that of semiconductors) -- although temperamental, crystals
were better than vacuum-tube diodes at rectifying the high
frequencies used by radar. So, during the war, much effort was put into
improving the semiconductors, mostly silicon and germanium, used in crystal
detectors. At about the same time, Russell Ohl
at Bell Laboratories discovered that these materials could be "doped"
with small amounts of foreign "impurity" atoms to create interesting
new properties.
Depending on the
selection of impurities (often called dopants)
added, semiconductor material of two electricallly-different types
can be created -- one that is electron-rich (called N-type, where N
stands forNegative), or one that is electron-poor
(called P-type, where P stands for Positive). Most of the
"magic" of semiconductor devices occurs at the boundary
between P-type and N-type semiconductor material --
such a boundary is called a P-N junction. Ohl and his colleagues found that
such a P-N junction made an effective diode.
Like many components,
diodes have a positive side or leg (a.k.a, their anode), and a negative
side (cathode). When the voltage on the anode is higher than on
the cathode then current flows through the diode
(the resistance is very low). When the voltage is lower on
the anode than on the cathode then
the current does not flow (the resistance is very high).
An easy way to remember
this is to look at the symbol for a diode -- the "arrow" in the diode
symbol points the direction in which it
allows current (hole flow) to flow.
The cathode of a diode is generally marked with a
line next to it (on the diode body). You can see a similar line in the
schematic symbols, above.
How Diodes work?
The diode operates when a voltage signal is applied across its terminals. The application of a DC voltage to make the diode operate in a circuit is called as ‘Biasing’. As already mentioned above the diode resembles to that of a one way switch so it can either be in a state of conduction or in a state of non conduction. The ‘ON’ state of a diode is achieved by ‘Forward biasing’ which means that positive or higher potential is applied to the anode and negative or lower potential is applied at the cathode of the diode. In other words, the ‘ON’ state of diode has the applied current in the same direction of the arrow head. The ‘OFF’ state of a diode is achieved by ‘Reverse biasing’ which means that positive or higher potential is applied to the cathode and negative or lower potential is applied at the anode of the diode. In other words, the ‘OFF’ state of diode has the applied current in the opposite direction of the arrow head. During ‘ON’ state, the practical diode offers a resistance called as the ‘Forward resistance’. The diode requires a forward bias voltage to switch to the ‘ON’ condition which is called Cut-in-voltage. The diode starts conducting in reverse biased mode when the reverse bias voltage exceeds its limit which is called as the Breakdown voltage. The diode remains in ‘OFF’ state when no voltage is applied across it.
A simple p-n juction diode is
fabricated by doping p and n type layers on a silicon or germanium wafer. The
germanium and silicon materials are prefered for diode fabrication because:
· They
are available in high purity. · Slight
doping like one atom per ten million atoms of a desired impurity can change the
conductivity to a considerable level.· The
properties of these materials change on applying heat and light and
hence it is important in the devlopment of heat and light sensetive
devices.
Diode circuit voltage measurements: (a) Forward biased. (b)
Reverse biased.
A forward-biased diode conducts current
and drops a small voltage across it, leaving most of the battery voltage
dropped across the lamp. If the battery’s polarity is reversed, the diode
becomes reverse-biased, and drops all of
the battery’s voltage leaving none for the lamp. If we consider the diode to be
a self-actuating switch (closed in the forward-bias mode and open in the
reverse-bias mode), this behavior makes sense. The most substantial difference
is that the diode drops a lot more voltage when conducting than the average
mechanical switch (0.7 volts versus tens of millivolts).
This forward-bias voltage drop exhibited
by the diode is due to the action of the depletion region formed by the P-N
junction under the influence of an applied voltage. If no voltage applied is
across a semiconductor diode, a thin depletion region exists around the region
of the P-N junction, preventing current flow. (Figure below (a)) The depletion region is almost
devoid of available charge carriers, and acts as an insulator:
Diode representations: (a) PN-junction model, (b) schematic symbol.
The schematic symbol of the diode is
shown in Figure above (b) such that the anode (pointing end)
corresponds to the P-type semiconductor at (a). The cathode bar, non-pointing
end, at (b) corresponds to the N-type material at (a). Also note that the
cathode stripe on the physical part (c) corresponds to the cathode on the
symbol.
If a reverse-biasing voltage is applied
across the P-N junction, this depletion region expands, further resisting any
current through it. (Figure below)
Depletion region expands with reverse bias.
Conversely, if a forward-biasing voltage
is applied across the P-N junction, the depletion region collapses becoming
thinner. The diode becomes less resistive to current through it. In order for a
sustained current to go through the diode; though, the depletion region must be
fully collapsed by the applied voltage. This takes a certain minimum voltage to
accomplish, called the forward voltage as illustrated in Figure below.
Inceasing forward bias from (a) to (b)
decreases depletion region thickness.
For silicon diodes, the typical forward
voltage is 0.7 volts, nominal. For germanium diodes, the forward voltage is
only 0.3 volts. The chemical constituency of the P-N junction comprising the
diode accounts for its nominal forward voltage figure, which is why silicon and
germanium diodes have such different forward voltages. Forward voltage drop remains
approximately constant for a wide range of diode currents, meaning that diode
voltage drop is not like that of a resistor or even a normal (closed) switch.
For most simplified circuit analysis, the voltage drop across a conducting
diode may be considered constant at the nominal figure and not related to the
amount of current.
Actually, forward voltage drop is more complex. An equation
describes the exact current through a diode, given the voltage dropped across
the junction, the temperature of the junction, and several physical constants.
It is commonly known as thediode equation:
The term kT/q describes the voltage
produced within the P-N junction due to the action of temperature, and is
called thethermal voltage, or Vt of the junction. At room temperature,
this is about 26 millivolts. Knowing this, and assuming a “nonideality”
coefficient of 1, we may simplify the diode equation and re-write it as such:
You need not be familiar with the “diode
equation” to analyze simple diode circuits. Just understand that the voltage
dropped across a current-conducting diode does change
with the amount of current going through it, but that this change is fairly
small over a wide range of currents. This is why many textbooks simply say the
voltage drop across a conducting, semiconductor diode remains constant at 0.7
volts for silicon and 0.3 volts for germanium. However, some circuits
intentionally make use of the P-N junction’s inherent exponential
current/voltage relationship and thus can only be understood in the context of
this equation. Also, since temperature is a factor in the diode equation, a
forward-biased P-N junction may also be used as a temperature-sensing device,
and thus can only be understood if one has a conceptual grasp on this
mathematical relationship.
A reverse-biased diode prevents current
from going through it, due to the expanded depletion region. In actuality, a
very small amount of current can and does go through a reverse-biased diode,
called the leakage
current, but it can be ignored for most purposes. The ability of a diode to
withstand reverse-bias voltages is limited, as it is for any insulator. If the
applied reverse-bias voltage becomes too great, the diode will experience a
condition known as breakdown (Figure below),
which is usually destructive. A diode’s maximum reverse-bias voltage rating is
known as the Peak
Inverse Voltage, or PIV, and may be obtained from the
manufacturer. Like forward voltage, the PIV rating of a diode varies with
temperature, except that PIV increases with
increased temperature and decreases as
the diode becomes cooler—exactly opposite that of forward voltage.
#Team Circuitready
No comments:
Post a Comment