welcome to Professor mad electronics for beginners
today we are going to learn about Diodes
Simply diode is a two terminal electronic component
that only conducts current in one direction
diode's behavior is analogous to behavior of a
hydraulic device for a check where a cheque valve
A cheque valve allows a fluid flow through it in only one direction
here the two cheqe valves try to flow water in different directions
but current flow is permitted in only one direction
current flow is prohibited in other directions
tech coils are pressure operated devices
they open and allow flow if the pressure across them is of
the correct polarity to open the gate
if the pressure of the opposite polarity
the pressure difference across the cheque valve will close and hold the gate
so that no flow will occurs.
Same as cheque valves diode are also pressure operated devices
electron pressure is called as voltage
the most common kind of diode in modern circuit design is the semiconductor diode
semiconductor diodes are symbolized in schematic diagrams such as the figure
The direction of the diode symbol's arrow head
points at the direction of the currents in conventional flow
the opposite is true when electron flow is use,
where the direction of electron is always against the arrow head
here we have two simple circuits
look at the diodes carefully they are in opposite directions
let's see what will happen after the switch is on...
only one circuit is working
that is because the diode will either allow prevent current through the lamp
depending on the polarity of the applied voltage
when the polarity of the battery is such
that current is allowed to flow through the diode,
the diode is said to be forward biased
battery is backward and the diode blocks the current
the diode is said to be reverse biased
semiconductor diodes are the most commonly used diode type today
and those are the first semiconductor electronic devices
semiconductor diode is just a P-N Junction
that connected to two electrical terminals p-type and n-type
inside a single crystal of semiconductor
The N side contains free electrons
that can move through the conductor
this electrons can act as the charge carriers
since electrons are negatively charged we call them as
negative type or n type
this is a silicon lattice
here every silicon atom has four electrons in valence band
and they are all bonded to atoms
if we remove a silicon atom and replace it by using phosphorous atom
that has five electrons in valance band,
it (phosphorus atom) also has to behave like a silicon atom
By bonding four electrons with surrounding silicon atoms
but what happens to the final electron
it is not bonded to any atom. It is a free electron
those can move freely
this is how n-type semiconductors are created
now let's talk about p-type semiconductors
the P side contains excess of holes
an electron hole is the lack of an electron at a position where one could exist
as an example think about this silicon latices
every atom is bonded to four other atoms
what if we remove a silicon atom and introduce a boron atom
that only has three electrons in its valence band
Boron atoms try to behave like silicon atoms
but it needs one more electrons to compete for bonds like silicon
so it's searching for an electron
that is what we called as a hole,
simply a lack of electron
since in a normal atom or crystal lattice
the negative charge of the electron is balanced by the positive type of the atomic nuclei
the absence of an electron leaves the
net positive charge at the holes location
holes are not actually particles but rather quasi particles
this is how p-type semiconductors are made
such as that we can introduce new atoms into non conducting atomic lattice and make it
the conductor this process is called semiconductor doping
now we know about n-type and p-type semiconductors
let's see what happens in the PN Junction
at the junction the free electrons in the n-type are attracted to positive holes in the p-type
for ease of understand let's pay attention to only one electron
due to this electron movement total charge of N side becomes positive one
and P side becomes negative one
this happens to every electron near the junction
so in the n-type a region near the junction becomes positively charged
in the p-type the region near the junction becomes negatively charged
the result is a region near the junction
that acts to repel the mobile charges away from the junction
through the electric field V1
the region near the P-N in interfase lose their neutrality
and most of their mobile carriers
forming the depletion layer
the depletion region is devoid of available charges and carriers
and act as an insulator
right now let's talk about forward biasing
in forward biased configuration the p-type is connected
with the positive terminal and the n-type is connected with the negative
terminal the positive potential applied to the p-type material attracts the
electrons those are in the depletion layer while the negative potential
applied to the n-type material repels the electrons this goes neutralization
of the depletion layer reducing its width
with increasing forward bias voltage the depletion zone eventually becomes thin
enough that the zones electric field cannot counter react charge carrier
motion that was the PN Junction this takes a certain minimum voltage to
accomplish called the "forward voltage". For silicon diodes the typical forward
voltage is 0.7v for germanium diodes the forward voltage is only 0.3v if
an external voltage that is greater than and opposite to the built-in voltage is
applied the current will flow and the diode is said to be turned on
right we know the depletion layer normally
prevents the current flow through the diode let's see what happens in reverse
bias configuration the p-type is connected with the negative terminal and
the n-type is connected with the positive terminal we say the diode is in
reverse bias more electrons in the n-type material near the junction are
pulled away from the junction and combined with the holes in the p-type
near the junction leaving behind the charged ion and causing the width of the
depletion layer to increase if a reverse biasing voltage is applied across the PN
Junction this depletion region expand further resisting any current through it
let's try to understand the characteristic curve of PN Junction
diode the characteristic curve of a junction diode is also called an IV
curve it is typically a graph showing the current flow at a different forward
voltages the current is typically on the y-axis and the voltage on the x-axis
We know diode has three regions p-type end n-type end and the middle depletion
layer let's set the n-type in at the 0v and make the voltage of P in
a variable we are using the voltage of P end to plot the graph
if we increase the voltage of the P end we can see the depletion layer is
becoming thinner because the diode is at forward bias but we still can't see any
current grow through the diode let's increase the voltage further more
now huge current can go through the diode because the depletion layer vanished it
is now like a conductor let zoom in the current flow started nearly at 0.7 volt
that is the forward voltage of this diode we know increasing the voltage of
the P end makes the diode a conductor
what happens to the diode if we decrease the voltage of P end , see nothing diode is
now reverse bias the depletion layer expands in the decreasing of the voltage
this blocks the current flow we can say any increase in reverse bias voltage
will keep the reverse current to almost zero until it goes to the breakdown
this is the maximum reverse voltage that diode can handle we can clearly identify three
regions of this curve forward region previous region and the breakdown region
the forward voltage is 0.7 volt for silicon diodes and 0.3 volt for
germanium diodes and normally the breakdown voltage can go up to minus 50
volts in practical cases diode allow very low current to go through it at the
reverse bias region it is called leakage current that is all about diodes see you
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