A diode is a crystalline piece of
semiconductor material which is formed by the combination of p type
semiconductor and n type semiconductor. (refer diode construction).
As n-type semiconductor has plenty of free
electrons to donate and p-type semiconductor has plenty of vacant positions to
accept electrons. So, if we excite or apply the voltage to the diode terminal
then these free electrons are also excited and ready to move. The movement of
these free electrons decides the direction of conventional current flow and the
behavior and working of the diode.
Here in this article, we will discuss this
phenomenon in detail which will help us to determine the behavior of a PN
junction diode.
Diode Working
A p-n junction diode working depends on
its Biasing. Biasing means applying voltage at diode’s terminal. Diode works in
different manner in different biasing conditions due to which diode is also characterized
as unilateral or unidirectional device.
Forward Bias Diode
A p-n junction diode is said to be Forward
biased when its positive terminal i.e. anode (p-type semiconductor side) is
connected to positive terminal of electrical supply and its negative terminal
i.e. cathode (n-type semiconductor side) is connected to negative terminal of
supply as shown in given figure.
In forward biased p-n junction diode,
electric current is flow from p-type semiconductor to n-type semiconductor, or
we can say electric current is entering from p-side of the p-n function diode.
resulting the flow of free electrons form n-type semiconductor to p- type
semiconductor through depletion region. As n-type semiconductor have plenty of
free electrons.
Let us discuss this phenomenon in details,
when we apply forward biased to a p-n junction diode then positive potential of
battery attracts electrons from p side this will result more holes or vacant
positions on p-side. These holes on p-side attracts more electrons from n-side
but these electrons require more energy to pass this depletion region as this
depletion region have no charge carriers.
So, when a voltage that is greater then,
this energy requirement of electrons is applied to diode terminals then
electrons from n-side are freely move to p-side and current start conducting in
opposite direction. This means diode start conduction and behave like short
circuit.
Note :- The minimum forward biased
voltage required by the diode, so that it start conduction is called Cut-In
Voltage, Knee Voltage or Threshold Voltage
Above figure represents the V-I
Characteristics of a diode during forward bias condition. As we discuss in
above paragraph, forward biased diode start conduction and behave like short
circuit (means V α
I)
after a certain applied voltage. We can clearly see that in above, that after
knee voltage Vd is proportional to Id.
Reverse Bias Diode
A reverse-biased p-n junction diode
refers to a situation where the voltage across the diode is applied in such a
way that the voltage applied to the anode is lower than that applied to the
cathode. In other words, the negative side of the voltage source is connected
to the diode's anode, and the positive side is connected to the cathode.
In reverse bias p-n junction diode,
electric current entered from n side of p-n junction diode, means electrons
flow from n-side to positive potential of the battery. This will result the
number of holes or the vacant position increases on n-side. This uncovered
positions or holes form layers near the junction. Similarly, the negative
potential of supply attracts more holes from p-side, resulting the number of
electrons increase on p-side. These free electrons on p-side form layers at the
junction.
So, when we apply voltage to a p-n
junction diode then number of electrons (-ve ion) layers on p-side and number
of holes (+ve ion) layers on n-side increases. Hence the width of depletion
region increases, means the potential barrier increases. That’s why a reverse
biased diode does not allow conduction. So, we can say that in reverse bias
condition diode act as open circuit or diode is switch off.
However, a small amount (almost
negligible) of current flows through the diode due to minority charge carriers
that are entering into the depletion region. This current is called reverse
saturation current and is denoted by Is.
Above figure shows V-I characteristics of
p-n junction diode during reverse bias condition. As we discussed in above
paragraph, that a reverse bias diode is behave like open and does not allow
conduction. We can clearly see in above characteristics figure that there is no
current (Id) flow while applying negative voltage. However, a small
Reverse Saturation Current is flow due to minority charge carriers.
Avalanche Breakdown in diode
This phenomenon occurs in reversed biased
diode.
When a sufficiently high reverse voltage
applied to diode terminal then minority charge carriers acquire energy from
applied voltage. These charge carriers collide with other atoms and imparts
sufficient energy to break their covalent bond. By doing this, these charge
carriers form new electron-hole pairs and generate additional charge carriers.
This cumulative phenomenon is called avalanche multiplication. It results the
flow of large reverse saturation current through diode. Actually, a breakdown
occurs called avalanche breakdown or reverse breakdown.
At this condition diode is said to be on
avalanche or reverse breakdown region.
Note:- Reverse bias voltage required to
derive the diode in reverse breakdown region or avalanche breakdown region is
called reverse break down voltage.
V-I Characteristics of P-N Junction Diode
- VD = Applied Voltage
- ID = Diode Current
- Is = Reverse Saturation Current
- VBR = Reverse Breakdown Voltage
- VT = Cut-in Voltage, Threshold Voltage, or knee voltage.
Above we study that, if forward bias
voltage is greater than the threshold voltage than diode start conduction means
current flow through the diode. In above characteristics graph we clearly see
that ID (Diode Current) rise significantly as applied voltage
increased.
And if reverse bias voltage is applied
than diode does not allow conduction, however a small amount of current
(reverse saturation current) flows through diode. We can see in above vi
characteristics graph that in reverse bias region a small current Is
flows through diode up to a certain voltage and if reverse voltage is greater
than reverse breakdown voltage than reverse saturation current increase sharply
at this condition avalanche breakdown occurs.
related tags
#_Diode Definition, Construction and Symbol
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