P-N junction Diode
The P-N junction diode is a semiconductor device that widely used in all
electronics applications and industrial manufacturers. It controls and
allows the current flow from the P-Type to the N-Type when it is connected
in forward bias and blocks its flow in the other direction from the N-Type
to the P-Type.
In previous articles like
semiconductors materials
and the P-N basics as well as the formation of the junction formation
between the P and N types. This article will discuss and explain the P-N
junction diode in detail. We will discuss its properties, advantages,
construction, and applications.
Introduction.
The P-N junction
diode is an electronic device with two leads anode and cathode based on the
P-N junction. The anode is connected to the P-Type side and the cathode is
connected to the N-Type. The fabrication of the P-N junction diode is made
through special processes. The symbols of the P-N junction diode are shown
in the following figure.
In the P-N junction diode, the P side has a hole as a majority carrier and
the electrons are the minority carriers while in the N side, the majority
carriers are electrons and the minority are holes. At the junction between
the P and N, there is a layer called the depletion layer which contains the
negative ions on the P side and positive ions in the N type. This depletion
layer creates an electric field called barrier potential.
P-N junction diode biasing.
The biasing process of the
P-N junction
diode is defined as the application of an external potential source to the
P-N junction diode. There are three states of the diode when biasing. The
first one is the equilibrium state, forward bias, and reverse bias state.
Let us go deeply into that.
Equilibrium state:
The equilibrium State is the state at which the P-N junction diode is not
connected to any external source or connected to an external source but the
applied voltage is zero. when the external potential source is zero or not
connected. When the P-N junction diode is at an equilibrium state, the
majority carriers and minority carriers are still the same.
The built-in electric field and depletion layers block the transfer of the
chargers through it and there is no charge diffusion or drifting inside the
P-N junction and as a result, there is no flow of current. The barrier
potential at the equilibrium State is about 0.6V and 0.3 V for Si and Ge
respectively.
The energy band of the P-N junction diode is shown in the next Figure where
the EF for the P-N junction is constant, the depletion layer and barrier
potential are established, and there is no diffusion or drifting of charge.
At the equilibrium state, the N-type side energy bands are shifted by the
value of barrier potential (VB).
Forward Bias.
In forward bias, the external source is connected by connecting the
positive terminal of the external source which has a high potential to the
anode of the P-N junction diode (P side), and the negative terminal of the
external source which has a lower potential is connected to the cathode
terminal of the P-N junction diode (N side).
When the external voltage starts to increase > 0, the positive charges
start to force the holes in the P side towards the depletion layer and the
electrons in the N side will be forced to move in the direction of the
depletion layer. So, the number of negative ions on the P side and the
number of positive ions on the N side in the depletion layer will be
decreased and as a result, the barrier potential and the depletion layer
will be decreased too.
When the external voltage exceeds the barrier potential (> 0.7 for Si
and > 0.3 for Ge at equilibrium), the current will flow exponentially
from the P side to the N side as shown in the figure. At this point, a small
increase in applied forward potential causes a sharp increase in the diode
current (exponentially).
For the energy bands at the forward bias, the energy band of the N-type
semiconductor will be shifted by the value of
(VB-VD) because of the diffused holes and electrons. The depletion and the
built-in electric field are decreased as shown in the next figure.
Reverse Bias.
The reverse bias is to connect the negative terminal of the external
source to the positive diode of the diode (anode) and connect the positive
terminal of the external potential source to the negative terminal of the
diode (cathode). In such case, the holes and electrons will be pulled out
to the external negative and positive terminals causing more negative ions
in the P side and positive ions in the N side.
So, the external potential source will be added to the barrier potential
that exists at the junction between P and N will be increased and the
depletion layer will be increased also. So, no current will pass through
the depletion layer. A very small current in micro-amps will flow in the
reverse direction due to the minority carriers which is the reverse
saturation current while the majority current is zero.
If the reverse potential increases to a very high value the breakdown
will happen in the diode and the current will flow in the direction from N
to P. The reverse voltage at the breakdown down is called avalanche
breakdown and the voltage is called the breakdown voltage. The reason is
that the electrons have a high kinetic energy when applying a high reverse
potential.
This high kinetic energy enables electrons to break the covalent bond and
become free. These free electrons strike other electrons. By repeating
this process, we have the rabid (exponentially) increase of the reverse
current. For the energy bands at reverse bias, the energy bands of the
N-Type were shifted by (VB-VD) but the shift is high as
the applied voltage is reversed as shown in the next figure. the shift is
large because of the movement of holes and electrons out of the
junction.
Conclusion:
- At equilibrium, there is no current flow, depletion layer, and built-in electric field (barrier potential) is established. At forward bias and when a positive potential is applied, the current flows from the P side to the N side when the applied voltage exceeds the barrier potentials, the depletion layer and the built-in electric field decrease.
- At reverse bias when a negative potential is applied, there is no current flow, the depletion layer and the built-in electric field are increased till the breakdown of the diode where the current follows in the reverse direction.
- Depletion Layer (forward) < Depletion Layer (equilibrium) < Depletion Layer (reverse).
- Barrier potential (forward) < Barrier potential (equilibrium) < Barrier potential (reverse).
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