Diodes are the simplest of all the semiconductor devices.
Semiconductor diodes are used in a variety of applications. These include communication systems, radio, TV, computers, power supplies and so on.
A semiconductor diode is a polarity-sensitive two-terminal device comprising a P-N junction formed between a P-type semiconductor material and an N-type semiconductor material as shown in the Figure below.
Semiconductor diode
A P-N junction is formed by introducing the donor impurities (N-type impurities) on one side and acceptor impurities (P-type impurities) on the other side of a single crystal of a semiconductor.
The circuit symbol of a semiconductor diode is shown in the figure below.
Circuit symbol of a semiconductor diode
The arrow is associated with the P-region and the vertical line with the N-region.
The P- and N-regions are referred to as the anode and the cathode, respectively.
Silicon and Germanium are the most commonly used materials for fabricating semiconductor diodes.
The electrons in the N-region and the holes in the P-region combine near the junction, resulting in a region near the junction that is devoid of free electrons and holes. Due to the depletion of free carriers in this region, it is referred to as the depletion region. The thickness of this region is of the order of 0.5 μm in the absence of an external bias voltage.
Electrons (majority carriers) in the N-region and negatively charged ions in the P-region repel each other near the junction. Also, holes in the P-region (majority carriers) and positively charged ions in the N-region also repel each other near the junction. This results in the development of an effective potential of the order of few tenths of a volt, across the depletion region. This potential is referred to as the contact potential or the barrier potential.
Figure below shows the diagram of the P-N junction with no applied bias.
P-N junction with no applied bias
From the figure, we can see that, in the absence of an applied bias voltage, the net flow of current in a semiconductor diode is zero.
The following phenomenon happen under zero-bias conditions.
-• Some of the holes and electrons have sufficient kinetic energy to overcome the contact potential and be able to pass through the depletion region. This results in a flow of electrons from the N-region to the P-region and flow of holes from the P-region to the N-region. This constitutes the majority carrier flow vector.
-• In addition, holes (minority carriers) that are present in the depletion region of the N-region pass to the P-region. Similarly, electrons (minority carriers) that are present in the depletion region of the P-region pass to the N-region. This constitutes the minority carrier flow vector.
-• The relative magnitudes of the minority and the majority flow vectors are such that the net flow of carries in either direction is zero. Therefore, there is no flow of current through the diode under zero bias conditions.
-• This is referred to as the open-circuit condition of the semiconductor diode where no bias voltage is applied to the diode.
There are two ways in which the potential can be applied to a semiconductor diode.
• Refer to figure below. A positive potential is applied to the P-region and a negative potential is applied to the N-region. This condition is referred to as the Forward bias condition.
Forward biased Semiconductor diode
• Refer to figure below. A positive potential is applied to the N-region and a negative potential is applied to the P-region. This condition is referred to as the Reverse bias condition.
Reverse biased semiconductor diode
When the diode is forward biased, the applied potential causes the electrons in the N-region and the holes in the P-region to combine with positive and negative ions, respectively, in the depletion region. This results in a reduction of the width of the depletion region as shown in Figure below. In addition, the potential barrier at the junction also decreases.
Depletion region of a forward-biased diode
As the applied bias is increased in magnitude, the width of the depletion region decreases until a point is reached where there is a sharp rise in the number of majority carriers crossing the junction. A large number of holes cross the junction from the P-region to the N-region and a large number of electrons cross the junction from the N-region to the P-region. This results in exponential rise in the current due to the majority carriers.
There are two components of current flowing through the diode in the forward biased condition. The first component is the current is referred to as the forward current. It is due to the flow of majority carriers and is in the range of few tens of milliamperes for normal diodes. The second component is due to the flow of the minority carriers and is referred to as the reverse saturation current or reverse leakage current. It is the same as in case of diode with no applied bias and is of the order of a few nanoamperes to a few microamperes. The reverse saturation current is in the opposite direction to the forward current. However, its magnitude is negligible as compared to the forward current.
Typically, the voltage across the forward-biased diode depends on the diode material and is less than 1V. As an example, the forward voltage for Silicon and Germanium diodes is typically 0.7 V and 0.3 V, respectively.
When a semiconductor diode is reverse biased, electrons and holes are drawn away from the junction due to the polarity of the applied voltage as shown in Figure below. This results in widening of the depletion region.
Reverse-biased semiconductor diode
When the semiconductor diode is reverse biased, the widening of the depletion region reduces the flow of majority carriers to approximately zero. The minority carrier flow remains the same as in case of diode with no applied bias and constitutes the reverse saturation current. It is of the order of a few nanoamperes to a few microamperes.
The reverse saturation current does not significantly change with change in the reverse-bias potential. However, when the applied reverse-bias is increased beyond the breakdown voltage of the diode, there is a sharp increase in the reverse current. The reverse saturation current is a strong function of the diode temperature and increases with increase in diode temperature.