PN Junction Diode

If you’ve ever been interested in the workings of electronic devices, you may have heard of a PN junction diode. A PN junction diode is a fundamental electronic component that is widely used in various applications. In this article, we’ll provide a comprehensive guide to help you understand what a PN junction diode is, how it works, its applications, advantages, disadvantages, and in the end characteristics of a PN junction diode.

What is a PN Junction Diode?

A PN junction diode is a semiconductor device that allows the flow of electric current in one direction and blocks the flow in the opposite direction. It is created by joining two types of semiconductors – P-type and N-type – together. The interface between the P-type and N-type regions is known as the PN junction.

Symbol of PN Junction Diode

The circuit symbol of the PN junction diode or simple diode is shown in the figure.

Formation of PN Junction Diode

PN junction diode is formed either from Ge or Si crystal. When a p-type material is intimately joined to an n-type a PN junction is formed. Actually, PN junction is fabricated by special techniques like growing, alloying, and diffusion methods.

A PN junction is illustrated in the below figure. The figure shows the p-type and n-type semiconductor pieces before they are joined. Before joining both types of semiconductors are electrically neutral. In the p-type material holes are the majority carriers and electrons are the minority carriers, In n-type material electrons, are the majority carriers and holes are the minority carriers.

Now, let us consider the PN junction produced from a single crystal of intrinsic silicon doped first with a pentavalent material ( to get a p-type semiconductor) as shown in the figure above. But more commonly a PN junction is produced by solid-state diffusion of one type of impurity (p-type) into existing (n-type) material.

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Also Read: Photodiode – Basics, Working Principle, and Applications

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PN Junction Diode under Open Circuit Condition

After joining P-type and n-type materials, the crystal becomes a single piece as shown in the figure below. The plane dividing the two is called the PN Junction. The p-type region has holes as the majority of charge carriers and similarly, the n-type region has electrons as the majority of charge carriers. In addition to these majority charge carriers, there are a few minority charge carriers in each region. In the p-region, electrons are the minority charge carriers, and in the n-region holes are the minority charge carriers.

The majority of carriers near or at the junction diffuse across the junction and recombine. After a few recombinations of majority carriers at the junction, the process stops. This is because the electrons crossing over the junction into the p-type material are repelled by the large negative ions.

Similarly, holes crossing over the junction are repelled by the large positive ions in the n-type material. The immobile ions at the junction create a zone depleted of majority carriers called the depletion region as shown in the figure above.

Thus there is no current flow under open circuit conditions. The thickness of this region is of the order of 10^-6 m. The potential difference across the depletion region is called the potential barrier. The potential barrier can be increased or decreased by applying an external voltage.

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Also Read: Tunnel Diode: Definition, Characteristics & Applications

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Working of PN Junction Diode

When an external voltage is applied to the PN junction diode, it is said to be based. In order to consider the working of a PN junction diode, let us consider the effect of biasing across the PN junction diode.

There are two types of Biasing:

1. Forward Bias
2. Reverse Bias

1. Forward-biased PN Junction diode

• A forward biasing can be applied to a PN junction diode by connecting the positive terminal of the battery to the p-type semiconductor and the negative terminal of the battery to the n-type semiconductor, as shown in the figure below.

• When an external voltage is applied to a PN junction diode, the chances of the potential barrier and permitting the current flow means, the junction is said to be in the forward-biased condition. If the forward bias is greater than the potential barrier, the majority of carriers move toward the junction and cross it resulting in a considerable current flow. The current that flows due to the majority of carriers is called forward current. It increases with forward bias.

2. Reverse-biased PN Junction diode

• A reverse biasing can be applied to a PN junction diode by connecting the positive terminal of the battery to the n-type semiconductor and the negative terminal of the battery to the p-type semiconductor as shown in the figure.

• When an external voltage applied to a PN junction diode increases the potential barrier means, then it is called reverse bias. The applied reversed voltage established an electric field that acts in the same direction as the potential barrier. Due to this, the resultant field at the junction gets strengthened and the barrier height also gets increased. This increased potential barrier prevents the flow of charge carriers across the junction.

• However, a very small amount of current flows in the circuit due to the motion of minority carriers. This current is called reverse current.

• Thus, when a PN junction diode is forward biased, the junction has a low resistance path, and hence current flows in the circuit. On the other hand, when it is reverse biased, it has a high resistance path and no current flows in the circuit. Hence a PN Junction diode can be used as a rectifier, i.e., for converting alternating current into direct current.

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Also Read: Light Emitting Diode (LED)

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VI Characteristics of PN Junction Diode

It is very important to study how a device responds when it is connected to an electrical circuit. The behavior of a diode can be obtained by means of a graph known as volt-ampere or VT characteristics.

VI Characteristics is a graph between the applied voltage across the terminal of the device and the current flowing through it.

The characteristics of a diode are studied under two conditions:

1. Forward Biasing characteristics
2. Reverse biasing characteristics

1. Forward Characteristics of PN Junction Diode

• The circuit diagram for obtaining the forward characteristics of a diode is shown in the Figure below:

• When the PN Junction diode is forward biased and the applied voltage is gradually increased in steps, at some forward voltage (V_f), 0.3V for Ge and 0.7V for Si, the potential barrier is altogether eliminated and the current starts flowing. This voltage is known as threshold voltage (V_th) Knee voltage or Cut-in voltage.

• The milliammeter readings are noted at various steps of the applied voltage and a graph is plotted between voltage and current, as shown in the figure above. From the graph, it is seen that practically no current flows until the barrier voltage (V_B) is overcome, THis is shown by point A in the graph. Once the external voltage exceeds the barrier potential or the threshold value, the current increases exponentially, as shown by portion AB in the graph. This portion is known as the linear operating region of the diode. At point A,

V_f = V_{th}=V_B

• From the graph, the forward resistance of the diode is R_f=\frac{\Delta V_f}{\Delta I_f}

• If the forward voltage is increased beyond a safe limit, damage is likely to occur to the diode due to overheating.

2. Reverse Characteristics of PN Junction Diode

• The circuit diagram for obtaining the reverse characteristics of a diode is shown in the figure below. When the PN junction diode is reverse biased, majority carriers are blocked and only a small current due to minority carriers flows through the diode. As the reverse voltage is increased from zero in suitable steps, the reverse current very quickly reaches its maximum or saturation value which is called reverse saturation current (I_r) or leakage current.

• The diode current is recorded at each step of reverse voltage (Vr) and a graph is drawn with reverse voltage along the horizontal axis and the diode current along the vertical axis. Curve OCD as shown above the figure is obtained. The curve OCD is called the reverse characteristics of the diode.

• It can be seen from the graph that when the applied reverse voltage is below the breakdown voltage (V_BR), the diode current is small and remains constant (portion OC of the curve). When the reverse voltage exceeds the breakdown voltage, the leakage current suddenly and sharply increases. The curve CD indicates zero resistance at this point. The reverse current is of the order of microampere (uA) for Ge and nano ampere (nA) for Si.

• Any further increases in voltage are likely to produce damage to the PN junction diode unless protected by a current-limiting resistor.

• The reverse resistance of the diode from the slope of the curve is R_r=\frac{\Delta V_r}{\Delta I_r}

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Combined Forward and Reverse VI characteristics of PN Junction Diode

The combined forward and reverse characteristics are shown in the figure below for both Ge and Si. It can be seen from the VI characteristics curve that the leakage current of the Ge junction is much more than that of the Si junction. Also, Note the difference in the scale of voltage and current in the forward and reverse characteristics.

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PN junction diode with specification

Specification	Value	Property	Description
Forward Voltage Drop	0.6 V to 0.7 V	Function	When a positive voltage is applied to the P-type region and a negative voltage to the N-type region, the diode is said to be forward-biased
Reverse Breakdown Voltage	50 V to 1000 V	Structure	Consists of a P-type and N-type semiconductor material that are fused together
Maximum Forward Current	1 A to 10 A	Operation	When a positive voltage is applied to the P-type region and a negative voltage to the N-type region, the diode is said to be forward-biased
Maximum Reverse Current	10 µA to 100 µA	Forward Bias	When a positive voltage is applied to the N-type region and a negative voltage to the P-type region, the diode is said to be reverse-biased
Maximum Power Dissipation	200 mW to 1 W	Reverse Bias	When a positive voltage is applied to the P-type region and a negative voltage to the N-type region, the diode is said to be forward-biased
Junction Temperature	-55 °C to +150 °C	Current Flow	In forward bias, current flows easily through the diode. In reverse bias, only a small reverse saturation current flows
Reverse Recovery Time	4 ns to 100 ns	Applications	Rectifiers, voltage regulators, signal detection, signal mixing, signal demodulation, LEDs, solar cells
		Advantages	Low power consumption, fast switching speed, small size, low cost
		Disadvantages	Temperature sensitive, voltage sensitive, high reverse leakage current

This table provides both the technical specifications and properties of a PN junction diode. It’s important to note that these specifications may vary depending on the specific type and manufacturer of the diode.

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Applications of PN Junction Diodes

PN junction diodes have numerous applications in electronic devices. Some of the common applications are:

Rectifiers: PN junction diodes are widely used as rectifiers in electronic circuits. They allow the current to flow in one direction and block it in the opposite direction, making them ideal for converting AC (alternating current) to DC (direct current).

Voltage Regulators: PN junction diodes can be used as voltage regulators by combining them with resistors or other electronic components. This allows the diode to regulate the voltage across a circuit and prevent voltage spikes.

Light Emitting Diodes (LEDs): LEDs are a type of PN junction diode that emits light when forward biased. They are used in various applications, such as lighting, displays, and indicators.

Solar Cells: Solar cells are devices that convert sunlight into electricity. They are made up of PN junction diodes that absorb photons from sunlight and generate a flow of current.

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Advantages and Disadvantages of PN Junction Diode

Advantages PN Junction Diode

• PN junction diodes are inexpensive and widely available.
• They are small in size, making them ideal for use in compact electronic devices.
• They have a fast response time and can switch on and off quickly.

Disadvantages PN Junction Diode

• PN junction diode has a limited current rating and voltage rating.
• They are susceptible to temperature variations and can be damaged by excessive heat.
• They have a high reverse leakage current, which can cause power dissipation.

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