pn junction and diode

In the field of electronics and communications, semiconductors play a vital role. Among various semiconductor devices, the pn junction stands as a basic building block, which is crucial for a wide range of applications. Understanding the pn junction is essential for anyone delving into electronics, as it is the key component in many semiconductor devices such as diodes, transistors, solar cells and others.

What is a semiconductor?

Before we dive into PN junctions and diodes, let's clarify what a semiconductor is. Semiconductors are materials whose level of conductivity lies between conductors, such as metals, and insulators, such as glass. Silicon (Si) and germanium (Ge) are the most commonly used semiconductors. The unique properties of semiconductors make them ideal for controlling electrical currents.

Intrinsic and extrinsic semiconductors

Intrinsic semiconductors

Intrinsic semiconductors are pure semiconductors that do not contain any significant impurity atoms. They have equal numbers of electrons and holes (missing electrons). At absolute zero, intrinsic semiconductors behave like perfect insulators.

Extrinsic semiconductors

Adding impurities to a semiconductor creates an extrinsic semiconductor. This process is called doping. Doped semiconductors are mainly of two types: p-type and n-type.

n-type semiconductor

In n-type semiconductors, the added impurity has more electrons than the semiconductor. For example, adding phosphorus (which has five electrons in its outer shell) to silicon (which has four electrons in its outer shell) creates an n-type semiconductor. The extra electrons from the phosphorus atom provide charge carriers.

Si + P → n-type Semiconductor

p-type semiconductor

In p-type semiconductors, the added impurity has fewer electrons than the semiconductor. For example, adding boron (which has three electrons in its outer shell) to silicon creates a p-type semiconductor. This creates "holes" or positive charge carriers.

Si + B → p-type Semiconductor

Formation of PN junction

When p-type and n-type semiconductors are brought together, they form a pn junction. This junction is the heart of many semiconductor devices.

Depletion zone

At the PN junction, electrons from the n-type region diffuse into the p-type region and recombine with holes. This process leaves behind charged ions and creates a region devoid of charge carriers, known as the depletion region.

Barrier capacity

The depletion region forms a potential barrier that prevents further flow of charge carriers. This is called the barrier potential. For silicon, the typical barrier potential is about 0.7 volts, while for germanium, it is about 0.3 volts.

Diode: Using pn junction

A diode is a semiconductor device that allows current to flow primarily in one direction. It is constructed from a single pn junction and has two terminals: anode and cathode. The fundamental characteristic of a diode is that it allows current to pass through only when the anode is at a higher potential than the cathode. This is known as forward bias.

Forward bias

In forward bias, a positive voltage is applied to the p-type material and a negative voltage to the n-type material, causing current to flow across the junction. In this case, the barrier potential decreases, allowing electrons and holes to cross the junction.

Reverse bias

In reverse bias, a negative voltage is applied to the p-type material and a positive voltage to the n-type material. The depletion region widens, and the barrier potential increases, effectively blocking current flow.

Diode characteristics

A diode has a characteristic curve known as the I-V (current-voltage) characteristic. In the forward bias region, the diode conducts current exponentially after crossing the threshold voltage. In the reverse bias region, only a small leakage current flows until breakdown.

V_d > 0 → I ≈ I_s * (e^(V_d/nV_t) - 1)

Here, I_s is the saturation current, V_d is the diode voltage, n is the ideality factor, and V_t is the thermal voltage.

Applications of diode

Diodes are used in a wide range of applications:

• Rectifiers: Used to convert AC to DC.
• Clippers and clampers: Used to shape waveform signals.
• Voltage multiplier: Used to generate higher voltage.
• Solar cells: Use the photovoltaic effect combined with diode properties.

Zener diode: A special type of diode

The Zener diode is designed to make the current flow in the reverse direction when the voltage exceeds a certain value known as the Zener breakdown voltage. It is commonly used as a voltage regulator.

Zener breakdown

In the reverse bias condition, at a certain reverse voltage, the Zener diode allows significant current without any damage. This phenomenon is known as Zener breakdown.

I_z = (V_z-V_l)/R_z

Here, I_z is the current flowing through the Zener diode, V_z is the Zener voltage, V_l is the load voltage, and R_z is the limiting resistor.

Light emitting diode (LED)

LEDs are special diodes that emit light when current flows through them. The wavelength (and thus color) of the light depends on the semiconductor material used.

Operation and application of LED

When the LED is tilted forward, the electrons recombine with the holes, releasing energy as light. LEDs are energy-efficient and are used in display systems, indicators, and lighting.

Conclusion

Understanding pn junctions and diodes is very important in the study of electronics. Diodes perform key functions in circuits, from controlling the direction of current flow to regulating voltage. As technology develops, the applications of semiconductors and diodes are increasing, making them even more important in today's electronic devices.

In this discussion, we covered the fundamentals of pn junctions, explored the functioning and applications of diodes, and touched upon some special types like Zener diodes and LEDs. Each of these components plays a vital role in the functioning of electronic circuits, paving the way for the advancement of technology.

Comments