semiconductor physics and Devices

Week 4: Diodes - Operation and Applications

Lecture Topics:

1. Introduction to Diodes

   • A diode is a semiconductor device formed by a single p-n junction.

   • The key function of a diode is to allow current to flow easily in one direction (forward bias) and to block current in the opposite direction (reverse bias).

   • Types of diodes:

     • Standard p-n junction diode

     • Zener diode

     • Light-emitting diode (LED)

     • Photodiode

2. Ideal Diode vs. Real Diode Characteristics

   • Ideal diode:

     • In forward bias, the diode conducts with no resistance.

     • In reverse bias, the diode blocks all current.

   • Real diode:

     • In forward bias, the diode requires a small threshold voltage (typically 0.7 V for silicon) to start conducting.

     • In reverse bias, a small reverse saturation current flows due to minority carriers.

3. Diode I-V Characteristics

   • Recap of the Shockley diode equation from Week 3:

• where I0​ is the reverse saturation current, V is the applied voltage, q is the electron charge, kB is Boltzmann’s constant, and T is the temperature.

• Reverse bias: The current is very small and approximately constant, known as the reverse saturation current I0​.

Examples:

• Derivation of the built-in potential V0​ for a p-n junction given the doping concentrations NA​ and ND​.

• Calculation of the width of the depletion region for a given p-n junction under equilibrium conditions.

• Plotting the I-V characteristics of a p-n junction in both forward and reverse bias.

Homework/Exercises:

1. Explain the differences between Zener breakdown and avalanche breakdown. Provide examples of where each might be used.

2. Calculate the forward current in a diode for a forward bias voltage of 0.7 V, given a reverse saturation current of I0=1 μA

3. Design a simple rectifier circuit using diodes to convert AC to DC and explain how it works.

4. Calculate the wavelength of light emitted by an LED with a band gap of 2 eV.

Suggested Reading:

• Charles Kittel, Introduction to Solid State Physics, Chapter 8: Semiconductors (continued).

Key Takeaways:

• Diodes are key components in many electronic devices, controlling the direction of current flow and performing functions such as rectification, voltage regulation, and light emission.

• Zener diodes allow current to flow in reverse once the breakdown voltage is reached, while LEDs emit light when forward biased.

• Photodiodes generate current when exposed to light, making them useful in sensors and solar cells.

This week focuses on the operation and applications of diodes, including standard p-n junction diodes, Zener diodes, LEDs, and photodiodes. Understanding the I-V characteristics and practical uses of these devices is essential for designing and analyzing electronic circuits.