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Semiconducting Devices: An Introduction, Lecture 5

PN Junction#Optoelectronics#Field effect transistor#Band Bending#Fermi Energy#Diode#Built-in Potential#Electric Field Effect
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💫 Short Summary

Semiconductors and semiconducting materials are important for various devices, including junction devices like pn junctions made from p-type and n-type semiconductor materials. In thermal equilibrium, the carrier concentrations of electrons and holes vary across the junction, leading to a change in potential energy. The built-in potential, which is the difference in conduction band edge between the p-type and n-type sides, plays a crucial role in semiconductor physics. Pn Junctions and Diodes: Introduction to pn junctions and diodes, including forward and reverse bias, the Shockley diode equation, and the characteristics of pn junction devices such as built-in potential and constant band gap. In this lecture on semiconductor physics, the professor discusses the formation of a capacitor in a p-type semiconductor when a positive voltage is applied, leading to the creation of an n-type channel. This concept is fundamental to understanding field effect transistors, particularly MOSFETs. The lecture also touches on the basics of optoelectronic devices, specifically how light emitting diodes (LEDs) work based on the behavior of electrons and holes in a forward-biased PN junction.

✨ Highlights
📊 Transcript
Junction Devices
00:02
Junctions dominate semiconducting device characteristics, especially in integrated circuits.
A pn junction is made from two types of semiconductor materials: a p-type hole-doped material and n-type electron-doped material.
In thermal equilibrium with no bias, the left side of the junction is full of electrons, the right side is full of holes, and diffusion occurs due to the density difference.
The potential energy for electrons is lower in the n-type region and higher in the p-type region, while the opposite is true for holes.
The built-in potential, which is the difference in the conduction band edge between the p and n sides, is a key concept in junction devices.
Energy Levels in Semiconductors
00:04
The energy of electrons on the p-type side is higher due to the repulsion from negative acceptor dopants.
The energy level diagram shows the conduction band edge, valence band edge, and the energy gap, which is the same across n and p types.
The built-in potential is the difference in the conduction band edge between the p and n sides, measured with laboratory instruments.
In this section, the video introduces the concept of fermi energy in a semiconductor.
00:00
Fermi energy is a characteristic energy of the semiconductor.
In thermal equilibrium, the fermi energy is a horizontal line.
The fermi energy is closer to the conduction band edge in an n-type semiconductor and closer to the valence band edge in a p-type semiconductor.
When a battery is connected to a semiconductor junction, the fermi energy level changes, causing the bands to bend.
49:00
The fermi energy level changes when a potential difference is applied across the junction.
The bands bend up to meet the new position of the fermi energy level.
Junctions can be made from two different semiconductor materials to create different band gaps.
Closing the switch and applying a positive potential to the p-type side and a negative potential to the n-type side results in forward bias.
01:58:00
Electrons are attracted to the p-type side due to the applied potential.
The built-in potential barrier is reduced, allowing electrons to more easily cross the junction.
This bias condition is known as forward bias.
The current voltage characteristic of the junction can be measured by varying the voltage and measuring the current.
The shockley diode model describes the behavior of a diode under forward and reverse bias.
03:37:00
Under forward bias, the current-voltage relationship is exponential.
Under reverse bias, the current levels off and is described by the reverse bias current (Isub0).
The ideality factor (eta) describes the deviation from ideal behavior, with real data indicating a stronger dependence on voltage for germanium-based diodes.
Two salient points about pn junction devices are the built-in potential (Vsubbi) and the constant band gap throughout the junction.
05:33:00
The built-in potential is a result of the doping level and creates a barrier for carriers to cross the junction.
The band gap remains constant regardless of doping or bias voltage.
The video briefly mentions two other device categories: field effect devices and optoelectronic devices.
07:09:00
Field effect devices are the basis of transistors used in integrated circuits.
Optoelectronic devices are another category that will be covered in this course.
Applying a positive voltage to a p-type semiconductor can create an n-type channel at the top layer, known as the threshold voltage.
17:00
Raising the voltage causes holes to exit the p-type semiconductor.
Electrons from ground are attracted to the electrode but form a layer at the top due to the air gap, creating an n-type region.
The voltage at which the n-type channel begins to establish itself is called the threshold voltage.
Exceeding the threshold voltage results in the semiconductor being in inversion, where the top layer has inverted to the other type.
Field effect transistors, specifically MOSFETs, are the dominant type of device in integrated circuits and operate based on the principle of the electric field effect.
19:00
Field effect transistors combine capacitors and PN junctions into a single device.
MOSFETs, or metal oxide semiconductor field effect transistors, are the predominant type in integrated circuit design.
LEDs (light emitting diodes) are based on PN junctions and work by electron-hole recombination, with the p-type side easily filling up with electrons to create bright light.
20:00
Forward biasing the PN junction allows electrons to move to the p-type side and recombine with holes, emitting photons.
LEDs depend on the fact that electrons are more mobile than holes, allowing the p-type side to fill up with electrons more easily.
Near the junction on the p-type side, there is a high population of both electrons and holes, leading to their recombination and the generation of bright light.
💫 FAQs about This YouTube Video

1. What are junction devices in semiconductors?

Junction devices are a key component in semiconducting device characteristics, especially in integrated circuits, and are primarily found in transistors.

2. How is a pn junction made and what is its function?

A pn junction is made from two types of semiconductor materials - a p-type hole-doped material and an n-type or electron-doped material. It functions as a diode and allows the flow of electrical current in one direction.

3. What is the carrier concentration across a semiconductor junction with the switch open?

With the switch open and no bias on the semiconductor, the carrier concentration shows a high population of electrons on the left side and a high population of holes on the right side, creating a potential barrier for the movement of carriers.

4. How is the potential energy of electrons and holes distributed across the semiconductor junction?

The potential energy of electrons and holes varies across the semiconductor junction, creating a built-in potential that forms a barrier for the movement of carriers.

5. What is the function of the built-in potential in a semiconductor junction?

The built-in potential in a semiconductor junction forms a barrier that controls the movement of carriers and allows the junction to function as a diode, enabling the flow of electrical current in one direction.

6. What is the essence of the electric field effect?

The essence of the electric field effect is the formation of an n-type channel in a semiconductor due to the attraction of electrons to an electrode plate with a positive charge, creating a field effect capacitor.

7. What is the significance of field effect transistors?

Field effect transistors, particularly MOSFETs, operate based on the electric field effect and are the building blocks of integrated circuits, playing a significant role in the design of electronic devices.

8. How are light emitting diodes (LEDs) related to the electric field effect?

LEDs are optoelectronic devices based on the electric field effect in semiconductors, where the recombination of electrons and holes in a pn junction creates light emission.

9. What are the key categories of devices discussed in the video?

The video discusses pn junctions, field effect devices (specifically MOSFETs), and optoelectronic devices (such as LEDs) as the key categories of devices based on semiconductor physics and the electric field effect.

10. What will be covered in the upcoming topics after discussing the electric field effect and semiconductor physics?

The upcoming topics will cover the physics of semiconductors, including carrier concentrations, doping effects, and their impact on semiconductor performance.