ece 635 advanced semiconductor devices

ECE 635

Advanced Semiconductor Devices

Gong Gu

Course website:http://web.eecs.utk.edu/~ggu1/files/GradHome.html

Fall 2021

http://web.eecs.utk.edu/%7Eggu1/files/GradHome.html

What this course is about

Selected advanced topics (TBD by student research areas and interests,e.g. wide-band gap semiconductor devices for power electronics, memristors. Possibly presented by students)

So, after a brief introduction to semiconductors and devices, you’ll introduce yourself to each other.

Student background survey: name, major, research field, advisor, etc.

• Broad overview of semiconductor device physics and of device concepts, complemented with in-depth discussions on selected topics

• Multidisciplinary: ask (and discuss) intriguing questions on topics that usually fall through the cracks between courses in typical curricula

• A forum: students from divers backgrounds to inspire each other and learn each other’s terminologies

• We welcome students from all STEM departments. • Electrical and computer engineering alone is already very broad and diverse.

Why Semiconductors?

Information acquisition(sensors)

Image, sound, temperature, pressure, …

Information processing(Amps, A/D, processors,

tranceivers…)

Information processing(tranceivers,

processors, …)Displays

Information transmission(wires, busses, cables, optical fibers, or just air!)

• Brains and muscles of the system are made of semiconductors

• Metals & dielectrics are used as transmission media

• Why?

Image, sound, temperature, pressure, …

Sometimes we say “solid state”

Slide from ECE634 Fundamentals of Semiconductors (Spring 2020)

What’s common for all the core components?

Light, sound, temperature, pressure, …

sensorVoltage, current

input

output

−AVinVin

VinVout

Vout

Vin

Vout

Modulation of some physical quantity (output) by some others

Some kind of gain, conversion ratio, sensitivity, etc


Example: Field-Effect Transistors (FETs)Semiconductor vs Metal

Vin Vout

Vin

Vout

FET’s are building blocks.

S D

G

Schematic illustration of a FET

For SiO2 dielectric, breakdown field Eb ~ 107 V/cm. No matter how thick it is, the maximum induced carrier area density is εrε0Eb/q = 2 × 1013 /cm2. (Recall Gauss’s law)

For a 1 µm thick Si channel, the intrinsic carrier density ni = 1.45 × 1010 /cm3,the background carrier area density is ni × 10−4 cm = 1.45 × 106 /cm2. (×2, considering both electrons and holes)

In principle, the area carrier density, and therefore the channel conductance, can be modulated by 7 orders of mag!!!


Example: Field-Effect Transistors (FETs)Semiconductor vs Metal

Vin Vout

Vin

Vout

FET’s are building blocks.

S D

G

Schematic illustration of a FET

For SiO2 dielectric, breakdown field Eb ~ 107 V/cm. No matter how thick it is, the maximum induced carrier area density is εrε0Eb/q = 2 × 1013 /cm2. (Recall Gauss’s law)

For a 1 µm thick Si channel, ni = 1.45 × 1010 /cm3,the background carrier area density is ni × 10−4 cm = 1.45 × 106 /cm2.

In principle, the area carrier density, and therefore the channel conductance, can be modulated by 7 orders of mag!!!

For Al, n = 1.8 × 1023 /cm3. Even for 1 nm thin (monolayers!) Al, the background carrier area density is 1.8 × 1016 /cm2. The conductance can only be modulated by 0.1%!!!

(Si’s next-door neighbor in the periodic table)

What makes this difference?


Exercise

Calculate the values in the two previous slides

1. Given a dielectric breakdown field Eb = 107 V/cm for a FET, calculate the maximum possible induced areal carrier density. Use Gauss’s law. Give your rationale; show that this value is independent of the dielectric thickness.

2. For Si, the background volume carrier density is ni = 1.45 × 1010 /cm3. What is the areal background carrier density for a 1 µm thick slab of Si? Compare your answer to the above maximum induced carrier density. For how many orders of magnitude can you moderate the carrier density?

3. For Al, the volume carrier density n = 1.8 × 1023 /cm3. Assuming you could have 1 nm thin Al, what is the background carrier area density? Compare to the above maximum induced carrier density, and show that the conductance can only be modulated by 0.1%.

Note: We mostly use the SI units, where the unit for length is m. In semiconductor device physics, however, we customarily use cm for length. Make sure you get the units right.

What are semiconductors??? -- Simplified pictures

https://infogr.am/how-does-bond-structure-affect-melting-point

https://keterehsky.wordpress.com/2010/03/10/9-2-semiconductor-diod/

NaCl

http://knowledgebase.lookseek.com/Chemistry-Bonds-Ionic-Bonding.html

Bondpicture

Bandpicture

metallic covalent ionic

Energy of electrons filledem

pty

Allowed states

metal semiconductor insulator

Metal: “electron gas”

This line is fuzzy





NaCl


Bondpicture

Bandpicture



pty

Allowed states



This line is fuzzyThe bond and band pictures can be unified. That is a topic for ECE634.




NaCl


Bond pictureBand picture



pty

Allowed states



This line is fuzzy

This energy is called the Fermi level, Fermi energy, chemical potential. Subtle terminology differences in different fields.

EF

Band diagrams

semiconductor

EF

Evac

EC

EV

For homogeneous semiconductor, we often draw such a band diagram. What do these lines mean?

H2+ molecular ion and H2 molecule

Potential energy

(antisymmetric)

(symmetric)

H2

Antibonding, unoccupied

Bonding, occupied

2 atomic states form 2 molecular states.There are many other H atomic states forming higher unoccupied states. Only the highest (and only) occupied lowest unoccupied molecular states are important.

Energy of electrons

N atomic states form N molecular states. Small N, e.g. N = 2 for H2

Large N for solids.~ 1023 /cm3

N is so large that N states originating from one atomic states form a quasi-continuous band. There are gaps between bands. Electron filling determines whether the solid is metal or semiconductor/insulator.Evac (vacuum level) is the lowest energy of electrons outside the crystal.

Energy of electrons

Evac

Evac




NaCl


Bond pictureBand picture



pty

Allowed states



This line is fuzzy

Fully occupied band does not conduct electric current. Why? Bound (localized) vs. delocalized? Heuristic vs. rigorous (in ECE 634)

EF

Band diagrams

Si

EF

Evac

EC

EV

What do these lines mean?

Lowest unoccupied band of antibonding states of 2s and 2p Si atomic states (orbitals)

Highest occupied band of bonding states of 2s and 2p Si atomic states (orbitals)

Energy of electrons

Does the horizontal axis of such a diagram (for homogeneous solid) bear any meaning?

A homogeneous piece of material is boring, not a useful device.

https://www.sciencedirect.com/topics/computer-science/energy-band-diagram

Richard H. Bube, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

p type n type

Band diagram of pn junction

Built-in voltage

Here, the horizontal axis is meaningful.

https://www.sciencedirect.com/referencework/9780122274107/encyclopedia-of-physical-science-and-technology

https://en.wikipedia.org/wiki/P%E2%80%93n_junction

Built-in voltage

https://www.sciencedirect.com/topics/computer-science/energy-band-diagram

Richard H. Bube, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

p type n type

= q∆V

Can you measure the built-in voltage with a voltmeter? If we short-circuit a pnjunction diode with a wire, will the band diagram change?

https://www.sciencedirect.com/referencework/9780122274107/encyclopedia-of-physical-science-and-technology

A common concept that often falls through the crack (between courses)

What is voltage? In electromagnetism, we were taught that voltage is an electric potential difference.

Later, we studied semiconductor devices. Here’s what we were told: The pn junction has a built-in “voltage” that you cannot measure with a voltmeter. If you try, the meter reads zero.

Then we moved on to study the pn junction under bias. Here’s what we were told: The voltage (applied voltage to the pn junction in a circuit) is the difference between the two Fermi levels.

The built-in voltage, however, is an electrostatic potential difference.

electric potential difference

What’s going on? What is voltage, after all?

Let’s do some Gedankenexperiments:A charged capacitor, a battery.Measure the voltage with a voltmeter.Drive a load…

Consider an even simpler case: junction between two metals

A B

EF

Evac

EF

Evac

Two metals isolated

⊖ ⊖⊕ ⊕

Bend metal (wire) A to contact the other end of B.

Why doesn’t opposite charges cancel each other at the contact?

Why are there charges across the gap (wide enough to prohibit electron transfer)?

q∆V

This built-in voltage is called the contact potential (difference). It is also an electrostatic potential difference. Can we measure it with a voltmeter?

What’s going on? What is voltage, after all?

Let’s do some Gedankenexperiments:A charged capacitor, a battery.Measure the voltage with a voltmeter.Drive a load…

We now have a better understanding of the simple concept “voltage”.It is ∆EF.

EF has different preferred names in different disciplines. Physicists refer to it as EF only at T = 0. We will revisit this later.

Homework 1

A B

EF

Evac

EF

𝑊𝑊𝐴𝐴𝑊𝑊𝐵𝐵

By our simple model, 𝑞𝑞Δ𝑉𝑉 = 𝑊𝑊𝐴𝐴 −𝑊𝑊𝐵𝐵 in the case we discussed. What if we use a metal made from metal C (different from both A & B) to connect A and B?

One volunteer, to present in next class.

We stopped here on Tue 8/19/2021.

A Digression: The Vast Field of Electrical Engineering

• Different areas are different levels of extraction• Device engineers are at the junction of many disciplines• Follow your passion

Solid-state

physics

Device physics circuits

Transistor level

Higher level

Information theory

Control theory

chemistry

Semiconductor physics

Materials science

Semiconductor processing

Economics

Core knowledge body of the device engineer

This course is about the big picture.This course can be tailored to suit your research interest.

Let’s get to know each other!

• Name, year• Previous exposure to quantum mechanics, solid-state physics, device

physics, material growth/synthesis, processing, microelectronic or RF/microwave or power electronics circuits

• Advisor• Research field, particular topic (if you have one already)

SyllabusTopics• Quick review/overview of semiconductor physics

- Crystal structures, band structures, - Carrier statistics, doping- Defects, phonons, scattering, mobility, eletrical transport

• Device concepts - pn junction- MOSFETs, MESFETs, MODFETs, TFTs - Heterojunction bipolar transistors (HBTs) - Semiconductor processing- Photodiodes, LEDs, semiconductor lasers (?) - Introduction to nanoelectronics? Nanoscale CMOS?

Evaluation• Classroom participation, performance (15%)

• Homework / Mini projects – simple, often students taking turns (35%)

• Term presentation – review of a selected specific topic, or oral exam in a scope of your choice (50%)

• The good news: It’s not tough.

Reference books• M. Grundmann, The Physics of Semiconductors: An Introduction Including Nanophysics

and Applications, 2006Relativelyy up to date. Very comprehensive. A book I use for ECE634, but Part III about devices. E-book available at UT Lib.

• Jasprit Singh, Semiconductor Devices: Basic PrinciplesBook devices but including semiconductor physics & processing.

• U. K. Mishra & J. Singh, Semiconductor Device Physics and DesignE-book available on line thru UT Lib.

• R. S. Muller & T. I. Kamins, Device Electronics for Integrated CircuitsAn undergrad textbook on Si microelectronics, but good to have. I go back to it sometimes.

• Ben Streetman, Solid State Electronic DevicesFrom basic physics to device concepts. Oldie goodie.

• Karl Hess, Advanced Theory of Semiconductor DevicesThin, but covers lots of stuff at advanced levels

• S. M. Sze (施敏), Physics of Semiconductor DevicesThe “Bible” of device engineers. Not for beginners. Keep it in mind or on your shelf; an excellent reference book for your future career. A few editions over decades.

• J. D. Plummer, M. D. Deal, P. B. Griffin, Silicon VLSI technology: fundamentals, practice and modeling

Best textbook on processing, by the people who developed many of the models.

Summary & Outlook

• Semiconductors are used in active devices in information systems because they provide large modulation ranges.

• We view solid-state materials from two complementary and unified perspectives: the bondpicture and the band picture.

• The band picture is powerful in understanding the electrical and optical properties of semiconductors:

• Conduction band, valence band, and the gap…• States are occupied by electrons up to the “Fermi level” (at zero temperature; there is a

distribution at nonzero temperatures). Keep in mind terminology difference between fields.• We can thank the band gap for the large modulation range. To be better understood later.• We re-discovered voltage. • Before we move on to devices, we will first review common concepts in semiconductor

device physics, and ask questions.

ece 635 advanced semiconductor devices

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