Semiconducting Devices: An Introduction, Lecture 5

Stephen Remillard2020-12-07

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

2K views|3 years ago

💫 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.

00:02 before we dive in deep to the physics of

00:04 semiconductors and semiconducting

00:06 materials

00:08 now stop here and describe some of the

00:10 devices that is

00:11 motivating that physics because it's a

00:14 lot easier to appreciate why you want to

00:16 know these things about the

00:17 carrier concentrations the density of

00:19 states the fermi energies and everything

00:22 else that comes into the physics

00:24 if you know a little bit about how

00:26 that's used in the design

00:28 of a device so i'll describe

00:31 three device categories starting with

00:34 junction devices

00:36 junctions dominate semiconducting device

00:38 characteristics

00:40 especially in integrated circuits where

00:42 we're

00:43 dealing with transistors they're filled

00:45 with junctions

00:46 if you don't have junctions then you're

00:48 not going to have non-linear

00:49 semiconductor devices

00:51 a pn junction is made from two types of

00:54 semiconductor materials

00:56 a p-type whole built material and n-type

00:59 or electron-doped material

01:01 the prototype we'll illustrate with here

01:04 is a single

01:05 ribbon of semiconductor all one material

01:08 perhaps

01:09 silicon and it was formed by diffusing

01:13 donor dopants and acceptor dopings into

01:16 it one at a time

01:17 first the right side was masked off and

01:19 donor dopants were diffused in

01:21 and then the left side was masked off

01:23 and hole dopants were diffused into

01:26 it the result is this ribbon that's

01:29 half n type and half p type a battery is

01:33 connected to it

01:34 with the positive terminal of the

01:35 battery connected to the p

01:37 type side and the negative terminal of

01:40 the battery connected to the end type

01:42 side but there's a switch between

01:44 the battery and the semiconductor we

01:47 will first talk about what

01:48 the carrier concentration across the

01:52 semiconductor looks like

01:53 when that switch is open and it is just

01:56 a

01:57 junction that was found sitting there

02:00 and we're going to look at how the

02:02 carrier concentration the number of

02:03 carriers per

02:04 cubic centimeter varies going from the

02:07 left to the right

02:08 where n is the electron concentration

02:10 and p

02:11 is the hole concentration for now let's

02:14 have the switch

02:15 open so we can answer the question of

02:18 how are the carriers distributed

02:20 throughout the semiconductor

02:22 when there is no bias on it no

02:25 current is able to pass through it that

02:28 condition is referred to as thermal

02:29 equilibrium

02:31 when current cannot pass through the

02:33 junction

02:34 the left side is full of electrons and

02:36 the right side is full of holes and so

02:39 we can sketch that accordingly

02:41 in this first graph which is a carrier

02:44 concentration

02:45 versus position along the junction

02:48 let's look at how the carrier

02:49 concentration varies across where n

02:52 is the electrons per cubic centimeter

02:55 and p

02:55 is the holes per cubic centimeter with

02:57 the switch

02:58 open electrons are a high population

03:02 over on the left side and low population

03:05 on the right you see these two types of

03:06 semiconductors are attached

03:09 which means that the electrons in the

03:12 n-type

03:12 will diffuse into the p-type because the

03:15 n-type being full of the electrons and

03:17 the p-type having

03:18 virtually any electrons will mean that

03:20 some of these electrons

03:22 will diffuse over because when you have

03:24 a dense population of one

03:26 particle type in one region and no

03:29 population in another region diffusion

03:31 naturally occurs

03:33 the holes will do the same thing because

03:35 you have a lot of holes on the right and

03:38 not many holes on the left

03:40 and so holes will diffuse into the

03:42 n-type region

03:43 now you might wonder why don't they

03:45 diffuse more than this

03:47 so the n-type region is donor doped

03:51 donor ions are positively charged

03:55 an n-type semiconductor all by itself is

03:58 electrostatically neutral there are as

04:00 many free electrons as there are

04:02 positive donor ions charge neutrality

04:06 will be maintained throughout this

04:09 semiconductor

04:11 so when the electrons move from the

04:14 charge neutral n-type side into the

04:17 charge neutral p-type side they

04:20 encounter not just a bunch of holes

04:23 which

04:23 should be attracting them they counter

04:26 an equal number

04:27 of negative acceptor ions and so when

04:30 one electron moves

04:32 from the left side to the right side

04:35 the right p-type side becomes negatively

04:39 charged

04:40 and when a hole moves from the right

04:42 p-type side into the left

04:44 n-type side the left n-type side becomes

04:47 positively charged for now we will

04:50 define the junction as the place where

04:52 the carrier concentrations have gone to

04:55 half their

04:56 value deep inside their respective

04:58 regions

05:00 we can look at the energy of this

05:01 situation we'll put that in the bottom

05:04 graph

05:04 as you go across the junction from the

05:06 left side to the right side

05:08 the potential energy experienced by

05:10 electrons and holes is going to change

05:14 the electrons have their preferred

05:16 location where the potential energy

05:18 that's therefore lower and the holes

05:20 have their preferred location where

05:22 their potential energy

05:23 is therefore lower in semiconductor

05:26 physics we always plot the electron

05:30 energy not the whole energy so the

05:33 electron

05:34 energy as you go across this junction

05:37 looks like this it's lower in the n-type

05:40 region and is higher in the p-type

05:43 region

05:44 for that same reason that i just

05:46 mentioned when an electron moves into

05:48 the p-type

05:49 it encounters all of those negative

05:52 acceptor

05:52 dopants and it finds itself being

05:55 repelled

05:56 back to the n-type and so there's a

05:59 energetically favorable region for

06:01 electrons

06:02 and conversely holes are energetically

06:05 favorable on the right

06:06 when the electron is on the right it's

06:08 energetically unfavorable

06:10 it is in a more negative electric field

06:13 meaning that the potential energy of the

06:16 electrons

06:16 which equals minus q times the

06:20 potential goes up just remember that

06:23 when

06:23 charge is in a region of

06:27 the same charge it goes up and so when

06:29 that

06:30 electron encounters acceptor dopants it

06:32 wants to go back home

06:35 so the potential energy of an electron

06:37 goes up

06:38 as it diffuses into the p-type region

06:40 and that's reflected in this graph

06:42 of the electrons energy as it goes

06:46 across the junction

06:47 it's lower on the n side and it's higher

06:50 on the p side and so the electrons

06:53 energy goes up as it diffuses into the

06:55 p-type region

06:57 and that's reflected in the energy level

06:59 diagram

07:00 of the conduction electrons as the

07:03 energy goes

07:04 up and amount delta e so the electrons

07:07 go into the p-type

07:08 because there's a low population of

07:11 electrons over there and that's what

07:12 diffusion

07:13 is going to where the population is low

07:15 but when they get there they find

07:17 themselves to be electrostatically

07:19 unwelcome and so they roll back downhill

07:23 so that's the conduction bandage all

07:26 free electrons

07:27 have an energy that is higher than this

07:30 energy of called e sub c

07:33 then there's the valence band edge all

07:36 bound electrons that are stuck in atoms

07:39 have an energy that is below

07:41 this level the separation between the

07:45 conduction band edge and the valence

07:48 band edge

07:49 is the energy gap and it's the same

07:52 throughout because doping doesn't change

07:55 the energy gap

07:56 so e sub g like 1.12 electron volts or

08:00 silicon

08:01 is the same everywhere across the n and

08:03 p

08:04 types okay so if i

08:07 took my electron tweezers and dragged an

08:10 electron from the left side

08:12 over to the right side i'll be dragging

08:14 it up a hill

08:16 a hill of height delta e

08:19 the built in energy which is

08:22 best referred to actually in voltage so

08:25 it's

08:26 q 1.6 times 10 minus 19 coulombs

08:29 times the built-in potential p sub b

08:32 i will encounter this quantity built in

08:35 potential a lot

08:37 over the next many quite a few chapters

08:40 it's simply the difference in the

08:41 conduction band edge

08:42 on the p side minus the conduction band

08:45 edge on the

08:46 n side fisa bi the built-in potential is

08:50 what you would actually

08:51 measure with the laboratory instrument

08:53 with contacts placed on either side of

08:55 the

08:56 junction to replace the battery with the

08:58 voltmeter

08:59 you would read visa bi so the electron

09:02 does have to climb a potential hill

09:04 to get from the n side to the p side and

09:08 that's a big deterrent

09:09 but you also have the diffusion force

09:12 pushing it over there

09:13 and so there's a trade-off between the

09:15 electrostatic repulsion

09:17 that it encounters when it gets there

09:20 and the fact that the concentration

09:22 gradient drops off

09:23 attracting therefore the electrons over

09:26 through there

09:28 i want to point out one more energy in

09:30 this diagram and that's this horizontal

09:32 dashed line labeled e sub f

09:36 f is for fermi doesn't show here so i'll

09:38 just spell it f e

09:40 r m i is the fermi energy

09:43 it's an energy that's characteristic of

09:45 the semiconductor

09:46 when there's no battery connected as in

09:49 this

09:50 case it's a horizontal line

09:53 when we close a switch and apply a

09:55 potential difference across

09:57 the junction the shape of that fermi

09:59 energy will change but for now it's just

10:01 a horizontal line

10:03 the fermi energy is in the gap

10:06 and it's always closer to the conduction

10:09 band edge in an n-type semiconductor

10:12 and it's always closer to the valence

10:14 band edge in a p-type semiconductor

10:17 and so you can think of that as an

10:18 explanation also for why these

10:20 bands have to bend up because that

10:23 condition has to be met with the fermi

10:25 energy as a horizontal line

10:26 it has to be closer to the valence band

10:28 on the right and the conduction band on

10:30 the left

10:32 so this is a one dimensional energy band

10:34 diagram following the

10:35 energy as you proceed in one dimension

10:37 from the left side to the right side

10:39 i do want to mention one thing about

10:42 this energy gap

10:44 so it's the same everywhere throughout

10:46 the junction provided the junction is

10:48 one base material so provided is

10:50 fabricated as i described

10:52 a piece of say silicon that's been

10:54 n-doped

10:55 on the left and p-doped on the right

10:58 quite frequently

10:59 junctions are made from two different

11:01 base semiconductor materials

11:04 in which case they're called

11:05 heterojunctions and that's done

11:07 specifically to get a different band gap

11:10 on the left and the right so maybe

11:11 silicon on one side and some alloy on

11:14 the right side

11:15 so that's with the switch open no

11:18 current passes through the junction

11:20 and we're in a condition called thermal

11:22 equilibrium

11:24 so think of those as synonymous no

11:26 current passenger

11:27 junction and thermal equilibrium when

11:29 you're in thermal equilibrium there's no

11:31 current passing through the fermi energy

11:33 is a horizontal line

11:36 it will be different when current is

11:37 allowed to flow we're not going to

11:39 address that until later

11:42 so let's go ahead and close the switch

11:45 when we close the switch now we've

11:47 placed a positive

11:49 potential on the right end on the p-type

11:52 side

11:53 and the negative potential on the left

11:55 end on the n-type side

11:57 so the n-type is filled with the

11:58 electrons but we've put

12:00 negative potential over here those

12:03 electrons

12:04 are chased to the right they feel

12:07 less of a problem with heading into the

12:10 p-type

12:11 because they really don't like it over

12:13 here anymore

12:14 so they have less of a potential hill to

12:16 climb that built-in potential

12:19 is no longer the barrier it once was

12:22 the height of the potential hill has

12:24 become less

12:26 it used to be q times built-in potential

12:28 but now we have to deduct the

12:30 voltage of this battery from the

12:32 built-in potential

12:34 to get the new voltage difference

12:37 between these two sides

12:38 it subtracts it to the opposite so now

12:41 there's a shorter hill

12:42 and less of a voltage drop across the

12:45 junction

12:46 the electrons have a much easier time

12:48 getting across

12:51 this bias condition is referred to as

12:53 forward bias where you put the positive

12:55 voltage on the p

12:57 side and the negative potential is in

12:59 contact with the

13:00 n side and current goes through much

13:03 more readily

13:05 i drew the fermi energy is still

13:07 horizontal that's not

13:09 really right if you have a significant

13:11 amount of current going through the

13:13 junction

13:13 it will change shape and that will be a

13:16 subject that we have to consider

13:18 later but we will consider it

13:20 extensively later

13:22 so for now we're not going to focus on

13:24 what the fermi energy looks like

13:26 when we have current going through the

13:28 gap doesn't change

13:29 the battery has no ability to change

13:32 band gap

13:33 so it's the same if this is silicon

13:36 based material it's the same 1.12

13:38 electron volts all the way across

13:41 now if we vary the voltage starting at

13:44 zero for the origin

13:46 and increasing it as we go to the right

13:49 we can measure the current and generate

13:51 in the current voltage characteristic

13:54 we can flip the battery around put the

13:56 positive terminal on the left and the

13:58 negative terminal of the battery on the

13:59 right and

14:01 b to the left of the origin have

14:03 negative voltage

14:05 so those two conditions are referred to

14:06 as forward biased when you have the

14:08 positive terminal on the p-type or

14:10 reverse biased when you have the

14:11 positive terminal on the end type

14:13 and if you measure current versus

14:15 voltage you get this exponential

14:17 behavior described by the shockley diode

14:20 model

14:22 in the shockley diode model you have qv

14:25 q the charge of the electron 1.69 minus

14:28 19 coulombs

14:29 v is applied voltage k is boltzmann's

14:32 constant

14:33 t is temperature in kelvin

14:36 i zero is the reverse bias current

14:40 when the voltage becomes negative so if

14:42 you flip the battery around and you have

14:44 a large negative voltage

14:45 this term very quickly becomes

14:48 negligible

14:50 and you just have i equals minus i zero

14:52 for large negative voltages

14:54 and it levels off so that's why i sub 0

14:57 is called the reverse bias current

14:59 and then there's the ideality factor eta

15:02 oftentimes eta is just left at 1 meaning

15:06 ideal

15:06 but in a real pn junction you have a

15:09 phenomenon at the junction going on

15:11 called

15:12 electron hole recombination

15:15 which alters the behavior of the diode

15:18 and this ideality factor is meant to

15:21 describe

15:21 that empirically for a lot of integrated

15:24 circuit devices we can go ahead and take

15:26 ada to be equal to

15:28 one especially since we're focused

15:30 primarily on the field effect

15:32 in integrated circuits rather than the

15:34 pn junctions

15:36 real data indicates a much stronger

15:39 dependence on voltage for a

15:41 germanium-based diode than a

15:43 silicon-based diode

15:45 a germanium based out will pass a lot

15:46 more current as well

15:48 they both follow the shockley diode

15:51 equation

15:52 but the reverse bias current for a

15:54 germanium diode is much larger a tenth

15:56 of a milliamp

15:57 compared to a tenth of a nano amp

15:59 perhaps for a silicon

16:01 based diode and the consequence of a

16:03 larger reverse

16:04 bias current is that you also have a

16:07 larger forward biased current

16:09 because of the just the structure of the

16:11 equation

16:13 so two salient points to remember about

16:17 pn junction devices is one

16:20 that there is a built-in potential fisa

16:22 bi

16:23 it's a consequence actually of the

16:25 doping level and it's going to keep

16:27 coming back to us so we'll have to get

16:29 comfortable with the idea that when you

16:31 connect a p

16:32 and an n-type semiconductor together

16:34 there's a potential hill

16:36 that carriers need to climb to cross

16:38 over to the other side

16:40 the second sailing point is that the

16:43 band gap is a constant

16:45 throughout the junction fabricated from

16:47 the same

16:48 piece of semiconductor and it does not

16:50 vary with either

16:52 doping or with bias voltage

16:56 now i'll speak briefly about two other

16:58 device categories the field effect

17:01 devices

17:01 and the opto electronic devices those

17:04 are the three device types we'll cover

17:05 in this course

17:07 the field effect is the really

17:09 fundamental basis of

17:10 the transistors that are used in

17:12 integrated circuits

17:14 imagine a p-type semiconductor and

17:18 they have an electrode that faces it and

17:21 there's an air gap between them

17:23 the electrode is held at a voltage v

17:26 the p-type semiconductor is grounded the

17:29 voltage is

17:30 positive so as i turn up that voltage

17:33 start at zero and then turn it up higher

17:35 and higher

17:36 i have this capacitor formed from it

17:39 where i have a

17:40 positive charge on this electrode plate

17:44 that chases the holes out of the p-type

17:48 as i slowly raise this voltage holes

17:51 begin to exit the p-type

17:52 but also as i slowly raise this voltage

17:55 electrons come

17:56 from ground into the p-type and they're

17:59 very

18:00 strongly attracted to that electrode

18:02 they just can't get to it because of the

18:04 air gap

18:05 so what they do is they form a layer at

18:08 the top of the semiconductor slab

18:11 and so you have this layer of electrons

18:14 in what is otherwise a p-type

18:16 semiconductor

18:17 so you have this top region of this

18:20 b-type semiconductor that has been

18:21 converted

18:22 into an n-type and it's called the

18:24 n-type

18:25 channel the voltage on that electrode

18:29 where the n-type channel begins to

18:32 establish

18:32 itself is called the threshold voltage

18:35 and once you exceed that voltage we say

18:38 that the semiconductor is in

18:39 inversion because now there is a layer

18:42 on the top of the semiconductor that has

18:44 inverted to the

18:45 other type this is the essence of

18:49 the electric field effect the

18:52 transistors that we're going to focus on

18:54 in this course

18:55 field effect transistors operate on the

18:57 principle of this effect

18:59 this sort of capacitor and pn junctions

19:02 are integrated together into a single

19:04 device called a field effect transistor

19:07 and we're going to be studying that in

19:09 quite a bit of detail

19:11 the device type that dominates the

19:13 design of integrated circuits is in fact

19:15 the mosfet the metal oxide semiconductor

19:19 field effect transistor

19:21 this field effect capacitor is the

19:23 beginning the building block for all of

19:25 that

19:26 when we finish pn junctions we will do

19:29 an entire

19:30 unit on optoelectronics leds and

19:33 led lasers light emitting diodes are

19:37 based on a pn

19:38 junction when you forward bias the pn

19:41 junction by putting the positive

19:43 external voltage on the

19:45 p side and the negative external

19:48 potential on the n side

19:49 the bands become less bent the

19:52 conduction band edge moves up in the end

19:54 type and there's less of a potential

19:56 hill for electrons to climb to get over

19:58 to the peace side so they do and they go

20:01 over to the peace side

20:03 when they're over there they find a hole

20:05 and what is a hole

20:06 except an electron vacancy so an

20:09 electron sees a vacancy down there in

20:11 the valence band where the energy is

20:13 much

20:13 lower so that's preferable so it goes

20:15 there and in the process of going down

20:18 to the vacancy it emits a photon

20:21 electron hole

20:22 annihilation produces a photon and you

20:25 might

20:26 say what's the chances of that because

20:28 the holes will

20:30 also diffuse over into the n-type side

20:33 and you might not have sufficient

20:35 electrons and holes in the same

20:36 region to get a lot of light and that

20:38 would actually be very true

20:40 leds depend on a fact that was going to

20:43 become important for us soon

20:45 that electrons are much more mobile than

20:48 holes they both

20:49 move but electrons move more readily and

20:53 so the p-type side can fill up with the

20:55 electrons more easily than the n-type

20:57 side

20:58 what you end up having then is in this

21:00 region near the junction on the p-type

21:02 side you have

21:03 a high population of both the electrons

21:05 and holes

21:06 and they recombine in that small region

21:09 creating a very bright light so that's a

21:12 very brief

21:13 overview to how an led works we will go

21:16 in

21:17 much more detail later when we have

21:20 finished up with pn

21:22 junctions and we'll look at the physics

21:24 of how that photon is generated

21:27 and how that affects the design

21:30 of an led as well as we'll look at what

21:33 the materials

21:34 issues are in making efficient

21:37 light emitting diodes so those are the

21:40 three device

21:41 types the pn junction the field effect

21:44 devices

21:44 and the optoelectronic devices the three

21:47 categories that we're going to spend

21:49 time on

21:50 this semester next we'll start talking

21:52 about the physics of semiconductors

21:54 what the carrier concentrations are why

21:57 they are

21:58 what they are how they can be controlled

22:00 how

22:01 doping can affect the carrier

22:03 concentration and what that does

22:05 to the performance of the semiconductor

22:07 that's all coming up next

💫 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.

🎥 Related Videos