Semiconductors and semiconductor devices

Question 1

What can carry charge? (2)

Answer 1

electrons in most solids

ions can also carry mostly in liquid solutions

Question 2

Why don’t the negatively charged electron bump into the +ve charged Cu ions?

Answer 2

Because the electrons are waves travelling in a periodic arrangement of Cu ions - the electron wave avoids the ions – that’s partly the reason we
have an energy gap – the energy gap is related to the electron waves that can’t propagate because they are reflected by the planes of atoms – like Bragg reflection of X-rays

Question 3

What happens in a microscopic model of conduction using Ohm’s law?

Answer 3

There will be defects which include dislocations, impurities, surfaces so the electrons accelerate for bit collide with a defect, stop or go off in a different direction and start again – this constant colliding is called scattering – the defect is called a scattering centre

• Even in a perfect crystal if T>0 the temperature induced motion moves the Cu ions from their usual position the crystal is not longer periodic and the electron is scattered

Question 4

What is the defect called scattering centre

Answer 4

the electrons accelerate for bit collide with a defect, stop or go off in a different direction and start again – this constant colliding is called scattering

Question 5

What is drift velocity?

Answer 5

the constant stop-go motion of the charge carrier (the electron) with an averaged velocity

Question 6

How is conductivity determined in a metal?

Answer 6

determined by the concentration of electrons in the conduction band and the mobility of the electrons

Question 7

How is mobility determined in a metal?

Answer 7

the mobility is determined by the number of scattering centres and the “effective mass” of the electron in the conduction band

Question 8

In a metal, as As the temperature goes up……..scattering and mobility…

Answer 8

scattering goes up and the mobility drops.

Question 9

If a metal has lots of dislocations the conductivity will be….

Answer 9

lower

Question 10

What are the charge carriers in semiconductors?

Answer 10

the charge carriers are both electrons in the conduction band and holes in the valence band – the mobility the electrons and holes is usually different in a given semiconductor and also varies between semiconductors.

Question 11

Conductivity in semiconductors increases when… temperature…..

Answer 11

The conductivity of semiconductors increases with temperature

Question 12

Why does conductivity increase in semiconductors when temperature increases?

Answer 12

because although their mobility is reduced the concentration of electrons and holes increases and that dominates

Question 13

When will charge carriers not scatter?

Answer 13

If we had a perfect crystal at T=0 then the charges carriers would not scatter and material would not obey Ohm’s law

Question 14

What effects the mass of the charge carriers?

Answer 14

effected by moving through a period array of ions that make up the crystal lattice and to take account of that we say that the charge carrier has “an effective mass” in a solid. Electrons behave as if they had a different mass from free space when they are in a solid.

Question 15

Apart from effective mass, what happens when you increase the number of defects and impurities in a solid?

Answer 15

increasing the number of defects and impurities in the solid reduces the mobility of the carriers and so reduces the conductivity.

Question 16

Does the Presence of imperfections increase or decrease resistivity?

Answer 16

increases resistivity

Question 17

What are examples of imperfections? (4)

Answer 17

grain boundaries dislocations

impurity atoms vacancies

Question 18

Why do imperfections increase resistivity?

Answer 18

As they act to scatter electrons so that they take a less direct path

Question 19

Resistivity increases with…..

Answer 19

temperature

• impurity
• -deformation

Question 20

What is the difference between intrinsic and extrinsic conduction (number of electrons and holes)?

Answer 20

• The number of electrons = number of holes in intrinsic

- the number of electrons does not equal the number of holes in extrinsic

Question 21

How do extrinsic conduction occur?

Answer 21

when impurities are added with a different number of valence electrons than the host

When the electri- semiconductor
cal characteristics are dictated by impurity atoms,

Question 22

In an n-type extrinsic is n greater or less than p?

Answer 22

n is greater than p

Question 23

In an p-type extrinsic is n greater or less than p?

Answer 23

p is greater than n

Question 24

Why does conductivity increase doping?

Answer 24

Imperfection sites lower the activation energy to

produce mobile electrons.

Question 25

How does intrinsic conduction occur?

Answer 25

those in which the electrical extrinsic behavior is based on the electronic structure inherent in the pure material

When temperature of an extrinsic increases to the point it becomes intrinsic. for T&raquo_space; 450 K: “intrinsic

Question 26

What are the three stages in a semiconductor?

Answer 26

freeze-out
extrinsic 
intrinsic
- for T < 100 K: "freeze-out, thermal energy insufficient to excite electrons.
- for 150 K < T < 450 K: "extrinsic" 
- for T >> 450 K: "intrinsic

Question 27

For intrinsic semiconductors conductivity increases with….

Answer 27

temperature, it is the opposite to metals.

Question 28

What does MOSFET stand for?

Answer 28

Metal Oxide Semiconductor Field Effect Transistor

Question 29

Why are MOSFET called field effect?

Answer 29

because the electric field in the channel controls the
number of charge carriers in the channel –the voltage on the gate determines
the field in the channel.

Question 30

What parts are there in a MOSFET? (6)

Answer 30

• Three metal pieces, the first one is the source, second is the gate and third is the drain
• SiO2 insulating layer
• p-type channel; two sections of p-Type Si
• n-type Si substrate

Question 31

How do you turn the MOSFET on and off?

Answer 31

• The voltage on the gate controls (switches on and off) the current between the source and drain

Question 32

What happens during MOSFET operation?

Answer 32

A positive voltage on the gate drives the holes out of the p channel, remember in a p-type material (σ = p e μh) a positive gate voltage makes p go to zero in the channel and so the conductivity goes to zero and the MOSFET switches off. The electric field at the gate drives the carriers out of the channel. When the carriers are removed the channel is said to be depleted.

Question 33

What are dielectrics on the MOSFET?

Answer 33

SiO2

Question 34

What are dielectrics?

Answer 34

Insulators do not conduct electricity at least not Direct

Current (DC) electricity

Question 35

How are dielectric properties measured?

Answer 35

Their dielectric properties are measured by their relative permittivity (or
dielectric constant), εr

Question 36

Why is SiO2 used in a MOSFET?

Answer 36

he SiO2 on the MOSFET is used because it’s a dielectric – easily
made on the Si by just exposing the surface of the Si crystal to hot O2 -
it has good dielectric properties. For SiO2, εr~3

Question 37

What is going on inside the dielectric in a MOSFET?

Answer 37

In a dielectric the charges are bound and cannot move – the electrons are in the valence band. Electric dipoles form in the material when a voltage is applied

Question 38

The total amount of charge stored relates to all the sum of all the……

Answer 38

The total amount of charge stored relates to all the sum of all these dipoles in the material.

Question 39

What happens when you apple a very large voltage to the dielectric?

Answer 39

if we apply a very large voltage we can give the bound charges enough energy to cross the energy gap and the dielectric will be come conducting – when this happens it’s called dielectric breakdown – lightning is an example of dielectric breakdown.

Question 40

The element P is a substitutional impurity in Si crystal. What type of dopant is it called?

Answer 40

Donor dopant, as P is a non-metal that receives electrons

Question 41

What type of elements are used for p-type doping?

Answer 41

Elements with 3 valence electrons are used for p-type doping

Question 42

What type of elements are used for n-type doping?

Answer 42

elements with 5 valence electrons for n-doping.

Question 43

What happens during p-type doping?

Answer 43

The 3-valent dopants can catch an additional outer electron, thus leaving a hole in the valence band of silicon atoms. Therefore the electrons in the valence band become mobile. The holes move in the opposite direction to the movement of the electrons.

Question 44

Why is p-type doping called p-type?

Answer 44

With the inclusion of an electron, the dopant is negatively charged, such dopants are called acceptors (acceptare, lat. = to add). Again, the dopant is fixed in the crystal lattice, only the positive charges can move. Due to positive holes these semiconductors are called p-conductive or p-doped. Analog to n-doped semiconductors, the holes are the majority charge carriers, free electrons are the minority charge carriers.

Question 45

What happens in n-type doping?

Answer 45

Four outer electrons combine with ever one silicon atom, while the fifth electron is free to move and serves as charge carrier. This free electron requires much less energy to be lifted from the valence band into the conduction band, than the electrons which cause the intrinsic conductivity of silicon. The dopant, which emits an electron, is known as an electron donor

Question 46

Why is n-type doping called n-type?

Answer 46

The dopants are positively charged by the loss of negative charge carriers and are built into the lattice, only the negative electrons can move. Doped semimetals whose conductivity is based on free (negative) electrons are n-type or n-doped. Due to the higher number of free electrons those are also named as majority charge carriers, while free mobile holes are named as the minority charge carriers.

Question 47

When a positive voltage is applied to the gate the conductivity of the channel drops to zero - why?

Answer 47

p = 0, the number of holes in the channel drops to zero because the positive charge carriers - the holes are repelled by the positive voltage on the gate