Electrochem + Semiconductors

Question 1

oxidation reactions is the ___ of electrons and happens at the ___

Answer 1

loss of electrons and happens at the anode (an ox)

Question 2

reduction is when a species ___ electrons and happens at the ___

Answer 2

gains electrons and happens at the cathode

Question 3

When given two reactions, how do we figure out which will happen?

Answer 3

The half reaction that is positive will proceed as written. The more negative one will proceed in the other direction. More positive reactions will proceed spontaneously.

Question 4

During charing, electrons flow from the

Answer 4

cathode (positive side) to the anode (negative side)

Question 5

During discharge, electrons flow from

Answer 5

the anode (negative) to the cathode (positive)

Question 6

Cathode is the

Answer 6

positive electrode (Li is moved from here to anode)

Question 7

Anode is the

Answer 7

negative electrode

Question 8

Given a voltage “flat”/”jump”/”smooth”, match the phase regions

Answer 8

flat = two phase region

jump = new phase

smooth = solid solution

Question 9

Electrons moving into the applied field have

Answer 9

Lower energy

Question 10

Electrons moving WITH the applied field have

Answer 10

higher energy

Question 11

Insulators have

Answer 11

A wider band gap and fewer electrons excited across the band gap

Question 12

Semiconductors have

Answer 12

a narrower band gap with more electrons excited across the band gap

Question 13

Semiconductors are different from metals in that

Answer 13

As opposed to conductivity decreasing with temperature rising, conductivity rises when temperature rises.

Question 14

What are the two types of electronic charge carriers

Answer 14

Electrons (negative charge, promoted to conduction band) and holes (positive charge and the vacant electron state in valence band)

Question 15

What are the two periodic trends of semiconductors

Answer 15

1. The wider the electronegativity between the elements the larger the energy gap.

The lower on the periodic table the compound, the smaller the band gap.

(reduced covalent overlap with increased interatomic distance makes it go from covalent to metallic . . . insulator to metal as move down periodic table)

Question 16

What’s the difference between n type and p type semiconductors

Answer 16

Both are extrinsic semiconductors created by doping or adding controlled amounts of impurities to form substitutional impurities with pure semiconductors (n type has electrons as majority carriers) while p type has holes as majority carriers

Question 17

How does a n type semiconductor add conductivity

Answer 17

The dopant forms an energy state in the band gap where a loosely bound electron is donated to the conduction band and leaves very little energy for excitation

Question 18

How does a p type semiconductor add conducitivity

Answer 18

P type semiconductor formas an energy acceptor in band gap. There’s a small hole which accepts the electron from the valance band by having state excited. No free electron is formed during excitation.

Question 19

How does temperature affect conductivity of intrinsic semiconductors

Answer 19

Conductivity is exponentially dependent on T because the electron has to jump across a band gap to conduct which requires energy supplied thermally. Number of charge carriers n increases exponentially with temperature T.

Question 20

How does band gap affect carrier concentration

Answer 20

Those with a smaller band gap will result in a higher carrier concentration (less energy required to excite electrons across the band gap)

Question 21

What effect does thermal vibrations and impuritites (dopant concentration) have on the lattice

Answer 21

Thermal vibrations and impurities will reduce the carrier mobility (mu) or u

Question 22

What are the three states of extrinsic semiconductor conductivity

Answer 22

Conductivity increases with doping because doped sites have low activation energy to product mobile electrons and holes.

There is a freeze out region in Si T < 100K

Where thermal energy is sufficient to excite dopant carriers

IN the Extrinsic doping region between 150K and 450K

All of the dopants have ionized and ni is much smaller than n

For the intrinsic region > 450K

Intrinsic electorns are excited across the gap and overwhelm the donor carrier contribution which converges to ni

Question 23

What is a p-n rectifying junction?

Answer 23

When we combine a p and a n type semiconductor, the p type and n type diffuse into the other ends respectively, the ionized donor and acceptor atoms left behind creates an electric field from donor to acceptor that opposes carrier diffusion. Under forward bias, the electric field eforward opposes the built in field and current flows when the field is stronger than the bias field

Question 24

Given a p/n junction, which side is positive and which side is negative

Answer 24

The p side or plus side is positive charge while the n side or donor side is negative charge.

Question 25

What are the 7 differents of a battery coin cell?

Answer 25

Casing, Gasket, Separator, Cathode, Anode, Electrolyte, Spring

Question 26

What are good properties of a battery casing?

Answer 26

We need something tough and durable that can hold up to the mechanical distress placed on the battery. Corrosion resistance is important as is a malleable metal. It needs to be conductive.

Question 27

What are good properties of a good gasket?

Answer 27

Gasket stopes the electrolyte from leaking out of the battery and prevents contaminants from entering the battery. We need a good physical barrier and diffusion barrier. The material has to be deformable and elastic so there is a good seal. Rubber is used

Question 28

What are good properties of a good cathode?

Answer 28

In a Li-ion cell, the cathode needs to be able to store a lot of Li to extend the lifetime of each battery charge. Think lithium cobalt oxide. If the Li can easily transport from near from the cathod material, it’s good. Pseudo tetrahedral structure allows for good diffusion pathways of Li. As long as preferred coordination is allowed, we’re good.

Question 29

What are good properties of an electrolyte

Answer 29

Good electrolyte allows for high diffusion of lithium atoms and electrical resistivity. Good reliability in the form of little degradation is also desired. New solid state electrolytes prevent flammibility of liquid based electrolytes

Question 30

What are features of a good separator?

Answer 30

A good separator needs to be electrically insulating while providing high LI diffusion so the Li can move from cathode to anode and back.

Question 31

What are good features of the spring

Answer 31

Anode will swell during operation, so we want to keep the battery together. Good spring does deform elastically but not plastically

Question 32

What are good features of a battery anode

Answer 32

There is a large volume change with absorption, so it is important to choose a material that will not crack or deform after repeated charing and discharging cycles. Silicon USED TO be ideal but expands too much. Lithium forms dendrites and becomes flammable. Try carbon.

Question 33

What are the units for the following:

Potential

Capacity

Energy

Power

Efficiency

Answer 33

Potential: E

Capacity: Q

Energy: E*Q

Power: E*Q/t

Efficiency: n

Question 34

What are characteristics about charge carries with regards to drift velocity, mobility, and charge magnitude

Answer 34

Each charge magnitude is the same

Each carrier has different drift velocity

Each carrier has different mobilities

Question 35

What is the influence of temperature T on intrinsic semiconductors

Answer 35

Raising the temperature will raise the conductivity because the thermal energy supplied allows for electrons to jump across the band gap (exponential dependence on T because of Arrhenius-like behavior)

Question 36

How is carrier mobility influenced by dopant concentration

Answer 36

Carrier mobility is decreased by thermal vibrations and impurities (dopant concentration) in the lattice

Question 38

How is carrier mobility influenced by temperature

Answer 38

Thermal vibrations will scatter electrons more as temperature rises, decreases carrier mobility

Question 39

Describe a p-n junction?

Answer 39