01 Semiconductors

Question 1

Define “Conductor”

Answer 1

Allows the flow of electrical current

Question 2

Define “Free Electrons”

Answer 2

Electrons in the highest (valence shell)

Question 3

How many electrons in a conductor valence shell

Answer 3

1 valence electron

Question 4

How many electrons in an insulator valence shell

Answer 4

8 valence electrons

Question 5

How many valence electrons in a semi-conductor

Answer 5

4

Question 6

Define Valence Saturation

Answer 6

When the valence shell is full of electrons

Question 7

Describe the effect temperature has on creation of holes and free electrons

Answer 7

As the temperature increases, an electron gets bumped to a higher orbit, leading to a free electron and a hole in the previous shell.

Question 8

Define Recombination

Answer 8

When an electron merges with a hole.

Question 9

What is the Lifetime of a free electron

Answer 9

The time between the creation and disappearance of a free electron.

Question 10

Define Extrinsic Semi-Conductor

Answer 10

Pure semiconductor, only contains atoms of that material.

Has an equal number of hole and free electron charge carriers.

Question 11

Define Extrinsic Semi-Conductor

Answer 11

Doped with other atoms.

Question 12

Describe the atomic make up and resultant majority carrier resulting from n-type doping.

Answer 12

Doped with pentavalent atom.

Majority carriers are free electrons.

Question 13

Describe the atomic makeup and resulting majority carrier resulting from p-type doping.

Answer 13

Doped with trivalent atoms.

Holes are majority carriers.

Question 14

Define a pn junction

Answer 14

A p-type semiconductor interfaced with an n-type.

Question 15

What is the net charge of a pn-junction

Answer 15

Net neutral charge

Question 16

Define Depletion Layer

Answer 16

Free electrons cross into the p region where they have a short lifetime and recombine with holes, creating dipole pair atoms. The dipoles form at the interface of the pn-junction, creating a region void of free carriers.

As dipole pairs form at the dpletion layer, they each have an electric field which repulses further free carriers from entering the p region and creating more dipoles. Eventually this reaches equilibrium and there is a stable depletion layer

Question 17

Define Barrier Potential

Answer 17

Voltage potential across the depletion layer, created by the electric field between the ions.

The voltage requierd to push free carriers across the depletion layer.

Question 18

Define forward bias

Answer 18

Positive source connected to the p-type material, negative source connected to the n-type material.

The source voltage pushed free electrons towards the depletion layer, against the electric field.

Current flows easily through a forward-biased diode.

Question 19

Define reverse bias

Answer 19

The negative terminal is connected to the p-type material, the positive terminal is connected to the n-type material.

Voltage applied to a reverse bias is called “reverse voltage”

This pulls the atoms away from the depletion layer, widening it, creating a larger barrier potential. The depleiton layer stops widening when the barrier potential equals the applied reverse voltage.

Question 20

Does a current still flow after a reverse bias has reached equilibrium?

Answer 20

Yes, through minority carrer current and surface-leakage current.

Question 21

Describe break down voltage?

Answer 21

When large reverse voltage is applied, the minority carriers are accelerated and knock electrons out of their valence shells, creating more free electons. This cascades until a large current is able to pass through the depletion layer.

Question 22

Describe the Conduction Band

Answer 22

When thermal energy moves electrons into the conduction band and their holes are left in the valence band. The conduction band is where free electrons are.

Question 23

Describe the difference between n and p-type energy bands.

Answer 23

n-type: majority carriers are free electrons in the conduction band, minority carriers are holes in the valence band.

p=type: majority carriers are holds in the valence band, minority carriers are free electrons in the valence band.

Question 24

Describe the effect of temperature on the barrier potential.

Answer 24

As the junction temperature increases, it creates more free electrons and holes in the droped regions, which diffuse into the depletion layer, shrinking it and reducing the barrier potential.

Question 25

Describe transient current

Answer 25

When a reverse voltage is applied and the depletion layer widens, it takes time to adjust to its new width, during which time transient current flows through the circuit.

Question 26

Describe RC time constant

Answer 26

the time before the transient current drops to 0 after a reverse voltage is removed.