Detector II

Question 1

What are the typical semiconductor detector materials?

Answer 1

Silicon, Gemanium and Cadmium telluride

Question 2

How does a basic detector work?

Answer 2

Radiation interacts with surface and gets ionised

E field allows charges to drift

The current flow is measured

There is an applied bias voltage

Question 3

What is the disadvantage of gas detectors?

Answer 3

Their energy resolution

30eV of energy required to produce 1 electron

Question 4

What makes semiconductor detectors benefical?

Answer 4

They generate a large number of charge carriers per event

<5eV to produce an electron

they have good energy resolution

Question 5

What does a crystal structure of pure Si include?

Answer 5

Electrons locked in covalent bonds

No electrons in partially filled shells

Question 6

What happens when energy is applied to pure Si?

Answer 6

Electron can be promoted to a higher shell and can move between atoms

This leaves a hole in the bond

Question 7

What is the band model for conductors and semi conductors?

Answer 7

Conductors: no band gap, electrons are free to conduct always (Eg = 0eV)

Semiconductors: small band gap, electrons need to be given energy to cross band gap and they can conduct
electrons are locked in valence band (Eg = 1eV)

Question 8

What is the band model of an insulator?

Answer 8

There is a big band gap between conduction and valence band which makes it difficult to jump (Eg = 5eV)

Question 9

When does a valence band get filled?

Answer 9

T = 0K

Question 10

When does an electron get promoted to conduction band?

Answer 10

When T > 0K , thermal energy can promote electron to conduction band

Question 11

What does the probability per unit of thermally generated electron-hole pair rely on?

Answer 11

T (absolute temp)
Eg (band gap energy)
C = constant characteristic of material

Question 12

What is the effect of an electric field on semiconductors?

Answer 12

Electron-hole pair is created

Electron move along the field towards +ve direction

Hole jumps between bonds towards -ve direction

This creates a current flow

Question 13

What happens after the electrons and holes are forced in opposite directions due to an electric field?

Answer 13

Electrons collected at +ve contact

Hole eventually replenished by electron at -ve contact

Question 14

What happens to carriers when there isn’t an electric field?

Answer 14

They have random thermal velocity as E field causes carriers to move with a net drift velocity parallel to the direction of the field

Question 15

What is the velocity of hole and velocity of electron?

Answer 15

v_h = µ_h E

v_e = µ_e E

µ_h = mobility of hole

µ_e = mobility of electron

E = electric field magnitude

Question 16

What parameter is similar for holes and electrons?

Answer 16

Mobility of holes and electrons is similar

Question 17

What is the total motion?

Answer 17

A combination of thermal and drift velocities

Question 18

What is an intrinsic semiconductor?

Answer 18

One electron in conduction band ≡ one hole in valence band

This is impossible to produce in practise

Question 19

What type of semiconductors do impurities give rise to?

Answer 19

n-type and p-type semiconductors

Question 20

How do n-type semiconductors work?

Answer 20

There is an addition of pentavalent impurity to silicon e.g. phosphorus is introduced (houses 5 electrons in outer shell so one extra electron which is weakly bound)

This weakly bound electron forms new level called donor level, close to the conduction band so electron can be donated to conduction band

Question 21

How do p-type semiconductors work?

Answer 21

There is an addition of trivalent impurity to silicon e.g. boron is introduced which has one less electrons than silicon and leaves a hole

This forms a new band, acceptor level near valence band, small amount of energy needed to take electron from valence to acceptor level, decreasing the band gap and increasing holes in valence band

Question 22

Which type of semi conductor requires energy to promote electron initally?

Answer 22

n-type requires energy to promote to conduction band is very small and thermal excitation means parent atom is ionised most of the time

Question 23

What happens in n-type semi conductors when an electron hole pair is produced?

Answer 23

Holes is filled quickly due to excess electrons so that conductivity is determined exclusively by flow of electrons

Electrons are majority carriers

Question 24

Which type of semi conductor requires energy to promote an electron from valence band?

Answer 24

p - type: From the valence band to fill the extra holes is small and the acceptor sites are occupied most of the time

Leave excess holes in valence band and recombination probability between conduction electrons and hole is increased

Electrons are minority carriers

Question 25

Why can n and p type semiconductors be joined?

Answer 25

They are both electrically neutral

Question 26

What happens when the n and p type semiconductors are joined?

Answer 26

The electron from n-type migrates to p-type and cloud of positivity is left and cloud of negativity is left in p-type

This leads to an electric field

Depletion region formed in the middle: vacant donor sites and filled acceptor sites

Question 27

What happens when ionising radiation passes through joint semiconductor?

Answer 27

Charge carriers are created by ionisation and the charges are swept out of the depletion region

Moving charge constitutes basic electrical signal

Question 28

Why do the joined p and n type semiconductors have poor performance?

Answer 28

Due to small E field

To fix this additional (reverse) bias is applied to increase field strength

Question 29

What does the creation of charge carriers depend on?

Answer 29

Band structure

(energy levels modified by dopants)