Detector II Flashcards
What are the typical semiconductor detector materials?
Silicon, Gemanium and Cadmium telluride
How does a basic detector work?
Radiation interacts with surface and gets ionised
E field allows charges to drift
The current flow is measured
There is an applied bias voltage
What is the disadvantage of gas detectors?
Their energy resolution
30eV of energy required to produce 1 electron
What makes semiconductor detectors benefical?
They generate a large number of charge carriers per event
<5eV to produce an electron
they have good energy resolution
What does a crystal structure of pure Si include?
Electrons locked in covalent bonds
No electrons in partially filled shells
What happens when energy is applied to pure Si?
Electron can be promoted to a higher shell and can move between atoms
This leaves a hole in the bond
What is the band model for conductors and semi conductors?
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)
What is the band model of an insulator?
There is a big band gap between conduction and valence band which makes it difficult to jump (Eg = 5eV)
When does a valence band get filled?
T = 0K
When does an electron get promoted to conduction band?
When T > 0K , thermal energy can promote electron to conduction band
What does the probability per unit of thermally generated electron-hole pair rely on?
T (absolute temp)
Eg (band gap energy)
C = constant characteristic of material
What is the effect of an electric field on semiconductors?
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
What happens after the electrons and holes are forced in opposite directions due to an electric field?
Electrons collected at +ve contact
Hole eventually replenished by electron at -ve contact
What happens to carriers when there isn’t an electric field?
They have random thermal velocity as E field causes carriers to move with a net drift velocity parallel to the direction of the field
What is the velocity of hole and velocity of electron?
v_h = µ_h E
v_e = µ_e E
µ_h = mobility of hole
µ_e = mobility of electron
E = electric field magnitude
What parameter is similar for holes and electrons?
Mobility of holes and electrons is similar
What is the total motion?
A combination of thermal and drift velocities
What is an intrinsic semiconductor?
One electron in conduction band ≡ one hole in valence band
This is impossible to produce in practise
What type of semiconductors do impurities give rise to?
n-type and p-type semiconductors
How do n-type semiconductors work?
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
How do p-type semiconductors work?
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
Which type of semi conductor requires energy to promote electron initally?
n-type requires energy to promote to conduction band is very small and thermal excitation means parent atom is ionised most of the time
What happens in n-type semi conductors when an electron hole pair is produced?
Holes is filled quickly due to excess electrons so that conductivity is determined exclusively by flow of electrons
Electrons are majority carriers
Which type of semi conductor requires energy to promote an electron from valence band?
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
Why can n and p type semiconductors be joined?
They are both electrically neutral
What happens when the n and p type semiconductors are joined?
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
What happens when ionising radiation passes through joint semiconductor?
Charge carriers are created by ionisation and the charges are swept out of the depletion region
Moving charge constitutes basic electrical signal
Why do the joined p and n type semiconductors have poor performance?
Due to small E field
To fix this additional (reverse) bias is applied to increase field strength
What does the creation of charge carriers depend on?
Band structure
(energy levels modified by dopants)
