Semiconductors

Question 1

How does any detector work?

Answer 1

All detectors work by converting incident primary quanta (radiation) into mobile secondary quanta (light photons, ion pairs, electron-hole pairs) within the detector. We then convert these secondary quanta into a measurable signal through a series of physical and/or chemical reactions.

Question 2

What can detectors measure?

Answer 2

Number of events, energy fluence of events, and, if the timing is short enough, the number and energy of each individual event. This third type is radiation spectroscopy.

Detector is able to spatially localize signal, it can be used to generate an image of the emitter/attenuator.

Question 3

Which two types of conversions do detectors have?

Answer 3

1. Indirect Conversion Detectors use a scintillator (NaI(Tl), often) to convert incident radiation into light photons, collected by photodiodes producing electrical signal
2. Direct Conversion Detectors use a layer of photoconductor to convert incident radiation into electron-hole pairs.

indirect conversion has a lower resolution (light spreads)

Question 4

What kind of bonds do atoms make?

Answer 4

Ionic, covalent and metallic bonds

Question 5

Describe ionic bond.

Answer 5

Ionic Bonds: between elements at extremes of the periodic table, an ionic bond will simply transfer an electron from one element to the other. This effectively ionizes each, causing coulomb forces to create a bond between them.

Question 6

Describe covalent bonds.

Answer 6

Covalent Bond: valence electrons are shared instead, leading to bonded elements effectively both having full valence shells (if possible).

Question 7

Describe metallic bonds.

Answer 7

Metallic Bond: akin to covalent bonds, the valence electrons are shared. They are now, however, shared amongst all the local atoms. Electrostatic forces between the dispersed electrons and metal ions hold it together.

Question 8

What kind of lattice structures are there?

Answer 8

(SC) Simple cubic

(BCC) Body centered cubic

(FCC) Face centered cubic

(D) Diamond structure

Question 9

How many atoms are in each lattice unit?

Answer 9

We can calculate via geometry, on average, how many atoms make up each cube-shaped unit of these lattices. To do so, we weight each atom (dot) by how many different adjacent cubes it is used in, then sum them up.

(SC) 1 atom

(BCC) 2 atoms

(FCC) 4 atoms

(D) 8 atoms

Question 10

How do we calculate atomic/electron density in a lattice?

Answer 10

Knowing the number of atoms per unit as shown, and the volume of each cube via a₀ (length of one side), we can calculate the density of atoms, electrons, and valence electrons for each material.

Question 11

What’s a typical size of a0 (length of one side of a cubic lattice)

Answer 11

usually 3-7 angstroms