Physics Department, Yarmouk University,Irbid Jordan

  

Semiconductor Diode DetectorsPresented by

Afaf Mustafa AL-Sheyyab Presented to

Dr:_Nidal Ershaidat  

Phys. 641 Nuclear Physics 1 First Semester 2010-2011

   

 

semiconductor is a material that is between conductors and insulators in its ability to conduct electrical currents, the most common semiconductors are Si, Ge, as a single element, and GaAs, as compound

semiconductors .

when certain atoms combine to form a solid material. They arrange themselves in as a crystal. The atoms within the crystal structure are held together by covalent bonds ,which are created by valence

electrons .Si is crystalline material .

Energy bands consisting of a large number of closely energy levels exist in crystalline

material .

Semiconductor properties A. Band Structure in Solids

Figure 1-1 shows energy bands for insulators and semiconductors. The lower band , is called the valence bands.

The next higher –lying band is called the conduction band.

The difference in energy between the valence band and the conduction band is called energy band gab, the size of which determines whether the materials are classified as a semiconductor or an insulator .

B.Charge carrierAt any nonzero temperature, some thermal

energy is shared by the electrons in the crystal .It possible for a valence electron to gain sufficient thermal energy to be elevated across the band gap into the conduction band ,this process represents the excitation of an electron .the excitation process not only creates an electron in the empty conduction band ,but it also leaves a vacancy (called a hole ) in the full valence band and the combination of the two is called an electron –hole pair

The probability per unit time that an electron –hole pair is thermally generated is given by

Where

Eg=band gab energyK=Boltzmann constant

C=proportionality constant characteristic of the material

T= absolute temperature

kT

CTp ET g

2exp2

3

C. Migration of Charge Carriers in an electric field

Both the electrons and holes will undergo a net migration ,if an electric field is applied to the semiconductor material.

The hole is a point positive charge because the hole actually represents the absence of a negatively charged electron, so the holes move in the direction of the applied field ,while electrons move in the opposite direction .the ratio of the velocity to the applied field is called the mobility

Where

E is the electric field magnitude.

EvEv

ee

hh

2 ) Effect of Impurities or Dopants

.AIntrinsic Semiconductors

In a completely pure semiconductor, all the electrons in the conduction band and the holes in the valence band would be caused by thermal excitation .Because under these conditions each electron must leave a hole behind, the number of electrons in the conduction band must exactly equal the number of holes in the valence band.

Where the quantity and are the intrinsic carrier densities .

such material is called an Intrinsic Semiconductors.

np ii

ni pi

Dopping The conductivity of semiconductors can be

increased by the addition of impurities to the intrinsic material .This process, is called dopping

B. n-Type Semiconductor

Silicon is tetravalent and in the normal crystalline structure forms covalent bonds with the four nearest silicon atoms .To increase the number of conduction band electrons in intrinsic silicon ,pentavalent impurity atoms are added.

In this type the impurities are called donor impurities

Increasing the concentration of electrons

in the conduction band increases the rate

of recombination, shifting the equilibrium between electrons and hole so

P-Type Semiconductor

To increase the number of holes in intrinsic silicon ,trivalent impurity such as an element from group 3 is added the trivalent atom can take an electron ,and create electron sites within the forbidden Intrinsic concentration of holes ,then the number of holes can be controlled by the concentration of acceptor, or energy gap ,it is called acceptor impurity.

The Semiconductor junction -4

A)Basic Junction Properties If we have a region of silicon has been doped with a uniform concentration of acceptor impurity(p-type) and anotherregion has been doped with uniform concentration of donor impurity (n-type)and

the regions are brought together In good thermodynamic contact a p n junction

forms between the two regions and the charge carriers are able to

migrate across the junction from regions of high concentration to those of low

concentration The region over which the charge imbalance exists is called the depletion region.

The Ionization Energy the average energy expended by the charged particle

to produce one electron –hole pair.

The dominant advantage of semiconductor detectors lies in the smallness of the ionization energy .The value of for either silicon or germanium is about 3eV compared with about 30eVrequired to create an ion pair in typical gas filled detectors . thus the number of charge carriers is 10 times greater for the semiconductor case ,for a given energy deposited in the detector.

Semiconductor as Radiation DetectorsPulse formation

- When a particle deposit energy in a semiconductor

detector equal number s of electron –hole pairs are formed with a few picoseconds along the track of the particle, and where the detector configurations ensure that an electric field is present throughout the active volume, both charge carriers feel electrostatic force that cause them to drift in the opposite direction The motion of them constitutes a current that will persist until those carriers are collected at the boundaries of the active volume .And because the differences in drift distance and carriers motilities', the charge collection times are not the same ,so one of the two currents will persist for a longer time than other .When these currents are integrated on a measuring circuit with long time constant , the time of the measured induced charge will also be that of the rise of the pulse produced by a conventional preamplifier used to process the pulses from the

detectors .

The increased number of charge carriers has two beneficial effects on the attainable energy resolution . The statistical fluctuation in the number of the carriers per pulse becomes a smaller fraction of the total as the number is increased. This factor is predominant in determining the limiting energy resolution of a detector for medium to high radiation energy ,and the greater amount of charge per pulse

leads to a better signal . The ionization energy depends on the nature of the

incident radiation where most detector calibration are carried out using alpha particle ,and is also temperature dependent , the value of increases with

decreasing temperature .

Reverse BiasingUn biased junction is a semiconductor diode

junction which n external voltage is applied ,in this junction the charges can be readily lost as a result of trapping and recombination . the thickness of depletion region is quite small ,and the capacitance of un biased junction is high .therefore , unbiased junctions are not used as pulse mode radiation detectors ,but instead ,an external voltage is applied in the direction to cause the semiconductor diode to be reverse biased

The p-n junction is most familiar in its role as a diode. It will readily conduct current when voltage is applied in the forward direction(where the negative side of voltage is connected to the n side of the pn junction and the positive side of the voltage is connected to the p side of the pn junction). the potential will tend to attract conduction electrons from the n side as well as holes from the p side , in both cases these are the majority carriers ,so conductivity through the junction is enhanced .But the pn junction will conduct very little current when biased in the reverse direction((where the negative side of voltage is connected to the p side of the pn junction and the positive side of the voltage is connected to the n side of the pn junction) ,now the minority carriers which are attracted across the junction, the reverse current across the diode is quite small

Semiconductor Detector Configurations A) Diffused Junction Detectors

-one common method for semi conductor diode detectors start with homogenous crystal of p-type material ,one surface is treated by exposing it to vapour of n-type material ,which then converts a region of crystal near the surface from p-type to n-type material ,therefore a junction is formed some distance from the surface at the point which the n-type and p-type impurities reverse their concentration, the depletion region extends primarily into the p side of the junction, because the n- surface layer is heavily doped compared with p-type .So much of the surface layer remains outside the depletion region and represent dead layer which the incident radiation must pass before reaching the

depletion region ,and this dead layer can be disadvantage because a portion of the particle energy will be lost before the active region of the detector is reached.

B) Surface Barrier Detectors

In Surface Barrier Detectors the depletion region behaves in much the same way for a diffused junction detectors.

The usual treatment is etching of the surface ,followed by evaporation of a thin gold layer for electrical contact ,under conditions that promote slight oxidation of the surface .then the resulting oxide layer between the gold and silicon plays an important role in

the resulting properties of the surface barrier . Surface Barrier can also be produced by

starting with p type material and evaporating aluminum to form an equivalent n-type contact. One potential disadvantage of surface barriers is their sensitivity to light .

C)Ion Implanted layers

Ion Implantation is expose the surface of semiconductor to a beam of ions produced by an accelerator . This method can be used to formor layers by accelerating ,for example ,either phosphorus or boron ions . following exposure to the ions beam ,an annealing step is normally carried out to reduce the effects of radiation damage caused by the incident ions .the advantage of this method is that the annealing temperature required( less than 500 ) is considerably lower than that needed for the thermal diffusion of dopant to form diffused junction ,and compared with surface barriers ion implanted detectors tend to be more stable and less subject to ambient condition.

Passivated planar Detectors

 

The newest method of fabricating Si junction detectors combines the techniques of ion implantation and photolithography to produce detectors with very low leakage currents.

The process begins with high purity Si that is mildly n type due to residual donor impurities, after the wafer has been polished and cleaned, the surface is passified through the creation of an oxide layer at elevated temperature. Next, the techniques of photolithography are used to remove selectively areas of the oxide where the entrance windows are located .the junction is then formed by converting a very thin layer of Si into p-type material through the implanting of acceptor ions using accelerator. One advantages of this detectors is that the

oxide passivated surface keeps leak age currents much lower than in surface barrier detectors

Application of Si Diode Detector Energy calibration

When applied to the measurement of fast electrons or light ions such as protons or alpha particles semiconductor diode radiation detectors respond very linearly, and the energy calibration obtained for one particle type is very close to that obtained using different radiation type. The most common calibration source is the alpha –emitting isotope Am (241).

CONCLUSINIntrinsic Semiconductor materials have

limited number of free electrons in the conduction band ,and holes in the valence band ,adding impurities to the Intrinsic materials increases the number of carriers

where this process is called dopping

Pn junction is formed when two different types of semiconductor material are joined .

The bias refers to applied an external voltage ,in relation to a pn junction there are two bias :forward and reverse