A semiconductor detector in ionizing radiation detection is a device that uses a semiconductor (usually silicon or germanium) to measure the effect of incident-charged particles or photons.
A semiconductor detector is a silicon or Germanium diode of the p-n type operated in the reverse bias mode.
As the applied voltage is in the same direction as the diffusion field potential, the resultant potential drop across the transition region is increased. The width of the depletion layer further increases. A very high field of the order of 10,000 V/cm is developed at the dance Junction.
Suppose an α particle is absorbed in the p-n junction, it will lose nearly all its energy in producing electron-hole pairs. The high electric field in this region sweeps the electrons towards the positive side and holds them toward the negative side.
They move with the drift velocities of the order of 10−5 ms−1approximately thereby registering a pulse of the order of a millivolt with a very short rise time of the order of 10−9 to 10−10 seconds. The rising pulse appears across R. The first solid-state detector was given by P. J . Van Hardeen in 1945.
Surface Barrier Detectors
Semiconductor radiation detector which are used these days is of surface barrier type. To form it, an n-type donor impurity is diffused into, say silicon, containing a very small concentration of a p-type impurity. Just below the surface where the n-type impurity cancels the p-type one, a junction is established.
Thin films of gold evaporated onto the diode enable electrical contacts to be made with it without hindering the entry of charged particles into the Junction.
If the semiconductor is cooled to liquid nitrogen temperature, a considerable reduction in noise pulse is obtained. It can be used as a spectrometer for those particles whose range is less than that of the junction thickness because in this case, the pulse height will be proportional to the kinetic energy of the incident particles. This makes the detector insensitive to γ rays.
Detection characteristics of surface barrier detectors or semiconductor detector
- High conversion efficiency
- High speed of response
- Small size
- Differential sensitivity
Linearity: Most of the experiments performed to verify the linearity of the response to various particles with different energies.
High conversion efficiency: The conversion ratio is about 10 times the number of parts in the air. The energy required to produce ion pair in Silicon is 3.5 eV.
Resolution: The high energy resolution means a narrow distribution of pulses around the mean i.e. FWHM (full width at half maximum). Cooling increases the resolution. It is therefore good to cool the detector to -30°C.
High speed of response: The Ion pairs move only a short distance before the collection and the pulse widths are in the order of nanoseconds.
Small size: They are usually small in size so are quite handy.
Differential sensitivity: They are relatively insensitive to neutrons or photons.
Lithium-ion Drifted Junction Detector
This was introduced by E.M. in 1960. The technique is to allow the Lithium, which acts as the electron donor, to diffuse into the p-type silicon or germanium at an elevated temperature (120 to 150°C) and under reverse bias.
Under these conditions, the ions drift for a considerable distance into the Crystal where the Lithium Donor exactly compensates the existing acceptor impurities in the p-region and an effective intrinsic semiconductor layer is produced between the n and p-regions.
With improved techniques, the layer thickness has been steadily increased to over 1cm, thus making this type of detector useful for studies of the spectra of γ rays up to energies 2.5 MeV.
The excess Lithium on the upper surface results in a highly doped n layer which serves as an electrical contact and a thin uncompensated layer remains on the opposite side.
Lithium drifted Germanium detectors are more suitable than Silicon detectors for the detection of γ-rays. This is because the photoelectric absorption cross-section for γ-rays is proportional to Z5 and therefore Germanium (Z=32) is more efficient than silicon (Z=14) for γ-rays detection.
High Purity Germanium Detector
For sufficiently pure Germanium, the desired region can also be achieved directly without compensation by the creation of a diode structure. This is obtained by evaporating Lithium on one surface of the p-type Ge and allowing it to diffuse into for a short time period and a short distance.
The intrinsic region is created by applying a reverse bias to the p-n junction which pushes the majority carriers from the junction on both sides. The process of the recession of the free charges proceeds on both sides of the junction until the electrostatic field introduced by charged atoms balances the field from externally applied electric potential.
The thickness of the depletion layer is related to the applied voltage and the impurity concentration in the material.
Advantages of the Semiconductor Detector
- High counting rates of the order of 5 * 104 counts per second as possible without any difficulty.
- These have low sensitivity to γ-background radiation.
- They do not need any window for letting the charged particles into the detector.
- These have very good energy resolution.
- The pulse height in this counter is proportional to the kinetic energy of the incident particles, provided the particle range is less than the junction thickness.
- The rise time of the pulse is very small.
- The applied voltage is relatively low as compared to the gas counters.