Semiconductor radiation detectors

1
Semiconductor radiation detectors

• Best energy resolutions are obtained by these
  sensors at present
• The principle is based on the generation of
  electron-hole pairs by the incident radiation,
  which are then collected by applying a voltage
  across two contacts
• Since the ionization voltage is only a few eV
  compared to 10s of eV for typical gases, so the
  efficiency of ion generation is much higher. This
  means higher energy resolution.
• There are several desirable properties for
  semiconductors that are needed for detection
• Excellent charge transport (high mobility)
• Low leakage current of darkj current
• Efficient production of electron-hole pairs due
  to radiation
• Fast response
• Low cost
• Radiation tolerance
• Ge and Si are good choices for radiation
  detection,
• but the former needs to be cooled, while the
  later is
• not good for ? detection (why?).
• A special geometry is needed to maximize
  radiation
• collection area so a cylindrical geometry is
  generally
• preferred.

Lithium-drifted pin-junction detector. Structure
of the detector (a) and coaxial configuration of
the detector (b)
2
Semiconductor radiation detectors New trends

• One of the latest semiconductor materials for
  radiation detection are CdTe and CdZnTe
• They offer high enough band gap of 1.5 eV (low
  dark current at room temp, resistivity up to 1010
  cm-3), and also large average Z of the
  constituent atoms for efficient interaction with
  ?-ray, and production of electron-hole pairs
• The principal challenges are poor material
  quality, which leads to performance challenges.
  Not yet comparable to cooled Ge detectors.

Detector with 20x20x10mm CZT crystal showing
planar cathode and guard ring. (Courtesy of
Quik-Pak, a division of Delphon Industries)
3
Chemical and biological sensors Classification

• Based on electrical and electrochemical
  properties
• Metal oxide semiconductor based
• Electrochemical sensors
• Potentiometric sensors
• Conductometric sensors
• Amperometric sensors
• Chem-FET
• Based on Changes in physical properties
• Mass change
• Stress change
• Work function change
• Capacitance change
• Thermal conductivity change
• Based on Optical property changes
• Luminescence
• Transmittance/Absorption
• Complex array based sensors for multiplexed
  detection

4
Chemical and biological analytes Classification

• Toxic gases and pollutants (NOx, SO2, NH3, O3,
  heavy metal, As)
• Volatile organic compounds (alcohol, toluene,
  benzene)
• Explosives (TNT, PETN, RDX)
• Chemical warfare agents (Sarin, Tabun, Mastard
  gas)
• Bio-analytes
• Blood parameters (urea, pH, lactic acid, glucose,
  pO2)
• Pathogens (virus, bacteria, fungi, proteins)
• Cancer/Tumor cells
• Specific disease markers (PSA)
• Intracellular matter (DNA, RNA, proteins)

5
Single and array based detection
Metal-oxide-semiconductor-based sensor response
to increasing and decreasing concentrations of
ethanol
Response of a capacitive VOC sensor array
containing seven differently absorbing
polymer-coated chemicapacitors to pulses of
acetone, methyl ethyl ketone, toluene, ethanol,
and water at 25 C
6
Sensor parameters

• Sensitivity (usually ppb, sometimes even ppt)
• Selectivity
• Response time (seconds to minutes)
• Repeatability
• Lifetime (at least 1 year)

All these parameters are expected to get better
with nanoscale material. Why?