Question

(a) Name two properties of Si that would make it unsuitable for specific device applications and suggest an alternative semiconductor that can be used in each application. (b) You need to make a detector that is very sensitive to light with wavelength of 10 um. In what part of the electromagnetic spectrum will this device need to operate ? (ii) Calculate the optimum energy gap for the detector material ? (iii) Explain why you would or would not use Si to make such a detector. (iv) Use the value of the energy gap calculated in (ii) above and calculate what the intrinsic carrier concentration of a material with such an energy gap would be at room temperature (300 K). As an approximation assume the same values of the constants Nc and Ny as you would use for Si. (v) Based on this value calculated for the intrinsic concentration in (iv) can you identify a potential problem in making these types of detectors ? Can you suggest a way in which this problem could be solved in a practical device? (iv) One option for making such a detector is to use a ternary compound semiconductor. A possible material is to produce a ternary compound (HgCd)Te from the binary compounds HgTe (E8 = -0.3 eV) and CdTe (Eg = 1.7 eV). Explain how a ternary compound can be formed from these two binary compounds. If we assume that the energy gap will vary linearly between the binary endpoints, calculate the desired composition of the ternary alloy that will give the optimum bandgap for detection of light at 10 um wavelength.

Answer 1

a)   Electron hole mobility::

electron hole mobility in silicon is very poor, and this is a barrier to higher performance – so much so that for several years manufacturers have had to boost it by including germanium with the silicon.

Performance degradation badly at high temperatures

Silicon’s second problem is that performance degrades badly at high temperatures. Modern ICs with billions of transistors generate considerable heat, which is why a lot of effort goes into cooling them – think of the fans and heatsinks strapped to a typical desktop computer processor. Alternative semiconductors such as gallium nitride (GaN) and silicon carbide (SiC) cope much better at higher temperatures, which means they can be run faster and have begun to replace silicon in critical high-power applications such as amplifiers.

while electrons in silicon are quite mobile, they are much more so in other semiconductor materials such as gallium arsenide, indium arsenide, and indium antimonide.

b) i) Thermal IR

Thermal radiation is electromagnetic radiation emitted from all matter that is at a non-zero temperature.

ii)

hc E

h=6.626X10^-34 Js C=3X10⁸ m / s

   

2 = 10pm

E=1.9878X10^-20 J ( or )    E = 0.124 eV

iii)  optoelectronic applications of Si are restricted to the visible and near-infrared spectral range due to its 1.12 eV-indirect band gap at room tempeature.

Conventional Si photodetectors typically show peak-photoresponse between
0.7μm
and
այ16:0
.

Conventional Si photodetectors typically show peak-photoresponse between 700 and 900 nm with very low dark currents due to the high crystalline quality and excellent passivation properties of Si..

but at high temperature Performance degrades for si.

Therefore you would not use si to make such a detector of energy gap 0.124eV

iv) n_i² = N_c Nve^(-Eg/kT)

The  value of n_i  comes to be 2.81X10²⁸/cm³ at T = 300 K and for E_g=0.124EV

v )yes. as the energy gap is low n_i is increasing in large scale .

compound semiconductor

Characteristics of Compound Semiconductors

• Sensitivity to light (for photo detectors)
• The ability to generate high frequency (microwave) signal.
• Low voltage operation (for lighter & longer lasting, low voltage batteries used for portable devices)
• Light emission (both visible and infrared light, for lasers and LEDs)
• High speed operation.
• Sensitivity to magnetism (useful in sensor applications)
• Resistance to heat and radiation (useful in space applications, such as solar cells for satellites)
• Sensitivity to magnetism (useful in sensor applications)Resistance to heat and radiation (useful in space applications, such as solar cells for satellites) .
• ternary compound is a compound containing three different elements. An example is sodium phosphate, Na₃PO₄.