3.30 Sspd_chapter 1_part 11_solid state of matter_continued_crystal  (Page 17/4)

SSPD_Chapter 1_ Part 11_Solid State of Matter continued_Silicon, Germanium and GaAs crystal structures

1.11.5. SILICON AND COMPOUND SEMICONDUCTORS.

Before the advent of Solid State Technology, i.e. before 1950, semiconductor was used as two terminal point contact diode and photodiode. With the invention of Bipolar Junction Transistor, Germanium rose to the position of eminence because of its unusually high mobility of electron = 3900 cm ² /(V-sec) [Table 1.10]. But with the invention of Integrated Circuit Chips it yielded ground to Silicon.

Today Silicon has become the staple of the Post- Industrial Society just as Iron became the staple of the Industrial Society.

Why Silicon has become the material of choice and the most studied semiconductor in Computer Era? It is primarily because of Silicon-dioxide layer which can easily be formed by oxidation either in dry Oxygen or in steam. Oxidation is the key process in I.C.Technology. SiO ₂ acts as passivating layer as well as a shielding layer during diffusion. As we know that Silicon is a 3-D orderly arrangement of Silicon atoms which is abruptly terminated at the surface boundary of the crystal. This abrupt termination of orderly arrangement leaves many covalent bonds incomplete. These incomplete bonds are known as dangling bonds. These dangling bonds cause surface states and these surface states tend to trap or release mobile carriers. This random release or trapping of mobile carriers cause 1/f noise or flicker noise. Passivation of the surface states minimizes the flicker noise, stabilizes the electrical characteristics of the device and provides a protective layer to the underlying device. Since SiO ₂ is a perfect insulator it is used for isolation of devices which considerably reduces the parasitic effects. Conventionally reversed biased diode is used which eats away a lot of the real estate on the Silicon Surface and gives rise to severe parasitic effects. The real estate is saved as well as parasitics are minimized by the use of SiO ₂ as isolation layer as well as the substrate as it is done in Silicon - on - Insulator (SOI) Technology.

The second reason for the popularity of Silicon is easy and cheap availability. It is obtained from silica and silica comprises 25% of Earth’s crust. Silicon is second most abundant after oxygen.

But Silicon is most unsuitable for photonics because of indirect band-gap and its accompanying non-radiative transitions though in recent years Silicon has been suitably modified to obtain photonic circuits on Silicon Substrate. Because of the unsuitability of Silicon, we have had to opt for Compound Semiconductors and its alloy as shown in Table(1.25).

Table(1.25) give the compound semiconductors which have application in Photonics.

Table. 1.25. Compound Semiconductors for Photonics application.