What are suitable materials for use of neutral cure silicon sealant?
Neutral cure silicone sealant are used in places where there is metal present. like galvanised iron , zincalume, aluminum flashing's, etc where acids can be harmful to the materials that hold the seal. however these silicone's are suitable for most sealing applications in any case.
Why silicon crystal are suitable for making a diode and have a junction of P-type and N-type material?
Silicon is used because it has semi-conductor properties. It can conduct or not, depending on the junction formed and the deliberate impurities added (doped). Other substances can be used such as Germanium. Silicon is an element and can be grown in a laboratory, to form a pure crystal structure, suitable for making semi-conductors. It is easier and cheaper to grow silicon crystals than some other semiconductor materials.
It sealant is probably a silicon product, if it is on the glass and the glass is smooth then a razor will do the job. If the glass is textured then the task is very difficult. If the surface is rough then use a razor to remove as much as possible and use new sealant (I prefer clear) to assure no leaks and a fresh look.