Question

Please distinguish between the following terms.

1) Intrinsic semi-conductor and extrinsic semi-conductor

2) Conduction band and valence band

3) Dark current and read noise

4) Quantum efficiency and detective quantum efficiency

5) overscan and bias frame

6) dome-flat and twilight-flat

Answer 1

(1) Intrinsic semi-conductor and extrinsic semi-conductor

a pure semiconductor without any impurity is called intrinsic semicondutor while as an extrinsic semiconductor is a a doped semiconductor . we delibrately add impurity to an extrinsic semicnductor to increase its conductvity .

(b) Conduction band and valence band

Valence Energy band: In this energy band there are large no of free electrons available. Filling of the valence band can be partial or complete. These bands never get empty.
The major disadvantage of the electrons present in this band is that these electrons are unable to attain energy from any external source of electric field.

Conduction Energy Band: The density of the electrons is very few. As compared to the electrons present in the valence energy bands, these electrons can gain energy from the external field of electric field and are available for conduction.

(3) Dark current and read noise

dark current is the relatively small electric current that flows through photosensitive devices such as a photomultiplier tube, photodiode, or charge-coupled device even when no photons are entering the device. It is referred to as reverse bias leakage current in non-optical devices and is present in all diodes. Physically, dark current is due to the random generation of electrons and holes within the depletion region of the device

Read noise is the amount of noise generated by electronics as the charge present in the pixels is transferred to the camera. It is a combination of all the noise generated by system components which convert the charge of each CCD pixel into a signal for conversion into a digital unit (ADU or grey-scale value).

(4)Quantum efficiency and detective quantum efficiency

The "quantum efficiency" (Q.E.) is the ratio of the number of carriers collected by the solar cell to the number of photons of a given energy incident on the solar cell.

The detective quantum efficiency is a measure of the combined effects of the signal (related to image contrast) and noise performance of an imaging system, generally expressed as a function of spatial frequency. This value is used primarily to describe imaging detectors in optical imaging and medical radiography.

5) overscan and bias frame

Overscan is the situation in which not all of a televised image is present on a viewing screen. A bias frame is essentially a zero-length exposure (or as close as possible to zero length) with the shutter closed. Each pixel will have a slightly different value, but except for a small amount of noise, the value for any one pixel will be consistent from image to image