Semiconductors-A Summary

The energy bands (e or e) in the semiconductor are space delocalized which means that these are not located in any specific place inside the solid. The energies are the overall averages. When you see a picture in which e or e are drawn as straight lines, then they should be respectively taken simply as the bottom of conduction band energy levels and top of valence band energy levels.

In elemental semiconductors (s ) the n-type semiconductors are obtained by introducing �dopants’ as defects. In compound semiconductors, the change in relative stoichimetric ratio can also change the type of semiconductor. For example, in ideal GaAs the ratio of Ga:As is 1:1 but rich or as rich gaas it could respectively be in general, the presence of defects control the properties of semiconductors in many ways.

In transistors, the base region is both narrow and lightly doped, otherwise the electrons or holes coming from the input side (say emitter in CEconfiguration) will not be able to reach the collector.

We have described and oscillator as positive feedback amplifier. For stable oscillations, the voltage feedback from the output voltage should be such that after amplification in should again become if a fraction is feedback then and after amplification it should again be equal to v this means that the criteria for stable oscillator to be sustained si . this is know as Barkhausen’s Criteria.

In oscillator, the feedback is in the same phase (positive feedback). If the feedback voltage is in opposite phase (negative feedback) the gain is less than 1 and it can never work as oscillator. It will be an amplifier with reduced gain. However, the negative feedback also reduces noise and distortion in an amplifier which is an advantageous feature.