(a) Transistor as an oscillator: An oscillator is an electronic device which converts direct current into alternating current of high frequency and constant amplitude.
Circuit diagram
Fig. shows the circuit diagram for a n-p-n transistor as an oscillator in common-emitter configuration. Common-emitter circuit is reverse biased by battery E. A coil L' is inserted in collector-emitter circuit and it is coupled with L which is connected in the emitter-base circuit.
Working: When key K is closed, a small collector current start rising through L' and a change in electric current hence change in magnetic flux in L' is produced so a small induced e.m.f. is produced in L which is coupled with L'. Thus a small current flows in the emitter-base circuit. If it produces forward biasinig in this circuit, then small emitter current causes a corresponding increase in the collector current and upper plate of capacitor C gets positively charged and more magnetic flux is linked with L' and hence with L, so emitter current is further increased. This process continues till collector current becomes saturated.
As the current reaches saturation value, the mutual inductance stops playing its part. The capacitor C gets discharged through inductance L, and emitter current falls, so collector current also falls and a decreasing magnetic flux is linked with coil L' and hence with L. The decreasing collector current induces voltage in L its reverse direction which decreases the emitter current further. Now the lower plate of the capacitor is positively charged. This process continues till collector current becomes zero. Again mutual inductance stops playing its part. The capacitor gets discharged through inductance L and emitter current increases. The whole process will be repeated again and again.
The frequency of oscillations produced will be given by
V = \(\frac{1}{2\pi \sqrt{LC}}\)
The frequency v can be varied by changing the value of capacitor C by a variable capacitor.
(b) In insulator: In insulators, valence band is completely filled and conduction band is completely empty. The energy gap (Eg) between the two is so large that the electrons cannot overcome it and, hence, conduction is not possible. Therefore, insulators are poor conductors of electricity. Electrons cannot gain energy from the applied field and current cannot flow. An important example of insulator is diamond with energy gap of about 5.4 eV Fig. (a)
Semiconductors
The energy and structure of a semi conductor is shown in Fig (b) It is similar to that of an insulator but with a comparatively small energy gap of about 1 eV. At absolute zero of temperature, the conduction band of semiconductor is totally empty, and all the energy states in the valence band are filled. The absence of electrons in the conduction band at absolute zero does not allow current to flow under the influence of an electric field. Therefore, they are insulators at low temperatures.
However, at room temperature, some valence electrons acquire thermal energy greater than the energy gap Eg and move to the conduction band where they are free to move under the influence of even a small electric field. Common examples of semiconductors are silicon (14), Germanium (32) with energy gap of about (1.12 eV) and 0.75 eV respectively. The gap (band) that separates conduction and valence band is called Forbidden Band.
Metals (Conductors)
The energy band structure of a metal is shown schematically in Fig. (c). The last occupied band of energy levels is only partially filled:
The available electrons occupy, one by one (Pauli exclusion principle) the lowest levels. This leaves part of the band (called conduction band unoccupied.
The highest energy level occupied at absolute zero by electrons is partially filled conduction band, is called Fermi level and the corresponding energy is called Fermi energy.
When an electric field is applied, electrons gain energy (about 108 eV). They can be excited to empty energy levels immediately about the Fermi level and some of the electrons are accelerated in the direction of the field. Thus electric current is conducted through the conductors.
