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CHAPTER–14: SEMICONDUCTOR ELECTRONICS:
Materials, Devices and Simple Circuits
SEMICONDUCTOR ELECTRONICS: Energy bands in conductors, semiconductors
and insulators (qualitative ideas only) Semiconductor diode - I-V characteristics in
forward and reverse bias, diode as a rectifier; Special purpose p-n junction diodes:
LED, photodiode, solar cell.
Energy Band Diagram:
In a solid, atoms are close to each other. So the outer orbits of electrons from neighbouring atoms would come very close or could even overlap. Hence, the nature of electron motion in a solid very different from that in an isolated atom. In the crystal, the interatomic interaction between N-atoms splits each single discrete energy level of individual atom into N-sublevels ; these energy N-sublevels are very close and appear to be almost continuous giving appearance of energy band. The energy band which includes the energy levels of the valence electrons is called the valence band. The gap between the top of the valence band and bottom of the conduction band is called the energy band gap (Energy gap Eg). For insulators Eg , for semiconductors Eg is 0.2 to 3 eV and for metals Eg.
Metals,
In metals, the conduction band and valence band partly overlap or conduction band is partially filled. When the conduction band overlaps on the valence band electrons can move freely into it. So, a large number of electrons are available for electrical conduction in metallic conductors.
Insulator,
In insulator, the conduction band is empty and valence band is completely filled and the forbidden gap is quite large Eg = 6 eV. No electron from valence band can cross over to conduction band at room temperature or even if electric field is applied.
Semiconductor
In semiconductor, the conduction band is empty and valence band is completely filled and the forbidden gap is quite small Eg = 1 eV. At room temperature, some electron in valence band acquire thermal energy, greater than Eg = 1 eV and jump over to conduction band enhancing conductivity. Semiconductors are the basic materials used in the present solid state electronic devices like diode, transistor, ICs, etc. Metals have low resistivity (10-2^ to 10-8^ Ωm), insulators have very high resistivity ( 10^8 Ωm), while semiconductors have intermediate values of resistivity. Semiconductors are elemental (Si, Ge) as well as compound (GaAs, CdS, etc.) (i) Elemental semiconductors: Si and Ge (ii) Compound semiconductors: Examples are:
- Inorganic: CdS, GaAs, CdSe, InP, etc.
- Organic: anthracene, doped pthalocyanines, etc.
- Organic polymers: polypyrrole, polyaniline, polythiophene, etc.
Types of Semiconductors:
Intrinsic semiconductor (Si, Ge) Extrinsic semiconductor (p-type or n-type)
Intrinsic semiconductor
Intrinsic semiconductors are pure semiconductors and they are basically group 14 elements e.g. Si and Ge. The charge carriers present in semiconductors are free electrons and holes which are obtained as a result of thermal excitation. The number of electrons (ne) and the number of holes (nh) in an intrinsic semiconductor are equal. electrons o holes
Extrinsic semiconductor
p-n Junction formation
Two important processes occur during the formation of a p-n junction: diffusion and drift. In diffusion, due to the concentration gradient across p-, and n- sides, majority charge carriers move to other side i.e. holes diffuse from p-side to n-side ( p →n ) and electrons diffuse from n-side to p-side ( n^ →^ p^ ). This motion of charge carries gives rise to diffusion current across the junction. The motion of majority charge carriers depletes the space-charge region on either side of the junction region of its free charges. Depletion layer : A thin layer on both the sides of p-n junction, which is devoid of free charge carries but has immobile ions is called depletion layer. The thickness of depletion layer at p-n junction is about 10-6m. When majority charge carriers diffuse, they leave impurity atoms as immobile ions, positive immobile ion on n-side and negative immobile ion on p-side and form depletion layer. Potential Barrier: Due to immobile ions on either side of junction, A potential difference of about 0.7 V is produced across the junction, which gives rise to a very high electric field (= 10^6 V/ m) directed from positive charge towards negative charge develops. The difference of potential across the junction of the two regions due to immobile ions on either side of junction, opposes further flow of majority charge carriers. Hence it is called Potential Barrier. Drift Due to the electric field, minority charge carrier electrons on p-side of the junction moves to n-side and holes on n-side of the junction moves to p-side. The motion of minority charge carriers due to the electric field is called drift. Thus a drift current, which is opposite in direction to the diffusion current starts. Forward bias: In forward bias, the p-type is connected with the positive terminal and the n-type is connected with the negative terminal. Width of depletion layer becomes thin, when p-n junction is forward bias.
Reverse bias : In reverse bias , the p-type is connected with the negative terminal and the n-type is connected with the positive terminal Width of depletion becomes thick, when it is reverse bias. Characteristics of p-n junction diode: A graph showing the variation of current flowing through a p-n junction with the voltage applied across it, is called V-I characteristics of a p-n junction. Forward bias characteristics: The V-I graph is not a straight line i.e. a junction diode does not obey ohm’s law. Knee voltage of p-n junction for Ge is 0.3 V and for Si it is .6 to .7 V. Reverse bias characteristics: Reverse bias voltage produces a very small current (μA). It is due to drift of minority charge carriers. Graph for forward and reverse bias Rectifier The p-n junction is a one way device i.e. it offer low resistance when forward biased and high resistance when reverse biased. So it can be used as a rectifier Half wave rectifier Full wave rectifier Zener diode :It was invented by C. Zener. It is designed to operate in the reverse breakdown voltage region continuously without being damaged. A zener diode has unique feature that voltage drop across it , is independent of current through it. For a proper working of a zener diode in any circuit ,it is essential that The zener diode must be reverse biased
