Semiconductors Assignment help




A Semiconductor is a material with electrical conductivity due to electron flow (as opposed to ionic conductivity) intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter. Semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, and many other devices. Such devices include transistors, solar cells, many kinds of diodes including the light-emitting diode, the silicon controlled rectifier, and digital and analog integrated circuits. Similarly, semiconductor solar photovoltaic panels directly convert light energy into electrical energy. In a metallic conductor, current is carried by the flow of electrons. In semiconductors, current is often schematized as being carried either by the flow of electrons or by the flow of positively charged "holes" in the electron structure of the material. Actually, however, in both cases only electron movements are involved.

Semiconductor Conduction Band:

Semiconductors are the materials which behave as insulators at low temperature and conductors at higher temperature. Conductance of semiconductors increases with increase in temperature. Silicon and germanium is the example of semiconductor. Semiconductor behaves as both conductor and insulator due to its conductivity in conduction bands at various temperatures.

Semiconductor Conduction Band:

Normally any material consists of following types of bands which involved in conduction of electrons, they are

  • Valence band
  • Conduction band
  • Energy gap

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Valence band:

  • Valence band is also one of the electron bands, which occurs beneath the conduction band.
  • Valence band contains the valence electrons of atoms.
  • When the temperature is increased, number of electrons in the valence band gain thermal energy, cross the energy gap and move into the adjacent conduction band.
  • Due to the promotion of the valence electrons, vacancies are created in the valence band and the material conducts the electricity.

Conduction band:

  • Conduction band is one of the electron energy.
  • Conduction band is responsible for conduction of the electric current in any electrical conductors.
  • Conduction band is located above the valence band and separated by band gap.

Energy gap:

  • In semiconductors, the energy gap occurs between valence band and conduction band.
  • During high temperature, the electrons from the valence bands cross this energy gap and reach the conduction band.
  • In semiconductor, the energy gap is short between conduction and valence band.

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           At high temperature, the electrons from the valence band crosses the energy gap, because of the short energy gap it reaches the conduction band fastly, hence conductivity increases.

           If the semiconductor is doped, the conductivity becomes higher. Sometimes both the valence and conduction band gets overlap, there is no energy gap.

Types of Semiconductors

Intrinsic semiconductors

  • Intrinsic semiconductors are pure semiconductor.
  • Pure silicon and germanium is intrinsic semiconductor.
  • Conductivity is low because of high energy gap between conduction and valence band.

Extrinsic semiconductors or impure semiconductors:

  • Extrinsic semiconductors are impure semiconductor.
  • Dope impurities like boron, aluminum, phosphorus and arsenic to pure semiconductor.
  • High conductivity because of less energy gap between conduction and valence band.

 

Semiconductor Diode

All semiconductor devices make use of the directional feature of current flow across a boundary between n-type and p-type semiconductors.

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When a donor impurity is added to one side and an acceptor impurity is added to the other side of a pure semiconductor, the first side becomes n-type and the other side becomes p-type. Thus a p-n junction is formed (Fig. 26.6). There will be diffusion of electrons and holes across the boundary. At equilibrium, this establishes a potential difference across the p-n junction, even though no external emf is connected across the junction. This potential difference is called junction or barrier potential. This potential barrier stops further flow of charge carriers across the junction unless an external source is used. At room temperature (about 300 K) the junction potential is 0.3 V for germanium and 0.7 V for silicon.

Formation of depletion layer

On p-side of the p-n junction, there is a high concentration of free holes and on n-side a high concentration of free electrons. On ac-of their motions and fuse towards the p-side; the free holes diffuse towards the n-side. These current carriers recombine with each other in the vicinity of the junction.

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Consequently, the region at the junction is devoid of free current carriers and the region is called depletion layer or barrier region. This region contains immobile positive and negative ions. However, there will be holes on the n-side of the border and electrons on the p-side of the border. Further crossing is prevented by repulsion of the same type. The depletion layer behaves like an insulator. This layer has a thickness of the order of micron (10~6m). count of their motions concentration gradient, free electrons from the n-side tend to dif-

Majority and Minority charge carriers

Silicon and germanium have all valence electrons in covalent bondage. At absolute zero, no excess electrons are free to drift through as current. This in theory, represents a perfect insulator. However, at room temperature some of the valence electrons possess sufficient thermal energy to break their bonds and jump from valence band to conduction band and become free electrons; these free electrons create equal number of electron vacant sites called holes in the broken covalent bonds. Such a pure semiconductor is called intrinsic semiconductor. In other words, an intrinsic semiconductor is one in which the free electrons are those which are excited from the valence band and the number of free electrons is equal to the number of free holes. Electric current in the intrinsic semiconductor is thus constituted by the electrons which are elevated into the condction band as well as the holes that are left behind in the valence band.

At room temperature, the intrinsic concentrations for silicon and germanium are 1.4 x 10l6/m3 and 2.5 x 1019/m3. In terms of temperature T and energy gap, Eg, the free electron concentration is given by n, = Ae~ E-/ 2kT where k = Boltzmann's constant and A is almost a constant. In extrinsic semiconductors, at room temperatures, all the electrons from donor atoms are excited to the conduction band. Hence at room temperature, density of free electrons n is given by n = nl + nD where n, - intrinsic electron concentration and nD = concentration of donor impurity. Usually = nD. Due to donor type doping, free electron concentration increases and those free electrons can occupy the holes. Therefore, the number of holes decreases.

In a n-type semiconductor (at room temeperature) number of free electrons being more than that of holes, electrons carry most of the current. Therefore, electrons are majority carriers and holes are minority carriers. In p-type semiconductors the situation is reversed. Hence in p-type, holes are majority carriers and electrons are minority carriers.

Forward and Reverse biasing

When an external source of emf (usually low value) is connected to a p-n junction, it is said to be biased. The applied voltage is called bias voltage.

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When the positive terminal of a cell (source of emf) is connected to the p side and the negative terminal to the n side of the p-n junction, it is said to be forward biased (Fig. 26.8 a). In this condition, the junction permits easy flow of current through it. The external voltage reduces the strength of depletion layer and hence they gainenergy to sermount the potential barrier(0.7V for silicon & 0.3V for germanium). In other words, the effective resistance is decreased providing an easy path for the current (of the order of mA.).

When the positive terminal of a cell is connected to the n side and the negative terminal to the p side of a p-n junction, it is said to be reverse biased. Due to external voltage, the thickness of the depletion layer increases and hence the potential barrier increases. In other words, the effective resistance increases blocking the flow of current. However there will be a leakage current (of the order of nA in silicon diodes and jxA in germanium diodes) due to minority charge carriers. Thus, the p-n junction passes current preferentially in one direction only i.e., p-n junction acts as a rectifier or a diode. Fig. 26.9 shows the circuit symbol of a

imgdiode. The p-side is called the anode and the n-side is called the cathode. The diode symbol looks like an arrow that points from the p-side to the n-side. Because of this, it is a reminder that conventional current flows easily from the p-side to the n-side.

Semicondcutor diodes have the following advantages over diode valves; (i) no heater filament is required (ii) they have long life (iii) they are small in size, rugged and easy to handle (iv) they are economical and highly efficient (v) they consume lesser power.

Semiconductor diode characteristics

Fig. 26.10 shows the circuit arrangement for studying the V-l characteristic of a semiconductor diode.

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A p-n junction diode is connected with low dc voltage source using a potential divider in series with a milliammeter (mA) as shown in Fig. 26.10a. The positive end of the battery is connected with p-region end and the negative of the battery is connected with n-region end of the diode, i.e., the diode is forward biased. By increasing the voltage V across the diode as read by the voltmeter, the corresponding current is recorded by the milliammeter.

A voltage of about 0.3 to 0.7 volt is sufficent to overcome the potential barrier at the junction and permit a current flow. Thereafter the current increases rapidly with increasing voltage and a voltage of one to two volts permit current of 2.5 to 10 mA (Fig. 26.10b); the variation is almost exponential.


When the battery voltage is reversed in polarity i.e., the p-n junction is reverse biased (fig. 26.11a), the flow of current stops almost completely. However, a small current of the order of microamperes flows across the junction. When the voltage is sufficiently high, the current increases abruptly to a large value (Fig. 26.11b). This always occurs at a fixed reverse poential for a heavily doped diode and is known as zener potential. Hence the name Zener diode for the heavily doped ones.

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When a diode is operated a constant forward bias voltage Vf and the corresponding current is If, then the ratio Vf / If is called forward dc resistance (Rs ) of the diode. If A If is the change in current for a change A Vf in forward bias voltage, the ratio A Vf / AIf is called forward ac resistance (rs) of the diode.

In the case of reverse bias, the ratio Vr / Ir is called reverse resistance (R0). Forward resistance is of the order of tens of ohms, whereas the reverse resistance is very high of the order of hundreds of kilo-ohms.



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