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When p-type and n-type junctions are combined to form p-n junctions, they possess a characteristic called rectification [23-26]. Rectification is a property to allow flow of current easily in one direction only [28,29]. In the case of p-type material, the Fermi level (E_F) is near the valence band edge and is close to conduction band edge in n-type material as shown in Figure 1.5. In p-type configuration, holes are the majority carriers, while electrons are minority carriers. Just the opposite happens in case of n-type materials in which electrons are majority carriers and holes are minority carriers. Upon joining, large carrier concentration gradients happen at the junction to cause carrier diffusion. Majority holes from the p-type are transported by diffusion into the n-type semiconductor, while majority electrons from n-type semiconductor are diffused towards the p-type. Holes continue to leave the side of p-type while electrons keep on moving from the side of n-type semiconductor till a saturation point is reached. In this exercise of charge carrier transportation, a minor concentration of negative acceptor ions

Figure 1.4 Figure showing the doping of two types of foreign atoms of B (p-type) and P (n-type) in Si to form semiconductor material with better conductivities.

Figure 1.5 Two semiconductor blocks of p-type and n-type before the formation of junction and also showing position of Fermi level (E_F) in the corresponding dopes semiconductor material.

and positive donor ions at the semiconductor junction remains unreacted. The holes possess high mobility whereas acceptor atoms are permanently fixed in the semiconductor lattice. Similar explanation follows in case of electrons leaving the n-type semiconductor. A saturation point is attained after transportation of both types of charge carriers to oppositely doped semiconductor blocks. The result of large concentration gradient is that a part of the free electrons coming from donor impurity atoms migrates across the semiconductor junction filling up holes in the p-type semiconductor material to produce negative ions. On moving from n-type to p-type, positively charged donor ions (N_D) are left behind on the side of n-type. Similarly, holes coming from acceptor foreign atoms are transported across the junction in an opposite direction having large number of free electrons. The transport mechanism of holes and electrons across the p-n junction is called diffusion. As a result, a space charge region is formed in the region combining p-type and n-type semiconductor blocks. On the side of p-type block, a negative space charge region is formed, while a positive space charge region is formed on the side of n-type semiconductor. The constitution of this space charge region in the junction develops an electric field that is directed from the holes towards the negative charge. The width of the p- and n-type layers is dependent on the degree of heavy doping of each layer with acceptor impurity atoms (N_A) and donor impurity atoms (N_D), respectively. Figure 1.6(a) below shows the space charge region formed between the joining of two semiconductor blocks of p-type and n-type. The electric field will be directed from positive charge towards the negative charge. Figure 1.6(b) shows the energy band diagram of a semiconducting p-n junction in thermal equilibrium. It needs to be pointed out that the flow of charge carriers can be due to both drift and diffusion. It is apparent from Figure 1.6(b) that the hole drift current flows from right to left, while hole diffusion current flows from left to right. The electron drift current flows from right to the left, while electron diffusion current flows from left to right. Thus, the free charge carriers (electrons and holes) produce current in two ways under the application of an electric field in a semiconductor, i.e. by drift and diffusion. The passage of charge carriers under the effect of an externally applied electric field generates a net current called as the drift current. In case of spatial variation of concentrations of charge carriers in the semiconductor, charge carriers have the tendency to move from regions of high concentration to regions of low concentration called as the diffusion current. The spatial variation in charge carrier concentration is called as the concentration gradient. Figure 1.7 shows the current-voltage characteristics of a typical p-n junction diode. When the junction is forward-biased (+ve terminal of the battery connected to p-type having positive vacancies as majority carriers), current (I) increases rapidly as a function of voltage (V). In case of application of reverse-biasing (-ve terminal of the battery connected to p-type having positive vacancies as majority carriers and +ve terminal of the battery connected to n-type having electrons majority carriers), zero current flows initially virtually. A schematic of the two biasing regimes, reverse Figure 1.8(a) and forward Figure 1.8(b). Only a small amount of current flows on increasing the reverse potential through the battery terminals. At a critical value of the reverse bias, the current suddenly increases which is called as the junction breakdown. The diode response is achieved at relatively lower voltages (~1 V) in forward-biasing case as shown in Figure 1.8(b). In reverse-bias, the breakdown voltage or reverse critical voltage generally varies from few volts to larger voltages. This is typically dependent on the amount of doping of foreign atoms to form two types of semiconductor blocks or layers and different device parameters [10].

Figure 1.6 Space charge region formed in between the joining region of p-type and n-type semiconductor blocks is shown in (a). The energy band diagram of a p-n semiconductor junction in thermal equilibrium is shown in (b).

Figure 1.7 Current-voltage (I-V) characteristics of a semiconductor p-n junction.

Figure 1.8 The two biasing regimes of a diode, (a) reverse (b) forward, are shown in the above schematic. In reverse bias, the diode acts as an open switch, while it acts as a closed switch in case of forward bias.

In the reverse bias mode, the diode device acts as an open switch such that the positive terminal of the source will attract free electrons from n-type and negative terminals will attract holes from the p-type. As a result, concentration of ions in both the regions will increase enhancing the width of the depletion region. In any case, minority carriers will enter the depletion region and cross to other sides of the junction causing a small amount of current called as reverse saturation current (I_S). The term “saturation” here means that there will not be any enhancement in the current on increasing the reverse bias potential. As can be seen from Figure 1.7, current change happens very quickly in small voltages initially reaching the saturation current and dependency of the current on further changes in voltages is lost. At a certain higher critical reverse voltage, usually after tens of voltages, a huge current is caused in the opposite direction. On increasing the reverse voltage, it creates an electric field impacting greater force on the electrons to move faster and an enhancement in kinetic energy (K.E.) of electrons follows. This higher K.E. is transported to valence shell of electrons of stable atoms by highly mobile electrons causing them to leave the atom and form the stream of reverse current flow. The critical voltage at which this rapid change happens is called the Zener voltage.

In forward biasing mode, an electric field forces free electrons in n-type block and holes in p-type block towards the depletion region. In this biasing, holes and free electrons recombine with ions in the depletion region to reduce the width of the depletion region. On increasing the forward voltage further, depletion region becomes thinner and a larger number of majority carriers are able to pass through the barrier. It needs to be pointed out that no net current flows in the diode in absence of an externally applied electric field.