In a relatively simplified view of semiconductor materials, we can envision a semiconductor as having two types of charge carriers-holes and free electrons which travel in opposite directions when the semiconductor is subject to an external electric field, giving rise to a net flow of current in the direction of the electric field. Figure 1 illustrates the concept. A p-n junction consists of a p-type and n-type section of the same semiconductor materials in metallurgical contact.
The p-type region has an abundance of holes (majority carriers) and a few mobile electrons (minority carriers); the n-type region has an abundance of mobile electrons and a few holes (Fig. 2). Both charge carriers are in continuous random thermal motion in all directions. Fig. 2. Energy levels and carrier concentrations for a p-type and n-type semiconductor before contact. 1 When a section of p-type material and a section of n-type material are brought in contact to form a pn junction, a number of interesting properties arise. The pn junction forms the basis of the semiconductor diode.
Electrons and holes diffuse from areas of high concentration toward areas of low concentration. Thus, electrons diffuse from the n-region to the p-region. , leaving behind positively charged ionized donor atoms. In the p-region the electrons recombine with the abundant holes. Similarly, holes diffuse from the p-region into the n-region, leaving behind negatively charged ionized acceptor atoms. In the n-region the holes recombine with the abundant mobile electrons. This diffusion process does not continue indefinitely, however, because it causes a disruption of the charge balance in the two regions.
As a result, a narrow region on both sides of the junction becomes nearly depleted of the mobile charge carriers. This region is called the depletion layer. It contains only the fixed charges (positive ions on the n-side and negative ions on the p-side). The thickness of the depletion layer in each region is inversely proportional to the concentration of dopants in the region. The net effect is that, the depletion region sees a separation of charge, giving rise to an electric field pointing from the n side to the p side.
The fixed charges create an electric field in the depletion layer that points from the n-side towards the p-side of the junction. The charge separation therefore causes a contact potential (also known as built-in potential) to exist at the junction. This built-in field obstructs the diffusion of further mobile carriers through the junction region. An equilibrium condition is established that results in a net contact potential difference Vo between the two sides of the depletion layer, with the n-side exhibiting a higher potential than the p-side.
This contact potential is typically on the order of a few tenths of a volt and depends on the material (about 0. 5 to 0. 7 V for silicon). The built-in potential provides a lower potential energy for an electron on the n-side relative to the p-side. As a result, the energy bands bend as shown in Fig. 3. In thermal equilibrium there is only a single Fermi function for the entire structure so that the Fermi levels in the p- and the n-regions must align. No net current flows across the junction. The currents associated with the diffusion and built-in field (drift current) cancel for both the electrons and holes.
Fig. 3. A p-n junction in the Thermal equilibrium at T > 0? K. The depletion-layer, energy-band diagram, and concentrations (on a logarithmic scale) of the mobile electrons n(x) and holes p(x) are shown as a functions of the position x. The built-in potential difference V corresponds to the energy eV where e is the electron charge. 0 0 2 The Biased p-n Junction An externally applied potential will alter the potential difference between the p- and n-regions. This in turn will modify the flow of majority carriers, so that the junction can be used as a “gate”.
If the junction is forward biased by applying a positive voltage V to the p-region (Fig. 4), its potential is increased with respect to the n-region, so that an electric field is produced in a direction opposite to that of the built-in field. The presence of the external bias voltage causes a departure from equilibrium and a misalignment of the Fermi levels in the p- and n-regions, as well as in the depletion layer. The presence of the two Fermi levels in the depletion layer, Efc and Efv represents a state of quasi-equilibrium. Fig. 4.
Energy band diagram and carrier concentrations for a forward-biased p-n junction. In effect, then, if one were to connect the two terminals of the p-n junction to form a closed circuit, two currents would be present. First, a small current, called reverse saturation current, is, exists because of the presence of the contact potential and the associated electric field. In addition, it also happens that holes and free electrons with sufficient thermal energy can cross the junction. This current across the junction flows opposite to the reverse saturation current and is called diffusion current.
Of course, if a hole from the p side enters, it is quite likely that it will quickly recombine with one of the n-type carriers on the n side. (Fig. 4) The net effect of the forward bias is to reduce the height of the potential-energy hill by an amount eV. The majority carrier current turns out to increase by an exponential factor exp(eV/kT). So that the net current becomes i = isexp(eV/kT) – is, where is is nearly a constant. The excess majority carrier holes and electrons that enter the n and p regions, respectively, become minority carriers and recombine with the local majority carriers.
To explain the mechanism of reverse conduction, one needs to visualize the phenomenon of avalanche breakdown. When a very large negative bias is applied to the p-n junction, sufficient energy is imparted to charge carriers that reverse current can flow, well beyond the normal reverse, saturation current. In addition, because of the large electric field, electrons are energized to such levels that if they collide with other charge carriers at a lower energy level, some of their energy is transferred to the carriers with low energy, 4 and these can now contribute to the reverse conduction process, as well.
This process is called impact ionization. Now, these new carriers may also have enough energy to energize other lowenergy electrons by impact ionization, so that once a sufficiently high reverse bias is provided, this process of conduction takes place very much like an avalanche: a single electron can ionize several others. Fig. 6. The reverse breakdown region The phenomenon of Zener breakdown is related to avalanche breakdown. It is usually achieved by means of heavily doped regions in the neighbourhood of the metal-semiconductor junction (the ohmic contact) .
The high density of charge carriers provides the means for a substantial reverse breakdown current to be sustained at a much lower specific voltage than normal diode, at a nearly constant reverse bias known as the Zener voltage, Vz. This phenomenon is very useful in applications where one would like to hold some load voltage constant for example, in voltage regulators. The response time of a p-n junction to a dynamic (ac) applied voltage is determined by solving the set of differential equations governing the processes of electrons and hole diffusion, drift (under the influence of the built-in and external electric fields), and recombination.
These effects are important for determining the speed at which the diode can be operated. They may be conveniently modeled by two capacitances, a junction capacitance and diffusion capacitance, in parallel with an ideal diode. The junction capacitance for the time necessary to change the fixed positive and negative charges stored in the depletion layer when the applied voltage changes. The thickness l of the depletion layer turns out to be proportional to v(Vo-V); it therefore increases under the reverse-bias conditions (negative V) and decreases under the forward-bias conditions (positive V).
The junction capacitance C=ЄA/l (where A is the area of the junction) is therefore inversely proportional to v(VoV). The junction capacitance of a reverse-biased diode is smaller (and the RC response time is therefore shorter) than that of a forward-biased diode. The dependence of C on V is used to make voltage-variable capacitors (varactors). 5 Experiment l(a) : i-v characteristics of a semiconductor diode Procedure Connect the diode according to the circuit diagram as shown in Fig. 8. Fig 8 Vary the voltage V on the power supply between 0-30V.
Alternately, the second concept is that the blocking action of an inductor stops the a. c. portion while the d. c. portion passes without much attenuation. Note: For filtering, large capacitance (hundreds to tens of hundreds microfarad) is needed. These are generally electrolytic capacitors, which consist of a repeating sandwich of aluminum sheets and a conducting paste, rolled into a cylinder for miminmun size. The aluminum sheets are polarized to form thin layers of aluminum oxide, a dielectric insulating material. The thinner the the dielectric the higher the capacitance will be.