Semiconductor devices and Diodes

SEMICONDUCTORS

A semiconductor is a material which has electrical conductivity to a degree between that of a metal (such as copper) and that of an insulator (such as glass). Semiconductors are the foundation of modern solid-state electronics, including transistors, solar cells, light-emitting diodes (LEDs), quantum dots and digital and analog integrated circuits.

Types of Semiconductor devices:

There are two types of Semiconductor devices:

i. Extrinsic semiconductors

ii. Intrinsic semiconductors

Extrinsic Semiconductors:

An extrinsic semiconductor is a semiconductor that has been doped, that is, into which a doping agent has been introduced, giving it different electrical properties than the intrinsic (pure) semiconductor.

n_i² = n_en_h

This is of types:

1. p-type semiconductors:

p-type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities (trivalent element) p-type semiconductors have a larger hole concentration than electron concentration. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. P-type semiconductors have Fermi energy levels below the intrinsic Fermi energy level. The Fermi energy level lies closer to the valence band than the conduction band in a p-type semiconductor

2. n-type semiconductors:

n-type semiconductors are created by doping an intrinsic semiconductor with donor impurities( pentavalent elements). Extrinsic semiconductors with a larger electron concentration than hole concentration is known as n-type semiconductors. In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers. In an n-type semiconductor, the Fermi energy level is greater than that of the intrinsic semiconductor and lies closer to the conduction band than the valence band

Intrinsic Semiconductors:

An intrinsic semiconductor, also called an un doped semiconductor is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the material itself. In intrinsic semiconductors the number of excited electrons and the number of holes is equal.

n_e = n_h

Semiconductor Diode:

A diode made of semiconductor components, usually silicon. The cathode, which is negatively charged and has an excess of electrons, is placed adjacent to the anode, which has an inherently positive charge, carrying an excess of holes. At this junction a depletion region forms, with neither holes nor electrons. A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for the application of an external voltage. It is a two terminal device. A p-n junction diode is symbolically represented as shown in Fig.

The diode symbol is shown in Fig. The P-type and N-type regions arereferred to as P–end and N–end respectively. The arrow on the diode points the direction of conventional current.

Forward bias characteristics:

The circuit for the study of forward bias characteristics of PN junction diode is shown in Fig. The voltage between P–end and N–end is increased from zero in suitable equal steps and the corresponding currents are noted down. Fig shows the forward bias characteristic curve of the diode. Voltage is the independent variable. Therefore, it is plotted along X–axis. Since, current is the dependent variable, it is plotted against Y–axis.

From the characteristic curve, the following conclusions can be made.

(i) The forward characteristic is not a straight line. Hence the ratio V/I is not a constant (i.e) the diode does not obey Ohm’s law. This implies that the semiconductor diode is a non-linear conductor of electricity.

(ii) It can be seen from the characteristic curve that initially, the current is very small. This is because, the diode will start conducting, only when the external voltage overcomes the barrier potential (0.7V for silicon diode). As the voltage is increased to 0.7 V, large number of free electrons and holes start crossing the junction. Above 0.7V, the current increases rapidly. The voltage at which the current starts to increase rapidly is known as cut-in voltage or knee voltage of the diode.

Reverse bias characteristics:

The circuit for the study of reverse bias characteristics of PN junction diode is shown in Fig a. The voltage is increased from zero in suitable steps. For each voltage, the corresponding current readings are noted down. Fig b shows the reverse bias characteristic curve of the diode. From the characteristic curve, it can be concluded that, as voltage is increased from zero, reverse current (in the order of microamperes) increases and reaches the maximum value at a small value of the reverse voltage. When the voltage is further increased, the current is almost independent of the reverse voltage upto a certain critical value. This reverse current is known as the reverse saturation current or leakage current. This current is due to the minority charge carriers, which depends on junction temperature.