Nature and the Characteristics OfSemi-conductors(diodes and doped) By: Amah Philip
You have learned in chemistry that atomsfollow the octet rule. An atom is most stablewhen there are eight electrons in itsoutermost shell. An atom with only one ortwo electrons in its outermost shell tends togive away these outer electrons to gainstability (i.e, it now has eight outermostelectrons). This means that one or twooutermost electrons are not tightly held to theatom and are fairly free to travel.
If you take a look at the periodic table ofelements, you will observe that the group numberindicates the number of outermost, or valenceelectrons, of an element. Thus, Groups III, IV and Vhave three, four, five valence electrons, respectively. Elements in Group IV share their valence electronswith the nearest neighbouring atoms (figurebelow), They form covalent bonds to attain a stablefilled orbital. Recall that the ability of any material toconduct electricity depends on the behaviour of theelectrons in the outermost shells.
This property makes Group IV elementsrelatively poor conductors of electricity. Theyare called semiconductors. Semiconductorsare substances which have a resistance inbetween that of conductors and insulators. Electronic components made mainly ofsemiconductors are called solid-stateelectronic devices. Such components includediodes and transistors.
Doped Semiconductor The importance of semiconductors in today’selectronic technology atoms from the fact thattheir electrical properties are very sensitive tosmall amounts of impurities. The process ofdeliberately adding very small amounts ofimpurities or foreign substances to an otherwisepure substance is called doping, and theimpurities are referred to as dopants. Dopingresults in an extrinsic semiconductor, or thatwhose electrical properties depend upon thepresence of certain impurities.
Doping silicon with phosphorus. Phosphorus has one extra electron in its outermost shell. Each of the four atoms of P participates in the bonding with nearby Siatoms, leaving the extra electron weakly bounded. This electron is easily excited and contributes to electrical conduction.
we increase the number of holes in intrinsic silicon, trivalent impurity atoms areadded. These are those atoms with three valence electrons such as Boron (B), indium (in), and gallium (Ga). Each trivalent atom forms covalent bonds with four adjacent silicon atoms. All three of the boron atom’s valence electrons are used in the covalent bonds; and, since four electrons are required, a hole results when each trivalent atom is added. Because the trivalent atom can taken an electron, it is often referred to as an acceptor tom. The number of holes can be carefully controlled by the number of trivalent impurity atoms added to the silicon. A hole created by this doping process in not accompanied by a conduction free electron.
The purpose of doping is to increase the number of free charges that can be moved by an applied voltage. Silicon and germanium crystals are widely used in the manufacture of semiconductor devices. They are intrinsic semiconductors, or pure semiconductors without doping. They belong to Group IV. Each element has four outer electrons per atom and doping them with an element with five outer electrons, such as phosphorus (P), frees the fifth electron so that the semiconductor has an excess electron. It is then known as an n-type (negative-type) semiconductor, because the major charge carriers are negative electrons.
Mixing the dopants of Group IV produces n-typesemiconductors. Take for example phosphorus andsilicon (Figure above). Doping with an element that has only three outerelectrons, such as boron (B) or aluminium (AI),produces a crystal lattice with spaces, known as holes,which electrons from nearby atoms readily fill. Thetype of semiconductor does not have free electrons,which is equivalent to an excess of positive charges.This is known as a p-type (positive-type)semiconductor. Combining the dopants of Group IIIelements with elements from Group IV results in p-type semiconductors. Figure above shows silicondoped with aluminium.
Doping an insulator like silicon into a viable(although not so great) conductor is the basictechnique in the production ofsemiconductors. The n-type and p-type semiconductors canbe put together to form another importantelectronic component- the diode.
Diodes The diode is the simplest semiconductor device. It allows a current to pass through it in only one direction. It is produced when crystals of pure silicon are doped so that a junction is formed between p-type and n-type regions. The p-type material meets an n-type material across a narrow layer depleted of charge carriers, known as the depletion layer. Material in this layer conducts very poorly.
If the diode is connected to a battery so thatthe negative terminal is joined to the n-typesemiconductor and the positive terminal to the p-type semiconductor, electrons and holes cancross the junction and produce current. Such ajunction is said to be forward-biased. On the other hand, if the battery is connectedthe other way around, so the its positive terminalis connected to the n-type semiconductor and itsnegative terminal to the p-typesemiconductor, the free electrons and holes areforced away from the junction.
Practically no current results from thisconnection. This is known as inverse-biased. Adiode oriented in this manner acts like a verylarge resistor that is almost an insulator.Diodes can therefore act as automaticswitches that can be turned on and offwhenever current. One major use of diodes isto convert alternating current (AC) to directcurrent (DC). Diodes built for this specificpurpose are called rectifiers.
A diode is also used to separate information fromtransmitted radio signals or carrier waves. Itdemodulates and detects the audio signals. Such adiode is called a detector. Diodes that detect and emit light are called light-emitting diodes (LEDs). LEDs are often used as indicatorlights on videos and cassette players. Diode lasers thatemit narrow beams of coherent, monochromatic lightor infrared radiation are used in CD players andsupermarket bar-code scanners. They are compact andpowerful light sources.
Diagram of the forward and reverse- biasedForward-Biased: Positive end of the Reverse-Biased: Negative end of thebattery meets the positive end (the anode) battery meets the negative end (theof the diode, causing current flow. cathode) of the diode, no current flow occurs.