Volt-Ampere (V-I) Characteristics Of Semiconductor Diode

Fig. 1.1 Shows a circuit of volt-ampere of a semiconductor diode.

Reverse Biased Diode

When the p-n junction is

P-N Junction As a Diode

Depletion Region:

The simplest semiconductor device

Silicon Vs Germanium

Both silicon and germanium have semiconducting properties.

Hall Effect

Let a bar of semiconductor carry a current I and lie in a transverse magnetic field B. because of a phenomenon known as hall effect, an electric field E is induced. The direction of electric field E is perpendicular to both I and B. This effect is used to find the type of semiconductor (P-type or N-type), carrier concentration and also to measure conductivity δ and hence find mobility µ.

Fig.1: Shows a bar of semiconductor having width "d" and thickness "t". the current "I" is in the positive "x" direction and the magnetic field "B" is the positive "z" direction. A force will be exerted on the carriers are electrons and these electrons will be forced downwards. Therefore, terminal 1 will become negative with respect to terminal 2. A potential difference
E_N (Known as Hall Voltage)

Minority and Majority Charge Carriers

In a P-type semiconductor the number of holes is larger than the number of free electrons in the conduction band. Therefore, in p-type material, holes are majority carriers and electrons are minority carriers. However, in an n-type material the number of free electrons in the conduction band is much larger than the number of holes. Thus, in an n-type semiconductor, electrons are majority carriers and holes are minority carriers.

Extrinsic Semiconductor: Donor and Acceptor Impurities

The conductivity

Intrinsic Semiconductors

Germanium and Silicon are two important materials used in electronic devices. They are known as intrinsic material Semiconductor. The germanium atoms has a total of 32 electrons of which 28 are tightly bound to the nucleus and 4 are valence electrons.

The tightly bound electrons do not leave the nucleus. Therefore, each nucleus and it’s tightly bound electrons can be represented by a circle as shown in a two dimensional representation in Fig 1.1. Each positive charge shown as +4 depicts the nucleus along with tightly bound electrons (Since 4 valence electrons have been taken out, a charge of +4, measured in units of electronic charge remains in each circle). Since both germanium and silicon have 4 valence electrons, this representation is same for both.

 

          Fig.1.1: Two dimensional representation of a crystal of intrinsic semiconductor

 
Each atom shares its 4 electron with four neighboring atoms and also shares one electron from each of these four neighboring atoms. These shared valence electrons shown by lines in Fig. 1.1. Thus each atom fills its valence orbit with 8 electrons, out of which four are it’s own and four belongs to the neighboring atoms. This forms the covalent bond between atoms. At “0 Kelvin” Temperature there are no free electrons and hence no conductivity. At higher temperatures some of these valence electrons get thermally excited and break the covalent bond. At room temperature the number of such electrons is very small and conductivity is low. The energy required to break the covalent bond is 0.72 ev for germanium and 1.1 ev for silicon. These dislodged electrons are free to move in a random fashion throughout the crystal.

As an electron breaks the covalent bond and leaves the vicinity of an atom, it leaves a positive charge behind. This positive charge, left behind when an electron-hole pair is created, two charge carrying particles are formed. One is negative another positive on the application of an electric field “E” in the semiconductor the holes and electrons drift in the opposite directions and current flows. The holes are positively charged and move in the same direction as the field. The electrons move in a direction opposite to that of the field.

The conductivity of germanium increases by about 6 percent per degree increase in temperature. For silicon the increase is about 8 percent per degree increase in temperature.

Fig. 1.2 shows the apparent motion of holes due to recombination of holes and electrons. Let an electron breaks its covalent bond at position A and drift to position B under the influences of electron field. If a hole existed at position B, the electron would combine with this hole to neutralize the charge. Now there is a hole at position A and no hole in position B. Thus the hole has moved from position B to A. The drift velocity of electrons and holes is proportional to electric field strength. Thermal agitation produces new electron-hole pairs.

 Fig.1.2: Motion of holes in intrinsic semiconductor

Conductor, Insulator, Semiconductor

Conductor:

In a conductor, the conduction and valence bands overlap as shown in Fig. 1. There is no forbidden energy band region. When an external electric field is applied, the electrons acquire additional energy and to higher energy level. These mobile electrons constitute a current. The overlapping of conduction and valence bands gives rise to high conductivity. The resistivity of conductors is of the order of 10^-7

Ω (ohm). Most metals are good conductors.

Fig:1: Conduction Band formation of Conductor

Insulator:

The energy band structure of insulators is shown in Fig. 2. A large forbidden band of several electron volts, exists between valence and conduction bands. The valence band is completely filled at absolute zero temperature. The valence electrons remain tightly bound to the nucleus. As the temperature of the insulator is increased, the added heat energy enables some valence electrons to jump the forbidden gap and occupy the unfilled level above. At room temperature the number of such electrons is negligible and conductivity is very small. The resistivity of an insulator is very high and is around 10¹⁰ to 10¹⁶ Ω (ohm).

Fig:2: Energy Band Diagram

Semiconductor:

In some substances the forbidden gap between conduction and valence bands is small (about 1 ev). These materials are known as semiconductors. Germanium and silicon are the two examples of semiconductor. At 0K temperature the forbidden gap in germanium is 0.785 ev and in silicon it is 1.21 ev. Even at room temperature some valence electrons have enough energy and can move into the conduction band. Therefore, these materials are slightly conducting. The forbidden energy gap decrease with increase in temperature. The resistivity of semiconductors is of the order of 1 ohm-m.