In this article we will discuss about:- 1. Types of Semiconductors 2. Properties of Important Semiconductors 3. Preparation of Semiconductor Materials 4. Preparation of P-N Junctions Semiconductor Diode 5. Open-Circuited P-N Junction Semiconductor Diode 6. Forward Bias Arrangement 7. Reverse Bias Arrangement 8. Characteristics 9. Static and Dynamic Resistance 10. Breakdown Mechanisms.

Contents:

  1. Types of Semiconductors
  2. Properties of Important Semiconductors
  3. Preparation of Semiconductor Materials
  4. Preparation of P-N Junctions Semiconductor Diode
  5. Open-Circuited P-N Junction Semiconductor Diode
  6. Forward Bias Arrangement of Semiconductor Diode
  7. Reverse Bias Arrangement of Semiconductor Diode
  8. Characteristics of a Semiconductor Diode
  9. Static and Dynamic Resistance of a Semiconductor Diode
  10. Breakdown Mechanisms of Semiconductor Diodes

1. Types of Semiconductors:

N-Type and P-Type Semiconductors:

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The only current carriers in an intrinsic semiconductor (e.g., a pure silicon) are the electron- holes pair. For most of the cases they do not produce a usable current. By the term doping we mean addition of impurity atoms to a crystal so that the number of conduction-band electrons or holes are increased. When a crystal is doped it becomes an extrinsic semi-conductor.

Pentavalent atoms, like arsenic, antimony, phosphorus, etc., having five electrons in the valence orbit when added to a pure silicon crystal, extra conduction- band electrons are obtained. After forming covalent bonds with four neighbours, the central atom has an extra electron left over which travels in the conduction-band orbit. This has been shown in Fig. 1.14(a).

Since each pentavalent atom contributes one conduction- band electron we can conclude that the conduction-band electrons can be controlled by the amount of impurity added. Silicon doped by pentavalent atom in this way is called N- type semiconductor. Fig. 1.14(b) exhibits the energy diagram of an N-type semiconductor. In the figure the valence band has a few holes while the conduction band has many electrons.

When a crystal is doped with trivalent atom like aluminium, boron, gallium, etc., we can get extra hole. After adding trivalent atom to a pure silicon crystal, as shown in Fig. 1.15(a), we find that one hole appears in each trivalent atom. By controlling the amount of impurity added, the number of holes in the doped crystal can be controlled.

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Silicon doped by a trivalent atom in this fashion is called P-type semiconductor. Fig. 1.15(b) exhibits the energy diagram of a P-type semiconductor. In the figure, the valence band has many holes while the conduction band has a few electrons.

2. Properties of Important Semiconductors:

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Among all the semiconductors, the elements germanium (Ge) and silicon (Si) have been most extensively studied. These are tetravalent and belong to Group IV of the periodic table comprising of carbon, silicon, germanium, tin and lead. All these elements are crystalline in the diamond- lattice structure.

Recently gallium arsenide (GaAs) has also been investigated. It is formed from elements of Group III and V of the periodic table and so named as III-V compound. It crystallizes in zinc blende lattice structure.

3. Preparation of Semiconductor Materials:

To use germanium and silicon in diodes those are required to purify to a very high degree. Out of the two it is easier to purify germanium than silicon.

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Commercially, germanium is obtained as germanium dioxide (GeO2). It is reduced to germanium powder at 650°C in a controlled atmosphere of hydrogen. In an inert gas atmosphere if it is heated above the melting point (936°C) the material can be obtained in polycrystalline form.

Further, purification is made by the process of zone melting which is based on the principle that when a molten semiconductor solidifies slowly, most of the impurities remain in liquid phase. These will be swept to the end which solidifies last. The semiconductor bar to be refined is kept in a graphite boat and then slowly pulled through a series of induction-heating coils.

When the bar is moved slowly from right to left along the quartz tube having a few induction-coil heating zones, the impurities will remain in the liquid zone. These are concentrated finally at the right end of the bar which may be cut off to reject. The molten zones are removed with a speed of one-half to several tenths of a cm per minute so that the impurities are not trapped in the recrystallizing solid interface. To increase the impurity of the desired degree the process is repeated several times.

Silicon has favourable semiconductor characteristics up to about 200°C. This is in contrast to germanium which fails as a semiconductor above 100°C. But due to great solubility of other materials in silicon its purification is too difficult and expensive. By the action of chlorine on a heated mixture of silica (sand) and carbon one can get silicon tetrachloride.

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This is then reduced with zinc, followed by hydrogen and other reducing agents to a purity of 99.9%. At the end of chemical purification, a zone refining process is applied to reduce the impurities to 1 part in 1010 or legs.

In order to fabricate diodes (or transistors) large single crystals are used which are grown by touching a small seed crystal to the surface of molten semiconductor material and then withdrawing it slowly as new lattice crystals. A typical crystal-growing arrangement is shown in Fig. 1.17, when Fig. 1.17(a) exhibits the apparatus for growing a large single crystal while Fig. 1.17(b) reveals the cross section of the crystal melt interface. The temperature of the melt is controlled accurately by induction-heating technique. It is possible to form alternate regions of N- and P-type semiconductors in the same crystal by an alternate doping of the melt with N- and P-type impurities.

The crystals thus grown are sliced by a diamond saw into small wafers, the surface of each of which is polished and etched with acids.

4. Preparation of P-N Junctions Semiconductor Diode:

The P-N junctions are prepared by a transition from P- to N-type to occur within a single crystal. Junctions are generally made by three different means.

These are:

(i) The grown-junction method,

(ii) The alloy-junction method, and

(iii) The diffused-junction method.

(i) The Grown-Junction Method:

In this method, first of all, the germanium (or the silicon) crystal is doped with a very small quantity of P-type impurity. N-material Then an N-type impurity is added at an appropriate instant in sufficient quantity. Thus a single crystal with a junction between P- and N-material is prepared, functions formed in this way have a relatively gradual transition from P to N and are classified generally as ‘graded’ junctions. After the preparation of the crystal it is cut into rods along its length, the surface of each of the rod is polished and etched. The rod is then mounted and sealed in an enclosure.

ii. The Alloy-Junction Method:

The fused or alloy junctions are mostly used because of its simplicity. First of all a wafer to be used for the base is selected and the surface treated. A piece of material of opposite impurity is kept on the surface and the wafer is heated. For N-type germanium, a pallet or dot of indium may be used to form the junction. For the purpose, the temperature is raised to about 900 °C for a minute or so.

Due to heating the indium pallet melts and dissolves the germanium to form an alloy. When the heating is completed the liquid begins to solidify. The junction thus produced has a very narrow transition from N- to P-type. This is usually classified as ‘abrupt’ junction and it depends upon the time- temperature cycle to a great extent.

iii. The Diffused-Junction Method:

In this technique, one surface of a slab of material of the type to be used for the base is exposed to a gaseous impurity of opposite type. The slab is next heated to a high temperature. The gaseous impurity diffuses slowly into the surface whose concentration is highest at the surface and decreases exponentially inwards. Under the surface a P-N junction is thus formed directly. This method is found to be very useful for making solar cells.

5. Open-Circuited P-N Junction Semiconductor Diode:

In the P-N junction diodes the rectifying action takes place at the boundary layer between the P- type and N-type materials. Fig. 1.21(a) shows a junction diode where the P-side has many holes and the N-side many conduction-band electrons. In practice, however, a few conduction-band electrons on the P-side and a few holes on the N-side exist (not shown in the figure). The diode in the figure is called unbiased which means that no external voltage has applied to it.

a. Depletion Layer:

Due to repulsion of each other the electrons on the N-side tend to diffuse in all directions. The electrons which diffuse across the junction and enter the P-region become a minority carrier. With so many holes around a minority electron it gets a short lifetime, since just after entering the P-region the electron falls into a hole. As a result the hole disappears and the conduction band electron becomes a valence electron.

Each time an electron diffuses across the junction, creating a pair of ions. Fig. 1.21(b) shows these ions on each side of the junction. Because of covalent bonding the ions are fixed in the crystal structure and cannot move around like conduction-band electrons or holes. In the figure, the circles with plus signs are the positive ions and those with minus signs are the negative ions.

Each pair of positive and negative ions is called a dipole. Thus by the creation of a dipole we mean that one conduction-band electron and one hole have been taken out of circulation. With the formation of the dipoles the region near the junction is emptied of movable charges. This charge-empty region is called the depletion layer.

b. Barrier Potential:

Each dipole has an electric field. The direction of force of a positive charge. So, when an electron enters the depletion layer, the field tries to push the electron back into the N-region. The strength of the field increases with each crossing electron until an equilibrium is attained.

At the same time we need to include the minority carriers also. The P-side has a few thermally produced conduction-band electrons. Those inside the depletion layer are pushed by the field into the N-region. This slightly reduces the field strength and let a few majority carriers diffuse from right to left to restore the field to its original strength.

The field between the ions in Fig. 1.21(b) is equivalent to a difference of potential, known as the barrier potential. At a temperature of 25°C, the barrier potential becomes approximately 0.3 V for germanium diodes and 0.7 V for silicon diodes.

The value of barrier potential changes with the temperature at the junction. With an increase of temperature a larger number of electron-hole pairs are created. As a consequence, the drift of minority carriers across the junction increases and hence the equilibrium occurs at a slightly lower barrier potential. It has been found by experiment that for each degree centigrade rise of temperature the barrier potential decreases by 2 mV which can be written mathematically as-

∆V = – 0.002 ∆T.… (1.2)

Equation (1.2) can be used conveniently to determine the effect of temperature change on diode (or transistor) circuits.

6. Forward Bias Arrangement of Semiconductor Diode:

If the negative terminal of a d.c. source is connected across the N-type material of a diode and the positive terminal to the P-type, then the connection is called forward bias arrangement. Such an arrangement has shown in Fig. 1.23. Here the d.c. source sets up an electric field which opposes the field of the depletion layer. Thus it pushes electrons and holes towards the junction and hence deionizes the edges of the depletion layer. This narrows the depletion layer. With an increase of the external voltage the depletion layer becomes narrower.

As the conduction-band electrons move toward the junction, they leave positively charged atoms behind. Therefore, the right end of the crystal becomes slightly positive. These positively charged atoms pull electrons into the crystal from the negative terminal of the d.c. source.

What happens to an electron can be summarised in the following few steps:

i. After leaving the negative terminal of the d.c. source, it enters the right end of the crystal.

ii. It moves through the N-region as a conduction band electron.

iii. Near the junction it recombines and becomes a valence electron.

iv. It moves through the P-region as a valence electron.

v. After leaving the left end of the crystal, it follows into the positive source terminal.

A forward bias arrangement lowers the energy hill. The position where the recombination takes place is not important; the result is same. A stream of conduction-band electrons goes towards the junction and falls into holes near it. When an electron falls into a hole it gives off energy in the form of heat, light, etc. The captured electrons, now valence electrons, move towards the left in a steady stream through the holes in the P-region. Thus a continuous flow of electrons through the diode is achieved.

7. Reverse Bias Arrangement of Semiconductor Diode:

If the positive terminal of a d.c. source is connected across the N-type material of a diode and the negative terminal to the P-type, then the connection is called reverse bias arrangement. Such an arrangement has shown in Fig. 1.25. Here the field produced by the external source is in the same direction as the depletion layer field. So the holes and electrons move towards the ends of the crystal, i.e., away from the junction.

The flowing electrons leave positive ions behind and the holes leave negative ions resulting the depletion layer wider. The newly produced ions enhance the difference of potential across the depletion layer. The wider the depletion layer, the greater is the potential difference. The depletion layer will not grow further when the potential difference becomes equal to the applied reverse voltage.

Even after the depletion layer settles down, a few minority carrier exists on both sides of the junction most of which recombine with the majority carriers. However, inside the depletion layer these may live for a long time to get across the junction. As a matter of fact, a small current flows in the external circuit. This idea can be explained by considering Fig. 1.25(b).

As soon as an electron hole pair is created inside the depletion layer, the depletion layer field pushes the electron to the right forming one electron to leave the right end of the crystal. The hole in the depletion layer is, on the other hand, pushed to the left. Due to this extra hole on the P-side one electron enters the left end of the crystal and fall into the hole.

As the thermal energy produces electron-hole pairs near the junction continuously, a small current always flows in the external circuit. This reverse current caused by the minority carriers is known as the saturation current. The saturation current becomes approximately double for each 10 °C rise in temperature.

Since the energy gap between the valence band and the conduction band is greater in silicon than in germanium, the thermal energy produces fewer minority carriers in silicon diodes than in the latter. This indicates that with the same junction area a silicon diode has smaller saturation current than a germanium diode.

The greater is the value of the reverse voltage, the steeper is the energy hill. When a conduction-band electron is made to fall down, this hill can attain a high velocity.  

8. Characteristics of a Semiconductor Diode:

In order to use the P-N junction as a circuit element a study of its current-voltage characteristic for both forward and reverse condition is required. The circuit diagrams for studying the forward and reverse characteristics of the diode are shown in Fig. 1.27(a) and 1.27(b) respectively.

For studying the forward characteristic of the diode, the forward voltage is increased from zero at suitable regular steps and the corresponding current is recorded by the milliammeter. On the other hand, for studying the reverse characteristic of the diode the reverse voltage is increased from zero at suitable regular steps and the corresponding current is measured by a microammeter. The nature of the curve obtained by considering both forward and reverse characteristics is shown in Fig. 1.28.

From the curve the following important conclusions can be drawn:

i. Forward Characteristics:

a. The forward characteristic does not show a straight line nature indicating that a semiconductor diode is a nonlinear conductor of electricity.

b. If the applied forward voltage VF is less than the internal potential barrier V0 of the diode, then lF becomes zero. For germanium diode V0 is approximately 0.3V while for silicon diode it is nearly 0.7V at 25 °C. When V0 > VF, the potential barrier prevents holes from P-region and electrons from N- region to flow across the depletion region in the opposite directions.

But when VF > V0 a small current flows as represented by OP in the figure. The forward voltage for which the forward current just starts is known as the break-point voltage. It is also called as the threshold, cut-in or the offset voltage. This threshold voltage becomes equal to the potential barrier of the diode.

c. With a further increase of the forward voltage the current is sharply increased as represented by the steep part PQ of the curve. Here due to an increase of the forward bias voltage the speed of the flow of electrons and holes increases. During the movement of the electrons with higher kinetic energy they collide with crystal atoms. As a result some covalent bonds of the atoms are broken and pairs of electron and hole are created causing an increase of the forward current.

Again, with the increase of the forward current the heating effect in the crystal increases. As the generation of pairs of electron and hole depends on temperature, a rise of temperature causes a further increase in the current. When VF is sufficiently large in comparison to V0, the value of IF rises exponentially with VF.

ii. Reverse Characteristics:

a. With an increase of the reverse voltage VR the reverse current IR increases and attains the maximum value I0. With a further increase of VR, the IR becomes almost independent of VR up to a certain critical value. This value of IR is known as the reverse saturation current. This is also called as leakage current which is due to a few minority carriers.

b. When VR is increased to the critical value corresponding to the point P in the figure, the reverse current rapidly increases due to the breakdown of the junction. The critical value of the reverse voltage is known as the turnover voltage. Beyond this voltage the junction is said to be in the breakdown region.

9. Static and Dynamic Resistance of a Semiconductor Diode:

The static resistance can be defined as the resistance offered by the diode when a steady direct current passes through it. Thus the static resistance R of the diode is the ratio of the applied voltage V to the steady current I.

Mathematically,

K = V/I (1.3) …(1.3)

If the forward current flowing through a diode changes about its some average value, then the dynamic resistance rƒ can be defined as-

The values of δV and δI are obtained from the curve as shown in Fig. 1.29. Sometimes, rƒ is also called as the incremental resistance.

10. Breakdown Mechanisms of Semiconductor Diodes:

We know that the reverse current of a P-N junction, under reverse bias condition, is due to the movement of electrons from the P-region and holes from the N-region of the semiconductor across the depletion region. With the increase of the reverse voltage a minority electron passing through the depletion region gains high kinetic energy from the applied voltage.

This electron when collides with a crystal atom, an electron in a covalent bond may acquire high energy to be free from the bond. In this manner the covalent bond is broken and a pair of electron and hole is created. Thus by the collision of one electron with a crystal atom, a pair of an electron and a hole is generated. Each of the carriers, by the same process, generates a pair of an electron and a hole.

In this manner by the process of collision and subsequent break of covalent bonds the number of free electrons and holes increases. This cumulative phenomenon is known as the avalanche multiplication. As a result of the avalanche multiplication a large reverse current passes through the junction. The diode is then said to be in the avalanche breakdown region. With an increase of the junction temperature the magnitude of the avalanche breakdown voltage increases.

Let the P-region of a P-N junction diode has very large concentration of acceptor impurity atom and N-region of donor impurity atom. With an increase of its reverse voltage the reverse current increases rapidly when the junction breaks down. Such a breakdown at low reverse voltage is not simply due to the avalanche multiplication of electron-hole pairs but by an electric field of high magnitude which exerts a large force on valence electrons (of silicon atoms) in the depletion region.

The process of removal of electrons from the valence band to the conduction band with the help of a strong electric field in an insulator was first proposed by C. M. Zener (1934) and hence it is called zener breakdown or zener effect. It has been found that the zener breakdown of an insulator occurs for an electric field of about 2 x 107 V/m.

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