What is Semiconductor Diode.

 1.      What is Semiconductor Diode?

               A diode is an electronic device made up of, when some doping is done in semiconductor like Silicon and Germanium. Diode has a property that is pass the current in one direction only (work as short circuits) and do not pass the current on other side (work like the open circuits).

Content:-

1.1 Semiconductor Diode.

1.2 Biasing and working of Semiconductor Diode.

1.3 Terminology used in Semiconductor Diode.

1.4 Voltage- Current Characteristics of Semiconductor Diode.

1.1   Semiconductor Diode:

When a single crystal of semiconductor is doped with acceptors on one side and donors on the other side a p-n junction is formed. A schematic diagram of  a p-n junction is shown in fig. 1.1. In this figure donor ions are denoted by plus signs and the electrons they donate are represented by the small filled circles. The holes are shown by unfilled small circles and acceptor ions by the minus signs.

 

Fig. (1.1)

              

               A p-n junction cannot be formed by simply joining the two pieces together because it would produce a discontinuous crystal structure. Special fabrication techniques are needed to form a p-n junction. An important property of semiconductor diode is that it allows the conduction current in only one direction. For this it is used as rectifier. A schematic symbol of crystal diode is depicted in fig 1.2. The arrow in the symbol represents the direction of easier conventional current flow.

 

Fig (1.2)

               A semiconductor diode contains two terminals. When it is connected in circuit, one thing to decide is whether the diode is forward or reverse biased. The p-side of the diode is always the positive terminal for forward bias and designated the anode. The n-side is called cathode and is the negative terminal when the device is forward biased. The holes and electrons are the mobile charges, while the ions are immobile charges.

1.2   Biasing and Working of Semiconductor Diode

1.2.1 Forward Bias:- When an external voltage is applied with the polarity i.e.  + V to the positive terminal and – V to the negative terminal of the diode, this condition is known as Forward Bias. An ideal p-n diode has a zero ohm voltage drop across the body of the crystal. For such a diode, the height of the potential barrier at the junction is lowered by the applied forward voltage V. The equilibrium initially establishes between the forces tending to produce diffusion of majority carriers and the restraining influence of the potential energy barrier at the junction will be disturbed. Therefore, the holes cross the junction from the p-type to the n-type and the electrons cross the junction in the  opposite direction. These majority carriers can then travel around the closed circuit and a relatively larger current will flow.

                                      The semiconductor diode works or conducts current in the forward bias only. The mechanism of current flow in a forward biased p-n junction can be described in the following way:-

(i)                The free electrons from the negative terminal continue to pour the n-region while the free electrons in the n-region move towards the junction.

(ii)              The electron travels through the n-region as free electrons i.e., current in n-region by free electrons.

(iii)             When these electrons reach the junction, they combine with holes and become valence electrons.

(iv)             The electrons travel through p-region as valence electrons i.e., current in the p-region is by holes.

(v)              When these valence electrons reach the left end of crystal, they flow into the positive terminal of the battery.

From this we conclude that in n-type region, current is carried by free electrons whereas in p-type region, it is carried by holes. However, in the external connecting wires the current is carried by electrons.

1.2.2 Reverse Bias:- When an external voltage is applied with the reverse polarity i.e  + V to the negative terminal and – V to the positive terminal of the diode, this condition is known as Reverse Bias or blocking bias. Due to this type of connection, holes in the p-type and electrons in the n-type move away from the junction. As a consequence, the region of negative charge density spread to the left of the junction and positive charge density region is spread to the right. However, this process cannot go on indefinitely because in order to have a steady flow of holes to the left, these holes must be supplied across the junction from the n-type germanium and there are very few holes in the n-type side. Therefore, nominally, zero current results.

In fact, a small current does flow because a small number of hole-electron pairs are produced throughout the crystal due to thermal energy. The holes formed in the n-type germanium will wander over to the junction. A similar remark applies to the electrons thermally produced in the p-type germanium. This small current is known as the diode reverse saturation current. This reverse current will increase with the increasing temperature.  Therefore, back resistance of the crystal diode decreases on increasing the temperature.

This mechanism of conduction in the reverse direction can be understood in another way. Let us say, when no voltage is given to the p-n diode, the potential barrier across this junction exists. If a voltage V is given to the diode in the reverse direction, then height of the potential-energy barrier is increased by the amount eV. Due to this increase in barrier height , the flow of majority carriers is reduced. But the minority carriers are unaffected by the increased height of the barrier.

1.3 Terminology used in Semiconductor Diode.

1.3.1 Depletion Layer:-  

The depletion layer is the layer where to the two semiconductor meet together and form the parallel plates of opposite charges, this is formed due to the uncovered charges with the depletion region. Therefore it is called depletion layer. The depletion layer behaves like an insulator. The depletion layer possesses capacitance because of the presence of fixed charge plates. The width of the depletion layer depends upon the doping level of the immunity in n-type and p-type semiconductors. The higher the doping level, the thinner will be the depletion layer.

1.3.2 Barrier Potential:-

 The depletion layer of a p-n junction contains fixed plates of oppositely charged ions on its two sides. Because of this charge separation, an electric potential is established across the junction, even when the junction is not connected to any external voltage source. This electric potential is known as barrier potential.

               The potential barrier exerts a repelling force on the mobile charge carriers, trying to crossover the junction. This force stops the mobile charge carriers to crossover the junction, unless the energy is supplied from an external source.

1.3.3 Transition Capacitance:-

The transition capacitance represents the change in charge stored in the depletion region with respect to a change in junction voltage. The increase in the level of reverse bias caused the width of the depletion region, W to increase. An increase in the width of the depletion region, W is accompanied by additional uncovered ions in the space charge or transition region. Because positive ions exist on one side of the junction and negative ions on the other, the transition capacitance, C_T is analogous to a

 C_T   =      A_ (F)

                W 

Where,

A= Junction Area.

€= Permittivity of the semiconductor.

C_T= Transition capacitance.

W= Width of the depletion layer which is inversely proportional to the square root of the reverse bias voltage.

 

We must note that W is a function of the reverse biased voltage so that transition capacitance, C_T is voltage dependent. For a step-graded junction the width of the depletion region, W is inversely proportional to the square root of the reverse bias voltage.  Under forward biased conditions, the value of transition capacitance is so small compared to diffusion capacitance that it is generally neglected. Similarly in a reverse biased diode, a small amount of carrier diffusion exists, but this capacitance is negligible when compared to transition capacitance.

 

1.3.4 Peak Inverse Voltage (PIV):-

During the negative half cycle of the input, the diode is reverse biased. The whole of the input voltage appears across the diode. When the input reaches its peak value, Vm in the negative half cycle, the voltage across the diode is also maximum. This maximum voltage is called the peak inverse voltage. It represents that the maximum voltage of the diode must withstand during the negative half cycle of  the input.

When diode is used as a rectifier, the peak inverse voltage is extremely important. In rectifier service, it has to be ensured that reverse voltage across the diode does not exceed its peak inverse voltage during the negative half cycle of input A.C. voltage.

 

1.3.5 Breakdown Voltage:-

               It is the minimum reverse voltage at which p-n junction breaks down with sudden rise in reverse current. Even at room temperature, some hole-electron pairs (minority carriers are produced in the depletion layer. With reverse bias, the electron move towards the positive terminal of supply. At large reverse voltage, these electrons acquire high enough velocities to dislodge valence electrons from semiconductor atoms. The newly liberated electrons in turn free other valence electrons. In this way, we get an avalanche of free electrons. Therefore, the p-n junction conducts a very large reverse current.

 1.3.6 Knee Voltage:-

               It is the forward voltage at which the current through the junction starts to increase rapidly. When a diode is forward biased, it conducts current very slowly until we overcome the potential barrier. For silicon p-n junction, the potential barrier is 0.7 V whereas it is 0.3 V for germanium junction.

 

Diagram1.3.6

 

1.4 Voltage- Current Characteristics of Semiconductor Diode:-

               For a p-n junction, the current I is related to the applied voltage V by the equation as follows:-

I = I_o( ^V/ V_T – 1)                 -------------- (i)

Where,

I_o = Reverse saturation current

V_T = Volt equivalent of temperature.

A positive value of current I mean that current flows from the p-side to n-side. The diode is forward biased if voltage V is positive and indicating that the p-side of the junction is positive with respect to the n-side. The symbole  is 1 for germanium and is approximatiely 2 for silicon at the rated current. The volt equivalent of temperature is obtained by the equation as

V_T = T / 11600                  ------------------------- (ii)

Where T is temperature in degree Kelvin, i.e. (^oK). At room temperature, T = 27 ^oC = (27+273) ^oK= 300 ^oK, V_T = 300/ 11600 = 0.026 V = 26 mV.

The volt-ampere characteristic described by equation (i) is shown in fig. When voltage V is positive and several times V_T, the unity within the bracket in equation (i) may be neglected. Hence the current I increase exponentially with voltage V except for very small values of voltage V. When the diode is reverse biased and IVI is several times V_T, then the current approximately equals to the negative value of reverse saturation current i.e. I  - I_o. The reverse current is therefore constant, independent of the applying reverse bias. As a consequence I_o is referred to as the reverse saturation current.

Diagram 1.4

The voltage at which the current starts to increase rapidly is called the cut-in voltage or knee voltage (V_o) of the diode, whose value is 0.7V for silicon and 0.3V for germanium. In the reverse bias, the diode current is very small, only few µA for germanium diodes and only a few nA for silicon diodes. It remains small and almost constant for all voltage less than the break down voltage V_Z. At breakdown the current increase rapidly for small increase in voltage.