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Semiconductors formed with differently doped p-type and n-type material on a single crystal forms the basic electronic devices like diodes, transistors, etc. These have their own significance in the development of modern electronic systems. Basics of electronics are dependent on the characteristics of such components. The characteristics of voltage and current help to analyze the equipment better. This article discusses what is a dc load line and its significance.

What is a DC Load Line?

A circuit supplied with dc power as the external source of the circuit. There exist both alternating and direct currents in the circuit. The reactive components of the circuits are made zero and the straight line is drawn above the voltage-current characteristics curves. Hence these results in the formation of intersecting point referred to an operating point. The straight that is drawn for this purpose is defined as the DC load line.

Q Point in a DC Load Line

For any non-linear circuitry, if the analyses of its characteristics curve were done it is based on voltage and current parameters. During this graphical analysis as the characteristic curve is plotted above that straight line is drawn. This straight line is termed as dc load line because at this point reactive components are zero.

This is the constraint applied to the non-linear element by the external circuit. Hence the characteristic curve and the straight line are dawn above each other so that they can intersect each other at a point. The intersection point formed in between is known as Q point. It is also referred to as the correct DC operating point.

Significance

• The linear analysis of the circuit is obtained for the non-linear devices like diode or transistors by using this concept.
• The main intention behind the analysis of the load line is to find the operating point referred to as the quiescent point (Q-point).
• The Q-points formed by the dc load line are the centers at which the voltage and current parameters equivalent to each other for both the parts of the circuit.
• For transistor like BJT, in this case, the operating regions formed here because of the load line analysis method makes the device to remain in an active region.
• The signals that should be amplified are lesser than millivolts. The active region concept define by load analysis method is important here so that the amplification process is carried out here.
• From this, the operating point obtained is essential while drawing the ac load lines.
• If the resistance is constant and the dc voltage applied to the circuit varies. The load lone concept is significant for analyzing the circuit in an efficient manner.

DC Load Line of a Diode and Its Equation

DC load line for a non-linear device is drawn by making the reactive components as zero. Hence a diode is considered as a non-linear device and its voltage and current characteristics are exponentially related to each other. The formation of the intersection point for the characteristic curve and the straight line or dc-load line can be analyzed better by considering the example for the diode as in forward bias condition.

Let us consider a diode connected to the resistor(R), source of voltage (V_DD) in series. The diode is connected in forward bias so that the forward current and the forward voltages flowing through the circuit. As per the Kirchhoff’s current law, the current flowing through the diode (I_D) and the resistor (I_R) is equal.

I_D = I_R

Analysis of the circuit is done by applying Kirchhoff’s voltage law (KVL). This law results in the formation of the final equation for the dc load line. Here the dc voltage is the biasing voltage of the circuit by keeping any further reactive components as zero.

diode-operating-in-forward-bias-for-the-analysis-of-dc-load

Once the Kirchhoff’s voltage law is applied to the circuit an equation is obtained for voltages and currents in the circuit.

V_DD = V_D + I_DR

V_D = I_DR-V_DD

Where V_DD, is the applied dc source voltage and V_D is the voltage across the diode. Hence the above can be considered as the equation for the diode. The voltage and current characteristics of the diode in forward bias condition can be drawn. By our previous analysis on the condition of the diode in forward bias applied a voltage and the generated current in the circuit are exponentially related to each other.

After a certain cut-off voltage, the diode starts operating in forward bias condition. To this slope, the technique is considered and a straight line on the v-i characteristics is drawn. The slope here for the above general circuit for the diode is V_DD/R. Then the intersection of the lines results in the formation of diode operating point as already discussed.

dc-load-line-and-the-formation-of-operating-point

In this way, the analysis for the dc-load line is done for the non-linear device like a diode. Depending on the type of non-linear device some part of the analysis differs but the technique remains the same. This type of method comes under the graphical analysis because here the characteristics curve is considered for the formation of the dc-load line.

Once the characteristics curve is formed the slope is identified then with respect to that line is drawn. Everything is graphical here. In this way, the analysis for this is done in the above article. The main purpose of the analysis of the dc load line is that operating point is formed that is accurate in all terms and helpful for the further processing of the circuit. But for the overall analysis of the circuit Kirchhoff’s laws play a major role.

This is for the diode but transistor is also a non-linear device then based on its configurations the operating point is formed. Then what must be the reason so that to operate BJT the operating point mostly should remain in the active region and why the active region is preferred above the remaining other regions?

A p-n junction formed by the doping f p-type and n-type material on a single crystal. The basic device formed from this p-n junction is of two terminals. This is referred to as the p-n junction diode. It is a basic semiconductor device that allows the flow of current in a unified direction. Either germanium or silicon is used as the semiconductor for the purpose of the formation of the diode. Once the voltage is provided to the p-n junction it is termed as biasing of the diode. It can be operated in forward bias as well as in reverse bias. As the voltage is applied the characteristics of the diode are non-linear and it behaves in an exponential way with respect to the applied voltage and generated current in the circuit. The above are some of the basics of the PN junction diode and this article covers about the VI characteristics of the P-N Junction Diode.

VI Characteristics of the P-N Junction Diode

There is the presence of carriers at the junction. The potential developed across the junction due to the influence of the carriers is termed as diffusion voltage. It can be represented as

diffusion-voltage

Because of the potential across the junction, a barrier is formed so that further movement of the carriers can be noticed. This movement in the depletion region is dependent on the applied voltage. This reflects the width of the depletion region. Poisson’s equation determines the relation of the width of the depletion region and the bias voltage.

poisson’s-equation

Here V represents the biasing voltage and ε represents the permittivity of the given semiconductor.

Once the voltage applied there is some amount of current generated in the circuit. It is represented as

generated-current

Here in the above equation, I_s represents the saturation current, where V is the biasing voltage and I the total current generated. Whereas k is the Boltzmann constant e is the charge on the electron and T is the temperature in the Kelvin scale.

Here the biasing voltage, width of the depletion region based on biasing and the current generated in the circuit are related to the general characteristics of the pn junction diode.

Zero Biased Condition in P-N Junction Diode

Zero bias is the condition of the diode without any external supply. Under the unbiased condition, the diode consists of both p-type and n-type semiconductor material and the process of diffusion takes place. This recombination leads to the formation of the electric field. As the electric field is formed it tends to oppose the further movement in the charge carriers. The region formed in between is termed as the depletion region. This leads to the formation of thermal equilibrium state in p-n junction diode.

P-N Junction Diode in Forward Bias

The diode is said to be in forward bias when the p-type is connected to the positive terminal and the n-type is connected to the negative supply of the supply. The width of the depletion region decreases in the p-n junction diode during forward bias.

The VI Characteristics of the PN Junction Diode in Forward Bias

In forward bias condition, the diode gets enough voltage so that it can exceed the value of threshold voltage and provides the carriers with sufficient energy so that it can overcome barrier potential. Hence the forward current is generated in the circuit. The diode tends to conduct once the threshold is crossed.

P-N Junction Diode in Reverse Bias

Once the p-type is connected to the negative terminal of the battery and the n-type is connected to the positive side of the supply the diode is said to be in reverse bias. The width of the depletion region is more in reverse bias of the p-n junction diode.

The VI Characteristics of the PN Junction Diode in Reverse Bias

Due to minority carriers, there is some generation of the leakage current in the circuit. After some excess reverse voltage, the diode tends to conduct for some time and undergoes breakdown condition. This type of current generated is referred to as Reverse Saturation Current. As the reverse voltage brings sudden changes in the current is termed as Reverse Breakdown Voltage.

V-I Characteristics of P-N Junction Diode

The above are the basics of the p-n junction diode. The voltage and current characteristics are well suited for the explanation of the functioning of the circuit. The circuit can be analyzed better and analysis becomes simpler. The functioning is evident but have you ever given a thought what are the devices that only functions either in forward bias or in reverse bias?

The fundamental unit that evolved because of the same crystal added with acceptor and donor impurities on it. This is highly beneficial because of its features paved the way for the development of modern electronics. This device consists of two terminals but the flow of current will be in a unified direction. The characteristic of this device is tested between the applied voltage and the way current generated in the circuit reacts to it. It can’t be linear but one can determine the relationship between the applied voltage and the circuit current is exponential. This article discusses about P-N Junction Diode theory its Biasing, Forward Bias & Reverse Bias.

Define P-N Junction Diode

A piece of semiconductor with two terminals is referred to as the word Diode. One side of the terminal is added with acceptor impurity and the other is doped accordingly with donor impurity. It is clearly evident that one side is p-type and the other end will be n-type. Hence this type of semiconductor is known as p-n junction diode. The main purpose o this type of device is to facilitate the flow of current in a single direction.

Symbol Representing P-N Junction Diode

The symbol has been designed in such a way that it has an arrowhead pointing in the direction showcasing the flow of direction of the current. A diode consists of two terminals one side of the terminal is known as anode and another side is for the representation of the cathode.

P-N Junction Diode Symbol

P-N Junction Diode Theory

A diode has two regions that are p-type and n-type where it is operated. But on the condition of the applied voltage, the biasing conditions are classified into three types.

Zero Bias

The condition of the p-n junction diode where the device doesn’t have any external supply is known as Zero bias or Unbiased.

Forward Bias

When the positive side of the supply is connected to the p-type and n-type is connected to the negative side of the supply. This bias condition is known as forward bias.

Reverse Bias

When the negative side of the supply is connected to the p-type and the n-type is connected to the positive side of the supply. This type of bias is known as reverse bias.

Working of P-N Junction Diode-type

P-N junction diode working is completely based on the type of bias as well as the p and n-types interact with each other. In p-type the majority carriers are holes and the minority concentration is of electrons. Whereas in n-type the majority of charge carriers will be electrons and the minority of the carriers will be holes.

Basing on the type of bias the diode working can be explained as follows:

P-N Junction Diode under Zero Bias Condition

Zero bias is the condition of thermal equilibrium where the number of charge carriers on both the ends is balanced. As there is no external power supply to the diode the electrons diffuse towards p-side and the diffusion of holes is towards n-side resulting in the generation of the electric field because of these charge carriers diffusion. This generated electric field generated by both the carriers tends to oppose each other. Further, there is no movement of the charge carriers in the depletion region. In this way, the diode is worked under zero bias in order to attain the state of thermal equilibrium.

P-N Junction Diode Working in Forward Bias

This is the condition in which the p-n junction diode is provided with positive supply because the p-type is connected to the positive terminal of the supply. As there is some majority of charge carriers concentration on both the sides tends to attract at the junction resulting in a decrease of the width of the depletion region. This condition results in the establishment of a low impedance path where the high amount of current flows in the circuit.

P-N Junction Diode Working in Forward Bias

P-N Junction Diode Working in Reverse Bias

When the p-type is connected to the negative terminal and the n-type is connected to the positive terminal of the supply leads to the condition of reverse bias. As the condition is established the holes in p-type gets distracted from the junction because the p-type is connected to the negative terminal. The same condition is inferred to the electrons in n-type as it is connected to the positive side of the battery and the charged ions left behind during this process. This condition tends to increase the width of the depletion region. Hence the diode becomes completely resistive to the flow of current. In this way, the diode works during reverse bias.

P-N Junction Diode Working in Reverse Bias

How Voltage and Current are Affected in Various Biasing Conditions?

When the curve is plotted between the voltage across the junction and that influences the flow of charged particles that results in the generation of current in the circuit. This voltage and current curve give the characteristics of the p-n junction formed. The respective voltage and current values get influenced by biasing conditions so based on biasing its characteristics varies

Under Zero Bias

In this type of bias, there is no external supply as there is the presence of barrier potential in this condition no carriers are in the movement because the condition of equilibrium is attained. Then the current flowing through the circuit is zero. Because the external voltage applied is zero.

V-I under forward bias

Once the p-n junction is in forward bias it means that forward voltages are applied. Due to this forward biasing the depletion region present at the junction its width tends to decrease. As the width decreases the impedance becomes low and the path established allows high currents to flow.

The point from which there is a sudden increment in the flow of current is termed as knee point. In forward bias, the flow of current through the junction and its magnitude is completely dependent on the supplied voltage.

The approximated current equation when the junction is in forwarding bias is

Current Equation

Here Is represents the saturation current; VT is the volt-equivalent temperature, and n is the emission coefficient.

V-I under Reverse Bias

Generally, in the reverse bias, the p-n junction will be offering the highest resistance to the flow of current. Still, some amount of leakage current flows through the junction. When the reverse voltage applied is of maximum value then the junction gets overheated due to which the breakdown condition occurs and a maximum flow of current is recognized in the circuit.

There is a very tiny amount of current is flowing through the junction as it is in reverse bias. Hence the equation for it is given as

Equation for Tiny Amount of Current

Whereas Is represents the saturation current; VT is the volt-equivalent temperature, and n is the emission coefficient.

Experimental Analysis of P-N Junction Diode

The analysis of the p-n junction diode can be done easily by considering it as a switch. This application of the diode is considered for the experimental setup because of its both biasing conditions execution.

Experimental Analysis of P-N Junction Diode

The biasing conditions of p-n junction’s diode clearly state that during forward bias the diode is in conducting mode acting like a closed circuit. Whereas during reverse bias the diode is in non-conducting or acting as an open circuit. This paved the way the diode to act as a switch. The switch is on during forward bias that is conducting and during reverse bias the switch will be off indicating that it is in non-conducting mode.

Energy Band Diagram of P-N Junction Diode

Energy bands are affected based on the biasing techniques applied. The Fermi levels vary when the diode is unbiased as well as it is forward biased or reverse biased.

Energy Band of Unbiased Diode

When the diode is unbiased the state of the junction will be at equilibrium. It indicates that there is no movement in the charge carriers because both p-type and n-type are at the neutral condition.

Energy Band of Unbiased Diode

Energy Band of Forward Biased Diode

During the forward bias condition, the p-type is connected to the positive side so that it is convenient for the movement of electrons to flow across the junction in order to combine with the vacancy present there near to the junction.

Energy Band of Reverse Biased Diode

Energy Band of Reverse Biased Diode

As the diode is in reverse bias it reflects the condition in energy band where the p-side is supplied with negative voltages that reduces the conduction.

Parameters of P-N Junction Diode

The parameters in the p-n junction diode are as follows:

Reverse Saturation Current – when the p-n junction diode is in reverse bias few amounts of current tends to flow through the diode this is because of the influence of minority charge carriers in it. This is defined as the reverse saturation current.

Barrier Voltage – The voltage that is responsible for the opposing of the flow of majority carriers at the p-n junction from both sides that is p-type as well as n-type is known as barrier voltage.

Reverse Breakdown Voltage – The basic minimum voltage applied to the diode when it is in reverse bias so that the behavior of the diode will change from insulator to conductor for some period of time is defined as the reverse breakdown voltage. The above are some of the parameters of the p-n junction diode.

Applications of P-N Junction Diode

• The p-n junction diode working under forward bias is very useful for the light emitting diode application.
• The p-n junction diode during the reverse bias is highly sensitive to the light so that it can be used in photodiode application.
• P-N junction during forward bias offers low impedance path whereas during reverse bias acts as an insulator. This process is known as Rectification. Hence p-n junction diode can be used as a Rectifier.
• For the purpose of DC restoration, the diodes are preferably used in clamping circuits.
• These diodes are also utilized in voltage regulation as well as voltage multipliers.
• The above mentioned are some of the uses of the p-n junction diodes.

This is all about the p-n junction diode. As it is like a base for modern electronics this concept is utilized in various applications. It’s been used for LED application as well. But for led’s have you ever thought it is operated in which bias?

Both p and n are the basic semiconductors arrived from the concept of doping. Hence the basic difference between p and n-type is the way or the nature of the doping involved. Semiconductors can be differentiated as intrinsic and extrinsic as per the matter of purity concerned.  P-type and N-type semiconductors come under extrinsic semiconductors. Although they are categorized under the same category of extrinsic there behaviors are extremely different based on its dopants. However, they have Fermi levels lying towards conduction or valence band based on its type.

Comparison Between P-Type and N-Type Semiconductors

The difference between p-type & n-type semiconductors is discussed below.

P-Type Semiconductor	N-Type Semiconductor
(1)P-type is formed because it is doped with a trivalent impurity.	(1)N-type is formed because it is doped with a pentavalent impurity.
(2) As the impurity inserted is of third group elements it is responsible for creating a vacancy of the electrons termed as holes.	(2) In N-type, the impurity inserted is of a fifth group providing an excess of electrons.
(3) Holes formed here are termed as Acceptors.	(3) Electrons in the n-type are termed as Donors.
(4) The examples of trivalent impurity added are Al, Ga, In, etc.	(4) The examples of pentavalent impurity added are  P, As, Sb, etc.
(5) The concentration of holes is more in p-type.	(5) The concentration of electrons is more in n-type.
(6)The major concentration of the carrier’s movement can be observed from a higher level of concentration to a lower level of concentration.	(6) The majority of the carrier’s movement can be seen from lower level to higher level.
(7) As the holes concentration is high p-type carries the positive charge.	(7) In n-type, the electrons are the majority carriers. Hence the n-type preferably carries a negative charge.
(8)  The energy level of acceptor lies nearer too valence band but away from the conduction band.	(8) The energy level for donor lies nearer to the conduction band but away from the valence band.
(9)  Fermi level presence can be observed between the energy level of acceptor and the valence band.	(9) In this case, the presence of the Fermi level is in between the energy level of donor and the conduction band.
(10) Basic example for the formation of P-type semiconductor. Formation of P-Type Semiconductor	(10) Basic example for the formation of N-type semiconductor. Formation of N-Type Semiconductor
(11) Because of majority carrier influence, the density of holes is comparatively greater than the density of electrons.	(11) Whereas the majority of carriers in n-type are electrons. So here the density of electrons is comparatively greater than that of holes.

One can easily analyze that basic P-type is considered majority as holes and as its charge is positive it is abbreviated as P-type. Whereas the carrier concentration of electron is more in n-type it is abbreviated because of its negative charge in it. Hence the densities of particular charges are also dependent on its types of semiconductors.

key Difference between P-Type & N-Type Semiconductors

Both p and n are the basic semiconductors arrived from the concept of doping. Hence the basic difference between p and n-type is the way or the nature of the doping involved. Semiconductors can be differentiated as intrinsic and extrinsic as per the matter of purity concerned.

P-type & N-type semiconductors both come under extrinsic semiconductors. Although they are categorized under the same category of extrinsic there behaviors are extremely different based on its dopants. However, they have Fermi levels lying towards conduction or valence band based on its type.

In this article, the basic differences involved in P-type and N-type semiconductors are classified is described. The factors such as Fermi levels, energy levels based on the types they move towards either conduction band or valence band is clearly observed. As the p-type and n-type is distinguished based on its addition of a type of impurity the existence of majority and minority carriers and the polarities are classified. As per the concentration concerned its density is described.

Drift current is the concept involved in a doped semiconductor, there are free charges available, once the voltage is applied we can notice the movement of the charge carriers based on the polarity of the charges it gets attracted towards the respective terminals. Hence the electric field is applied due to which the motion of charge carriers observed results in the production of current. The velocity required for the movement of charge carriers is referred to as drift velocity. In order to describe this type of current direction, it is dependent on the collision occurring with the atoms. Generally, the flow of electrons will be in the Brownian motion. But here the electric field is responsible for them to arrange in a single direction.

What is Drift Current?

The current generated because of the application of external voltage that results in the movement of charge carriers is defined as Drift current.

The main reason behind the occurrence of this current is because of the application of external forces it can be either electric field or voltages.

Density in Semiconductors

As the external supply is provided there is the movement in the majority of the concentration of the carriers. Based on the type of semiconductors its majority of the carriers vary.

However, both holes and electrons in the majority or in minority are present in any type of extrinsic semiconductor.

Movement of Charge Carriers Based on the Supply Provided

Holes are the positive carriers and electrons being negatively charged, holes always prefer the direction with respect to electric field whereas the direction of the flow of electrons can be evident opposite to the path of an electric field.

This net motion in the concentration of the particles that are either holes or electrons can be referred to as the drift current of that particular semiconductor.

In this way, this current is calculated. It is measured in terms of Amperes. The velocity achieved by the charge carriers due to the application of external voltage is referred to as Drift velocity.

The drift velocity for the carrier concentration of electrons is

Vn = µ_nE

Where Vn is the drift velocity representing the carrier concentration of electrons.

µ_n is the mobility of the particular electrons and

E represents the electric field applied to the n-type of the semiconductor.

Drift velocity of the holes concentration is

Vp= µ_pE

Here Vp is the drift velocity referred to the concentration of holes

µ_p refers to the mobility of the concentration of holes and E representing the electric field acting upon it.

General Relation between Current and Drift Velocity

Suppose that there are n number of electrons per cubic centimeter as well as the drift velocity (V_d). In a certain moment of time, the electron tends to move this is referred to as the distance V_d∆t. Its volume is

AV_d∆t

Then the motion of a number of free electrons in that particular region is

AV_d∆t

Similarly, the charge crossed across the area is given by

∆q = nqAV_d∆t

Therefore

I= ΔQ/ Δt = nqaV_d

Hence the above equation shows the relationship between current and the drift velocity.

Derivation for Density

The current caused by the movement of charge carriers either by free electrons or holes per square unit area is referred to as the density of the drift current or Drift current Density.

The density is variable based on its type of charge carriers. The drift current is preferably measured in terms of Ampere per square centimeter.

The drift current density with respect to the concentration of electrons is

Jn = q_nµ_nE 

The drift current density based on the concentration of holes is

Jp = qpµ_pE 

Here n represents the number of electrons present per cubic centimeter
p represents the number of holes per cubic centimeter
q is related to the charge carried by the concentration of carriers

Therefore

q = 1.6X10^-19 coulomb
µ_n is the mobility of electrons its units are cm²/ Vs
µ_p is the mobility of holes and its units are cm²/ Vs
E is the electric field applied on it and it is measured in terms of V/ cm

Then the overall density is calculated by the sum of the individual current densities of electrons and holes. Therefore

J = Jn + Jp

J = qnµ_nE + qpµ_pE

J = q (nµ_nE+pµ_pE)

This the final equation obtained for the overall current density. As per the analysis made can the presence is detected in the unbiased condition of the diode with p-n junction?