at the negative terminal of battery. A A diode has two state either on [forward conduction .7 volts. semiconductor, n-type with positive terminal of voltage source V connected to the P-side and the negative terminal to the N-side. it has a p-type side and a n-type side, and their connection matters. A junction diode acts as a closed switch when it is forward … biased diode, Reverse
Yes, it will work. Change its value to 1.4V and simulate. Assume that each diode shown in the figure has a forward bias resistance of 50 Ω and an infinite reverse bias resistance. Reverse Bias. How do contact potential and the transition region width across a P-N junction vary with forward biased? Forward bias is the condition that allows current through the pn junction. However, the voltage present across a diode during reverse biasing does not produce any significant flow of current. While in reverse bias the potential difference increases the strength of the barrier which prevents the charge carrier to move across the junction. A diode can be connected to the source in several ways. This external bias voltage is designed as V BIAS. There is a formation of electron-hole pair. diode and forward biased diode, Under In a reverse biased condition, the diode will not conduct and current will not flow. In a forward bias setup, the P-side of the diode is attached to the positive terminal and N-side is fixed to the negative side of the battery. number of free electrons and holes further reduces the
The first area is when there are low levels of diode voltage (V D) and the associated current is very small. Properties of P-N Junction in Forward Bias, Forward Current Equation of PN Junction Diode. a) Forward Bias of PN Junction Diode: The Cut in Voltage or Knee Voltage (Vγ) of 1N4007 is _____Volts. In a standard diode, forward biasing occurs when the voltage across a diode permits the natural flow of current, whereas reverse biasing denotes a voltage across the diode in the opposite direction. V F =0. Take the readings and note Voltmeter reading across Germanium diode and Ammeter reading. Under this condition. holes or vacancies in the p-type semiconductor and fills
region breakdown, Diode terminal. semiconductor) to a lower concentration region (n-type
If the external forward
The resistor limits the forward current to a value that will not damage the diode. region slightly decreases. Controlled Rectifier, Electronics which cross the depletion region finds the large number of
negative terminal of the battery supplies large number of
voltage applied on the p-n junction diode is increased from
then enters into the p-semiconductor. from one point to another point in the p-n junction diode. A diode forward voltage refers to the voltage drop that occurs when an electrical current passes through a diode in an electrical circuit.It is generally independent of the amount of current passing through the diode, and will be the same for a given family of diodes. number of holes finishes their journey at the negative
Hence, they not used for any practical
Pnoà Thermal equilibrium minority carrier hole concentration in the N-side. In This bias condition is known as forward bias. When the electrons are moving in the correct direction to pass through the diode, the diode is said to be forward biased. of battery or terminates at the positive terminal of
Forward Bias Last updated; Save as PDF Page ID 5969; References; When an external voltage holds the p-type material at a higher potential than the n-type material, we say that the p-n junction diode is under forward bias. number of holes finishes their journey at the negative
Conversely, when the battery is backward and the diode blocks current, the diode is said to be reverse-biased. at the negative terminal of battery. The process by
the depletion region carry the electric current from one
When the diode is arranged in this manner, there is a reduction in the potential barrier. The condition of the p-n junction diode where the device doesn’t have any external supply is known as Zero bias or Unbiased. Thus, the negative charge carriers (free electrons) that are
A representation of this condition is shown below. If we want to use a 1. A dc voltage source connected by a conductive material (contacts and wire) across a diode in the direction to produce forward bias. In p-type semiconductors, the valence
The ideal diode has zero resistance for the forward bias polarity, and infinite resistance (conducts zero current) for the reverse voltage polarity; if connected in an alternating current circuit, the semiconductor diode acts as an electrical rectifier.. In a standard diode, forward biasing occurs when the voltage across a diode permits the natural flow of current, whereas reverse biasing denotes a voltage across the diode in the opposite direction. positive terminal. Np0à Thermal equilibrium minority carrier e– concentration on p-side. depletion region (positive ions) near the p-n junction. Understanding a forward-biased PN junction not only helps in understanding how a diode works but practically any semiconductor devices - transistors, thyristor, TRIAC, photocouplers, etc. diodes, The process by
free electrons, which fills the holes in positive ions
The Find static and dynamic resistances in both forward and reverse biased conditions. holes or vacancies will becomes valence electrons and then
The negative terminal of the battery provides a consistent flow of electron. p-type semiconductor, they get repelled from each other and
Forward Bias. zero to 0.1 volts, the, Electron and
Forward bias or biasing is where the external voltage is delivered across the P-N junction diode. The injected minority carrier concentration will be maximum at the edge of the depletion layer on the opposite side and then they diffuse into the region. This voltage is called cut-in voltage. their journey at the negative terminal whereas the large
What is the main requirement to forward bias diode? voltage applied on the p-n junction diode is increased from
In order to forward bias a p-n junction , the positive terminal of the battery is connected to to the p-type and negative terminal of the battery is connected to the n-type as shown in the figure below. terminal. With this forward bias, the holes in the P-region and the electrons in the N-region move closer to the junction. attracted to the negative terminal of battery or terminate
A junction diode acts as a closed switch when it is forward … free electrons, which begin their journey from the negative
Electrons carry a current in the N region. Hence, very small electric
The Dynamic or AC Forward Resistance is represented as shown below: The value of the forward resistance of a crystal diode is very small, ranging from 1 to 25 Ohms.. of depletion region, P-N Due to this reason, a covalent bond will break and an electron generated from the covalent bond move towards the positive terminal. In forward bias Zener diode works same as that of the normal p-n junction diode. applied on the p-n junction diode is further
the p-n junction diode is forward biased with
Actually there are some diodes that breakdown in forward bias -- these are called Esaki diodes or tunnel diodes. the p-n junction diode increases with the increase in
The direction of arrow is the direction of conventional current flow (under forward bias). P-N junction during forward bias offers low impedance path whereas during reverse bias acts as an insulator. depletion region (negative ions) near the p-n
Remember, 1 Answer. Reverse Resistance (R R). The value of forward current (I F) across the diode can be found by using the value of limiting resistance and biased voltage using Ohm’s Law. is apposite to the direction of positive electric field of
The semiconductor diode is not ideal. remaining holes will cross the depletion region and
Diode is an unipolar device, i.e. Figure 5: Effect of varying IS parameter value for a Schottky diode . in p-type semiconductor. As the minority carrier of p and n regions cross into the opposite regions, they become injected minority carriers. The number of holes and electrons are combined with each other once the junction is crossed. When a diode is connected in forward biased, current flows through the diode and it conducts. Under this condition, the In the case of large forward bias, a good approximation of the ideal diode equation is to simply set the second term of Equation \ref{eq1} to zero. A p-n junction is said to be forward biased when the external d.c voltage applied in such a direction that it cancels … The free electrons,
Under this condition, the negative terminal of the battery supplies large number of free electrons to the n-type semiconductor and attracts or accepts large number of holes from the p-type semiconductor. If its the other way we call it reverse biased and will block current flow . Thus, the depletion region or negative ions (negative
crossing the depletion region carry the electric current
(from p-side to n-side). Forward Bias. Model Editor will open and you will find the property Vj (Forward Voltage). In relation to the forward voltage, once you have chosen and placed a Diode in the schematic, click on it and then with Rifht Mouse Button select Edit PSpice Model. If the external forward
depletion region (positive and negative ions). Figure 2 shows a PN diode with forward bias V i.e. the electric current. Reverse Resistance (R R). Forward bias is the condition that allows current through the pn junction. Give a reason? Np(0)à Injected e– concentration entering into the p-side or injected e– concentration crossing the edge of the depletion layer on p-side. remaining free electrons will cross the depletion region and
If the voltage moves in the opposite direction, we call that orientation a “reverse bias.” In reverse bias, current flow is nominally blocked as a sort of electronic check valve. A diode is a two-terminal semiconductor device. The potential difference is of two types namely – forward bias and the reverse bias. In the forward bias region there exist two important areas to distinguish in relation to the amount of current observed through the diode. Forward Bias Region . 5 V battery to forward bias the diode, the resistor used in series with the diode so that the maximum current does not exceed 5 m A is field. The forward-bias and the reverse-bias properties of the p–n junction imply that it can be used as a diode. then enters into the p-semiconductor. When the diode is forward-biased, the resistance of the diode is so small. Share It On Facebook Twitter Email. large number of free electrons from the n-type
begins their journey at the positive terminal whereas the
0 votes . voltage. electric field) at p-type semiconductor near the p-n
zero to 0.1 volts, the depletion electric current in the p-n junction diode increases. However, before crossing the depletion
Fig-1: A forward bias diode. of Kansas Dept. The If In case of a p-n junction there are two bias conditions. We'll discuss forward bias, its effect on the depletion region and the effect barrier potential has during forward bias. Under forward bias, the field because of the space charge region and forward voltage V, When P-N junction is forward bias, the barrier height reduces by |V. Pn(0)à Injected hole concentration entering into the N side of injected minority carrier concentration crossing the edge of depletion layer on N-side. atoms. negative ions, which lose the electrons, become neutral
You can see that as IS decreases, for the same Forward Current (I) we get greater Forward bias Voltage (V). This large
junction. b) Reverse Bias of PN Junction Diode: Forward Biased PN Diode. The Diodes are devices that pass electric current in one direction only, and therefore may be thought of as a kind of one-way valve for electrons. Such as : Forward biasing; Reverse biasing; Forward Biasing of p-n Junction Diode. 8/22/2005 The Junction Diode Forward Bias Equation.doc 2/6 Jim Stiles The Univ. electrons (from n-side to p-side). © 2013-2015, Physics and Radio-Electronics, All rights reserved, SAT One of those several methods of connecting the diode with the source is given by the Forward Bias definition. If its the other way we call it reverse biased and will block current flow . try to move from higher concentration region (p-type
Thus,
A diode connected in forward biased is one in which the positive voltage of a voltage source is connected to the anode of the diode and the negative or ground of the voltage source is connected to the anode of the diode. no voltage or unbiased condition, the p-n junction diode
However, the voltage present across a diode during reverse biasing does not produce any significant flow of current. holes are nothing but vacancies created when the electrons
from one point to another point in the p-n junction diode. To bias a diode, we apply a dc voltage across it. There is a minimum threshold voltage required to overcome the depletion region, which for most silicon diodes is a significant 0.7 volts. free electrons, which begin their journey from the negative
In a forward-biased p-n junction, the current up to the edge of the depletion layer is due to drift of majority carrier. If each diode in Fig. current flows in the p-n junction diode. When the diode voltage is less than the barrier potential, a small current flows through the diode. What kind of charges present on the opposite face of the junction? 8/22/2005 The Junction Diode Forward Bias Equation.doc 2/6 Jim Stiles The Univ. atoms. to a lower concentration region (p-type semiconductor). When the diode voltage exceeds the barrier potential, the current flowing through the diode increases rapidly. Thus, the negative charge carriers (free electrons) that are
these electrons get attracted towards the positive terminal
I F =V BIAS /R LIMIT. increased, then even more number of free A forward-biased diode conducts current and drops a small voltage across it, leaving most of the battery voltage dropped across the lamp. diodes, p-type With a battery connected this way, the holes in the p region and the free electrons in the n region are pushed toward the junction. the depletion region carry the electric current from one
battery. large number of free electrons finishes their journey at the
actual direction of current is the direction of free
of Kansas Dept. 4. These electrons enter in the n-type area of diode. A diode is a two-terminal semiconductor device. However, the conventional
junction capacitance, P-n to the large number of positive charge carriers (holes) at
positive ions, which gain the electrons, become neutral
When a P-N junction is forward biased the majority carrier of P and N region will be moving toward the junction and this will reduce the region of immobile charges and therefore the width of the depletion layer is reduced. A diode can be connected to the source in several ways. If the diode is flipped around so that the positive voltage is supplied to the cathode and negative voltage to the anode, the diode is said to be reverse biased. remaining holes will cross the depletion region and
Zener diode acts as a closed switch above the cut in voltage in forward bias. Above this voltage the slope of the current-voltage curve is not infinite (on-resistance is not zero). Therefore from the graph we see that the diode starts conducting when the forward bias voltage exceeds around 0.3 volts (for Ge diode). negative terminal of the battery supplies large number of
Reverse bias increases a diode's resistance, and forward bias decreases a diode's resistance. of EECS Now, say a voltage v 1 across some junction diode results in a current i 1.Likewise, different voltage v 2 across this same diode a diode of course results in a different current i 2. However, before crossing the depletion region, free
When a diode is connected in forward biased, current flows through the diode and it conducts. Hence, holes are considered as the
2 m A when forward bias voltage is changed by 0. In other words, the large number of free electrons begins
Forward Bias vs. crossing the depletion region carry the electric current
Applying forward bias to the PN junction diode causes a low impedance path for the junction diode, allows for conducting a large amount of current known as infinite current. approximately 0.7 volts for silicon diode or 0.3 volts for
A diode may be thought of as like a switch: closed when forward-biased and open when reverse-biased. move from higher concentration region (n-type semiconductor)
characteristics of diode, Depletion 5. 5 V which is assumed to be independent of current. Emitting Diode, P-N current. Over this reverse voltage diode will go for breakdown due to its minority carriers. germanium diode, the p-n junction diode starts allowing
Below Figure shows a dc voltage source connected by conductive material (contacts and wire) across a diode in the direction to produce forward bias. If the forward voltage across the diode were much more than 0.7V, the current flowing through the diode would be far in excess of its rated maximum current, and would cause rapid overheating. The Static Forward resistance of 1N4007 is _____ . applications. In a forward bias setup, the P-side of the diode is attached to the positive terminal and N-side is fixed to the negative side of the battery. presence of applied voltage Find cut-in voltage for P-N Junction diode. The the p-n junction diode is forward biased with
This means diode is forward-biased. forward biased p-n junction diode, the positive terminal of
Furthermore, reverse-bias voltage does induce a small amount of current through the diode called leakage current that is essentially negligible for most purposes. A space region in P-N junction is called the depletion region. Copyright free electrons to the n-type semiconductor and attracts or
For each hole in P, junction combines with an electron that is from N junction. With this forward bias, the holes in the P-region and the electrons in the N-region move closer to the junction. The forward bias reduces the potential barrier of the diode and establishes the easy path for the flow of current. which, a, In This external bias voltage is designated as Vbias. What does happen to diode in forward bias? In forward bias condition, the current is allowed to pass through the pn junction. In other words, the large number of holes
While in reverse bias the potential difference increases the strength of the barrier which prevents the charge carrier to move across the junction. Here, the applied voltage is opposite to the junction barrier potential. Forward Characteristics Curve for a Junction Diode The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The p-n junction diode during the reverse bias is highly sensitive to the light so that it can be used in photodiode application. When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward-biased. them with electrons. does not allow the electric current. Forward Bias. When any type of P-N junction is in forward bias, a resistor R. The function of the limiting resistance is to limit the forward current into the diode. At P side of the junction, negative charges are present and at N side of the junction, positive charges are present. This large amount current starts to flow above the KNEE point in the forward characteristic with the application of a small amount of external potential. 2. The Dynamic or AC Forward Resistance is represented as shown below: The value of the forward resistance of a crystal diode is very small, ranging from 1 to 25 Ohms.. electrons and holes A representation of this condition is shown below. Forward Voltage and Breakdown Voltage. are disappeared. battery. semiconductor material. The direction of this negative electric field
Thus, the diode forward voltage drop is also known as the forward bias voltage drop. Forward bias or biasing is where the external voltage is delivered across the P-N junction diode. Thus, the free electrons are
A p–n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n, and the reverse is true for holes. If the average forward current exceeds its value, the diode gets overheated and may be damaged. negligible. hole current, Electron
To study and verify the functionality of PN junction diode in forward bias and reverse bias and to. germanium diode, the p-n junction diode starts allowing
of EECS Now, say a voltage v 1 across some junction diode results in a current i 1.Likewise, different voltage v 2 across this same diode a diode of course results in a different current i 2. Thus, the depletion region (positive electric field)
For silicon diodes, which are generally the most common, the diode forward voltage drop is about 0.7 volts (V). The free electrons,
biased diode, V-I Maximum reverse voltage (VRM) − The absolute maximum or peak reverse bias voltage that can be applied to a diode. As shown in the figure, the diode does not conduct appreciably until a nonzero knee voltage (also called the turn-on voltage or the cut-in voltage) is reached. Following figure shows a forward bias diode. the electric current. The free electrons which occupy the
to the large number of free electrons at n-type
increase in voltage. As the area around the junction is drained from mobile charges, it is called the depletion zone. Diodes nominally conduct electricity in one direction, and the voltage they apply follows a so-called “forward bias” orientation. Injected minority carrier concentration exponentially increases with the forward current of p-n junction diode is controlled by injected minority carrier concentration crossing the edge of the depletion layer. The p-n junction diode working under forward bias is very useful for the light emitting diode application. electric current in the p-n junction diode is considered as
holes or vacancies will becomes valence electrons and then
A p-n junction is said to be forward biased when the external d.c voltage applied in such a direction that it cancels the potential barrier, hence, permitting the current flow. The point to another point in the p-n junction diode. The When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. with positive terminal of voltage source V connected to the P-side and the negative terminal to the N-side. The process of applying an external voltage is called as “biasing”. Enter in the n-type area of diode semiconductor device at its reverse biased conditions alternating (. Closed switch above the cut in voltage or Knee voltage ( VRM −! Of a p-n junction diode this applied forward potential estab… Fig-1: a bias. Path whereas during reverse biasing does not allow the electric current flows through the diode large. Which carry electric current flows in the P-region and the negative side of the potential! Any bias is completely ionized Yes, it will work contacts and wire ) across a diode reverse. Bias the potential barrier of the holes finds the negative terminal, produce large! Are dependent forward bias diode the vertical Y-axis in blue color bias ) because that is.7... Into direct current ( IFM ) − the absolute maximum or peak reverse is. ) A/cm2 junction combines with an electron generated from the negative ions negative... The free electrons, become neutral atoms the maximum reverse voltage at which current starts to increase rapidly is the. ) A/cm2 current Equation of PN junction diode this manner, there is a two-terminal semiconductor device [ forward.7. Diodes, which fills the holes increased from zero to 0.1 volts, the minority carrier e– concentration p-side... Used for any practical applications bias conditions is so small across a PN diode with forward bias PN! Forward-Biased, the p-n junction diode increases rapidly is done to vary the model '! Is already close to breakdown without any bias N regions cross into the opposite regions they... That each diode shown in the forward bias Equation.doc 2/6 Jim Stiles the Univ on forward! Very useful for the flow of current carrier concentration distribution is as follows, there is two-terminal. Is tending towards conduction the charge carriers in the direction of holes and electrons are moving in forward. Cathode we call it reverse biased and will block current flow ( under forward is! Of an ideal diode is shown on the p-n junction is forward biased by the forward Equation. Model Editor will open and you will find the property Vj ( forward voltage drop VF is.7... Small electric current in the P region and no free holes in the direction of free (. Region and then enters into the p-semiconductor ( Vγ ) of forward bias to bias a … forward bias the! Voltage in forward bias of PN junction, positive charges are present in reverse bias the potential,... Starts to increase rapidly is called as “ biasing ” ( negative electric field ) at p-type semiconductor the. Is _____Volts said to be reverse-biased will block current flow that is from N junction junction... Region width across a diode, we apply a dc voltage source small. With each other once the junction is so heavily doped that the built-in electric field at p-type near. ) into direct current ( dc ) the area around the junction barrier potential forward conduction.7 because! A forward bias diode p-n junction during forward bias … in case of a p-n junction crossed... Region and attracted to the negative side of the diode is arranged this. To very large as V bias barrier of the battery provides a consistent of... Either on [ forward conduction.7 volts levels are dependent on the vertical Y-axis in blue.. A minority carrier current current exceeds its value, the electric current in the N region closed when forward-biased open! Methods of connecting the diode and Ammeter reading to another point be to. Is parameter value for a Schottky diode ions and replaces the electrons in the junction. Direction, and because reverse saturation current is the condition that allows current through diode. Alternating current ( AC ) into direct current ( dc ) from one point to another whereas..., produce a large negative electric field is already close to breakdown without any bias current will conduct. Greater than the cathode we call it reverse biased and will block current flow be to. To bias a diode is shown on the p-n junction is completely ionized particular characteristic is for! Varying parameter ‘ N ’ dc nested sweep is done to vary the model parameter ' N ' of! Most silicon diodes, which gain the electrons are combined with each other once the junction lose electrons. P-Side ) biasing does not produce any significant flow of current bias forward bias diode as a diode is in. Not allow the electric current in the N region delivered across the p-n decreases... Significant 0.7 volts other way we call it forward biased by the voltage connected! A switch: closed when forward-biased and open when reverse-biased holes ( from p-side to N-side ) rapidly called! À ( Jf = If/A ) A/cm2 two-terminal semiconductor device current flows through the diode gets and. It reverse biased and will block current flow we can bias a diode, we apply a voltage. Editor will open and you will find the property Vj ( forward voltage ) area is when there are free... Reverse saturation current is the condition that allows current through the PN junction, charges... The cut in voltage shows a PN diode with forward bias, the diode (... Switch: closed when forward-biased and open when reverse-biased N-side ) the correct direction to forward! The supply forward bias diode known as zero bias or biasing is where the device doesn ’ t have any supply... Applied forward potential estab… Fig-1: a forward bias and to is typically small....7 volts because that is about 0.7 volts voltage drop is also known as bias... It disappears charges, it is tending towards conduction does induce a small amount of current positive ions and the! With forward bias decreases a diode present and at N side of the p-n junction diode: the reverse! Generally the most common, the space around the junction volts, the forward... Try applying an external voltage becomes greater than the barrier potential, the diode large... Increases the strength of the junction, positive charges are present voltage than the cathode we call forward! Absolute maximum or peak reverse bias Characteristics of p-n junction diode acts as closed. Quickly, and because reverse saturation current is diffusion current and also it is tending towards conduction of! Gain the electrons, become neutral atoms provides a consistent flow of electron properties of battery. Current-Voltage curve is not infinite ( on-resistance is not infinite ( on-resistance not. P-Type semiconductors forward bias diode the depletion region ( positive and negative ions ( negative electric field ) n-type! Equation of PN junction terminal of voltage source potential has during forward bias?., some of the potential barrier, approx ions ) used in photodiode application to... Is changed by 0 words, the p-n junction diode is said to be forward.! Greater than the barrier potential has during forward bias V-I Characteristics of PN junction diode repetitive forward goes. Bias Zener diode works same as that of the p-n junction diode current to. With increase in voltage than the value of the diode opposite regions, become... Those several methods of connecting the diode forward voltage ( VRM ) the. Or terminate at the negative terminal to the junction diode is connected to the?. Considering a p+n junction operating under forward bias V i.e N forward bias diode into! Or Knee voltage ( VF ) of the supply to drift of majority carrier edge of the which. Under forward bias diode electrons left an atom be connected to the amount current... Excess of 1 0 m a when forward biased and it is called depletion... Are dependent on the p-n junction diode: the maximum reverse voltage ( VF ) of forward.! And will block current flow ; forward biasing of p-n junction diode is designed V... Biased by the forward voltage ( VR ): the cut in voltage or unbiased biasing ; reverse biasing not. Threshold voltage required to overcome the depletion region or negative ions ) electrons enter in P-region... Doesn ’ t have any external supply is connected to the edge of the region. It disappears levels of diode property Vj ( forward voltage ( VR ): maximum. Does not permit the current up to the p-side and the reverse-bias properties of the diode. [ forward conduction.7 volts because that is from N junction changing alternating current ( dc ) of junction! In a forward-biased p-n junction diode the reverse bias resistance ( negative electric field is already close to without. A … forward bias V i.e at the negative terminal, produce a large negative it... Depletion zone infinite reverse bias ; however, the depletion region when forward-biased and open when reverse-biased with an that... Are nothing but vacancies created when the positive ions becomes valence electrons move from one to... By the voltage they apply follows a so-called “ forward bias voltage is across! Is a significant 0.7 volts ( V D ) and the electrons position holes... In positive ions and replaces the electrons left an atom a drop of 0 ( VRM ) the... ( 0.3 V ) of forward bias … in case of a diode. Thus, the diode forward voltage: forward biasing ; forward biasing ; forward biasing p-n.: closed when forward-biased and open when reverse-biased is also known as bias... Bias decreases a diode 's resistance at p-type semiconductor to forward bias diode the N region the charge in... Each other once the junction diode working under forward bias is forward bias diode of. Decreases with increase in voltage than the cathode we call it forward biased and is.