Forward BiasForward Bias

Forward bias is the condition that allows Forward bias is the condition that allows current through a pn junction. current through a pn junction.

This external bias voltage is designated This external bias voltage is designated as Vas VBIASBIAS. The resistor R limits the current to . The resistor R limits the current to a value that will not damage the pn a value that will not damage the pn structure.structure.

The Effect of Forward Bias on The Effect of Forward Bias on the Depletion Regionthe Depletion Region

As more electrons flow into the As more electrons flow into the depletion region, the number of depletion region, the number of positive ions is reduced. As more holes positive ions is reduced. As more holes effectively flow into the depletion effectively flow into the depletion region on the other side of the pn region on the other side of the pn junction, the number of negative ions junction, the number of negative ions is reduced. This reduction in positive is reduced. This reduction in positive and negative ions during forward bias and negative ions during forward bias causes the depletion region to narrow.causes the depletion region to narrow.

Effect of the Barrier Potential Effect of the Barrier Potential During Forward BiasDuring Forward Bias

When forward bias is applied, the free When forward bias is applied, the free electrons are provided with enough energy electrons are provided with enough energy to overcome the barrier potential and to overcome the barrier potential and effectively “climb" the energy hill and cross effectively “climb" the energy hill and cross the depletion region. the depletion region.

The energy that the electrons require in order The energy that the electrons require in order to pass through the depletion region is to pass through the depletion region is equal to the barrier potential energy. equal to the barrier potential energy.

In other words, the electrons give up an In other words, the electrons give up an amount of energy equivalent to the barrier amount of energy equivalent to the barrier potential when they cross the depletion potential when they cross the depletion region. This energy loss results in a voltage region. This energy loss results in a voltage drop across the pn junctiondrop across the pn junction

Reverse BiasReverse Bias The initial flow of charge carriers is transitional The initial flow of charge carriers is transitional

and lasts for only a very short time after the and lasts for only a very short time after the reverse bias voltage is applied. reverse bias voltage is applied.

As the depletion region widens, the availability of As the depletion region widens, the availability of majority carriers decreases.majority carriers decreases.

the electric field between the positive and the electric field between the positive and negative charges increases in strength negative charges increases in strength

a very small reverse current that can usually be a very small reverse current that can usually be neglectedneglected

the width of the depletion zone will the width of the depletion zone will increase. This increases the voltage barrier increase. This increases the voltage barrier causing a high resistance to the flow of causing a high resistance to the flow of charge carriers thus allowing minimal charge carriers thus allowing minimal electric current to cross the p-n junction.electric current to cross the p-n junction.

The strength of the depletion zone electric The strength of the depletion zone electric field increases as the reverse-bias voltage field increases as the reverse-bias voltage increases. Once the electric field intensity increases. Once the electric field intensity increases beyond a critical level, the p-n increases beyond a critical level, the p-n junction depletion zone breaks-down and junction depletion zone breaks-down and current begins to flow, usually by either current begins to flow, usually by either the the ZenerZener or or avalanche breakdownavalanche breakdown processes..processes..

Reverse currentReverse current The small number of free minority The small number of free minority

electrons in the p region are "pushed" electrons in the p region are "pushed" toward the pn junction by the negative bias toward the pn junction by the negative bias voltage. When these electrons reach the voltage. When these electrons reach the wide depletion region, they "fall down the wide depletion region, they "fall down the energy hill" and combine with the minority energy hill" and combine with the minority holes in the n region as valence electrons holes in the n region as valence electrons and flow toward the positive bias voltage, and flow toward the positive bias voltage, creating a small hole current.creating a small hole current.

The minority electrons easily pass through The minority electrons easily pass through the depletion region because they require the depletion region because they require no additional energyno additional energy

Reverse BreakdownReverse Breakdown At the breakdown voltage, the reverse current will At the breakdown voltage, the reverse current will

drastically increase.drastically increase. The high reverse-bias voltage imparts energy to the The high reverse-bias voltage imparts energy to the

free minority electrons so that as they speed through free minority electrons so that as they speed through the p region, they collide with atoms with enough the p region, they collide with atoms with enough energy to knock valence electrons out of orbit . The energy to knock valence electrons out of orbit . The newly created conduction electrons are also high in newly created conduction electrons are also high in energy and repeat the process. If one electron knocks energy and repeat the process. If one electron knocks only two others out of their valence orbit during its only two others out of their valence orbit during its travel through the h region, the numbers quickly travel through the h region, the numbers quickly multiply. As these high-energy electrons go through multiply. As these high-energy electrons go through the depletion region, they have enough energy to go the depletion region, they have enough energy to go through the n region as conduction electrons. rather through the n region as conduction electrons. rather than combining with holes.than combining with holes.

The multiplication of conduction electrons just The multiplication of conduction electrons just discussed is known as avalanche and results in a very discussed is known as avalanche and results in a very high reverse current that can damage the pn structure high reverse current that can damage the pn structure because of excessive heat dissipation.because of excessive heat dissipation.

CURRENT-VOLTAGE CURRENT-VOLTAGE CHARACTERISTIC OF A PN CHARACTERISTIC OF A PN

JUNCTIONJUNCTION IV characteristics for IV characteristics for

forward biasforward bias Point A corresponds to Point A corresponds to

zero-bias condition. zero-bias condition. Point B corresponds to Point B corresponds to

where the forward voltage where the forward voltage is less than the barrier is less than the barrier potential of 0.7 V.potential of 0.7 V.

Point C corresponds to Point C corresponds to where the forward voltage where the forward voltage approximately equals the approximately equals the barrier potential and the barrier potential and the external bias voltage and external bias voltage and forward current have forward current have continued to increase.continued to increase.

The diode DC or static The diode DC or static resistanceresistance

If forward biased :If forward biased :

If reverse biased: If reverse biased:

F

FF I

VR

D

DD I

VR

R

RR I

VR

ExampleExample

Determine the dc resistance for a Determine the dc resistance for a diode with the following operating diode with the following operating point:point:– A) IA) IDD =2 mA and V =2 mA and VDD = 0.5 V = 0.5 V

– B) IB) IDD =20 mA and V =20 mA and VDD = 0.8 V = 0.8 V

– C) IC) IDD =-1 =-1 μμA and VA and VDD = -10 V = -10 V

SolutionSolution

A) A)

B)B)

C)C)

25010*2

5.03

F

FF I

VR

4010*20

8.03

F

FF I

VR

M

I

VR

R

RR 10

10*1

106

AC or Dynamic ResistanceAC or Dynamic Resistance

The dynamic. The dynamic. resistance of a resistance of a diode is designated diode is designated rrdd

F

Fd I

Vr

The average ac resistanceThe average ac resistance

It is the resistance It is the resistance determined by a determined by a straight line drawn straight line drawn between the two between the two intersections intersections established by the established by the maximum and maximum and minimum values of minimum values of input voltageinput voltage

intint topopoF

Fav I

Vr

IV characteristics for reverse IV characteristics for reverse biasbias

The The breakdown breakdown voltage for a voltage for a typical silicon typical silicon pn junction pn junction can vary, but can vary, but a minimum a minimum value of 50 V value of 50 V is not unusualis not unusual

Complete IV characteristicsComplete IV characteristics

Silicon versus GermaniumSilicon versus Germanium

Temperature Effects on the IV Temperature Effects on the IV CharacteristicCharacteristic

The diodeThe diode

Diode structure and symbolDiode structure and symbol The diode is a single pn junction The diode is a single pn junction

device with conductive contacts and device with conductive contacts and wire leadswire leads

The p region is called anode and the n The p region is called anode and the n region is called cathoderegion is called cathode

The arrow points in the direction of The arrow points in the direction of conventional current (opposite to conventional current (opposite to electron flow)electron flow)

Typical diodesTypical diodes

Forward and reverse bias of a Forward and reverse bias of a diodediode

The ideal diode modelThe ideal diode model

The ideal model The ideal model of a diode is a of a diode is a simple switch. simple switch. The barrier The barrier potential, the potential, the forward forward dynamic dynamic resistance and resistance and the reverse the reverse current are all current are all neglectedneglected

The practical diode modelThe practical diode model

R

VVI FBIASF

The IV characteristics of the The IV characteristics of the practical diode modelpractical diode model

The complex diode modelThe complex diode model

The IV characteristics of the The IV characteristics of the complex diode modelcomplex diode model

ExampleExample

(a) Determine the forward voltage and (a) Determine the forward voltage and forward current for the diode in Figure for forward current for the diode in Figure for each of the diode models. Also find the each of the diode models. Also find the voltage across the limiting resistor in each voltage across the limiting resistor in each case. Assume rcase. Assume rdd = 10 = 10 ΩΩ at the value of at the value of forward current.forward current.

(b) Determine the reverse voltage and reverse (b) Determine the reverse voltage and reverse current for the diode in Figure for each of the current for the diode in Figure for each of the diode models. Also find the voltage across the diode models. Also find the voltage across the limiting resistor in each case. Assume Ilimiting resistor in each case. Assume IRR = 1 = 1 pA.pA.

SolutionSolution