Forward Biased P-N Junction

     

Forward biasing the p-n junction drives holes to the junction from the p-type material and electrons to the junction from the n-type material. At the junction the electrons and holes combine so that a continuous current can be maintained.

                                                             

        

Show energy bands. Compare to reverse bias.

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► Semiconductor concepts

► Semiconductors for electronics

Reverse Biased P-N Junction

                       

The application of a reverse voltage to the p-n junction will cause a transient current to flow as both electrons and holes are pulled away from the junction. When the potential formed by the widened depletion layer equals the applied voltage, the current will

cease except for the small thermal current.

                                                            

              

Show energy bands. Compare to forward bias.

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► Semiconductor concepts

► Semiconductors for electronics

The P-N Junction Diode

                       

The nature of the p-n junction is that it will conduct current in the forward direction but not in the reverse direction. It is therefore a basic tool for rectification in the building of DC power supplies.

                                                           

                                           

Diode varieties

PIN diode

Step-recovery diode

Diode applications

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► Semiconductor concepts

► Semiconductors for electronics

The PIN Diode

               

The PIN diode has heavily doped p-type and n-type regions separated by an intrinsic region. When reverse biased, it acts like an almost constant capacitance and when forward biased it behaves as a variable resistor.

The forward resistance of the intrinsic region decreases with increasing current. Since its forward resistance can be changed by varying the bias, it can be used as a modulating device for AC signals. It is used in microwave switching applications.

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Step-Recovery Diode

            

In the step-recovery diode the doping level is gradually decreased as the junction is approached. This reduces the switching time since the smaller amount of stored charge near the junction can be released more rapidly when changing from forward to reverse bias.

The forward current can also be established more rapidly than in the ordinary junction diode. This diode is used in fast switching applications.

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DiodeDiode

The ideal diode:

(a) diode circuit symbol; (b) i-v characteristic; (c) equivalent circuit in the reverse direction; (d) equivalent circuit in the forward direction.

Diode CircuitsDiode Circuits

(a) Rectifier circuit. (b) Input waveform. (c) Equivalent circuit when (d) Equivalent circuit when v1 > 0 (e) Output

waveform.

During positive cycle, diode allow current to pass through and output voltage is positive

Diode CharacterizationDiode Characterization

The i-v characteristic of a silicon junction diode.

► Diodes have a nonlinear Diodes have a nonlinear response to voltageresponse to voltage

► We model different regions of We model different regions of operationoperation

Forward BiasForward Bias

Reverse BiasReverse Bias

BreakdownBreakdown

1/ TnVvs eIi

1/ TnVvs eIi

sIi

Forward-Bias RegionForward-Bias Region

► Terminal voltage Terminal voltage v v is positiveis positive

► k=Boltzmann’s constant=k=Boltzmann’s constant=► T=absolute temparature in K=273+ CelciusT=absolute temparature in K=273+ Celcius► q=magnitude of electronic charge=q=magnitude of electronic charge=► 1<n<2, depending on material, assume n=11<n<2, depending on material, assume n=1

► EXAMPLE: Diode with n=1 displays forward voltage EXAMPLE: Diode with n=1 displays forward voltage of 0.7V at 1mA. Findof 0.7V at 1mA. Find

► SOLUTION: SOLUTION: ► For n=1: For n=1: ► For n=2: For n=2:

)1( / TnVvs eIi

q

kTVT

231038.1 x

Cx 19106.1

ST

nVvS I

inVveIi T ln/

SITT nVv

SnVv

S ieIeIi //

AAxeIS151625/7003 10109.610

AAxeIS91050/7003 10103.810

Reverse-Bias RegionReverse-Bias Region

► Terminal voltage Terminal voltage v v is negativeis negative

v v is negative and a few times larger than is negative and a few times larger than

► Current in the reverse direction is constant and equal to Current in the reverse direction is constant and equal to

and called and called saturation currentsaturation current. . ► Typically, real life diodes exhibit much larger reverse Typically, real life diodes exhibit much larger reverse

currentscurrents

sIi

TV

SI

AIS1514 1010

Break Down RegionBreak Down Region

► Knee of diode Knee of diode I-vI-v characteristic, the voltage characteristic, the voltage is less than Zener voltage. is less than Zener voltage.

Diode ZenerDiode Zener

Diode Zener Characterization Diode Zener Characterization

Equivalent Diode ZenerEquivalent Diode Zener

Diode ModelDiode Model

► As a resistor with value equal to As a resistor with value equal to reciprocal of the slope of the “reciprocal of the slope of the “i-v”i-v” curvecurve

► Linearization around bias point VD and Linearization around bias point VD and time varying signal vd(t) is time varying signal vd(t) is superimposed. superimposed.

► Current due to VD:Current due to VD:

► Approx.Approx.

)/( TD nVvSD eII

)/()(()()()( TdD nVtvVSDdDD eItitvVtv

)1()(1T

dDD

T

d

nV

vItI

nV

v

Small Signal ApproximationSmall Signal Approximation

Equivalent circuit modelEquivalent circuit model► For small changes around For small changes around

bias point bias point Q Q is reciprocal of is reciprocal of tangent :tangent :

► VDVD0 is the intercept of the 0 is the intercept of the tangent on the tangent on the vDvD axis axis

DD IiD

Dd v

ir

/1

)(1

0DDD

D vvr

i

Separate DC and small signal analysis

Basic Limiting CktsBasic Limiting Ckts