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Turn-On Transient

x

Consider a p+

n diode (Q p >> Q n):

t

i (t )

t

v A(t )

0p x x

n

qADi

dxdp

n

For t > 0:

D p n( x )

x n

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By separation of variables and integration, wehave

If we assume that the build-up of storedcharge occurs quasi-statically so that

then

0for

t Q

I Q

idt

dQ

p

p F

p

p p

pt p F p e I t Q

/1)(

pkT qv

pdiffusion p Ae I I t Q 1)(/

0

pt F A e I I

qkT

t v/

0

11ln)(

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Summary of Important Concepts

Under forward bias, minority carriers are injectedinto the quasi-neutral regions of the diode.The current flowing across the junction iscomprised of hole and electron components.

If the junction is asymmetrically doped (i.e. it is one -sided) then one of these components will bedominant.

In a long-base diode, the injected minority carriersrecombine with majority carriers within the quasi-neutral regions.

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The ideal diode equation stipulates therelationship between J N(-xp) and J P (xn):

For example, if holes are forced to flow acrossa forward-biased junction, then electrons mustalso be injected across the junction.

sideni

side pi

An p

D pn

n P

p N

nn

N L D N L D

x J x J 2

2

)()(

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Under reverse bias, minority carriers are collected into the quasi-neutral regions of the diode.

Minority carriers generated within a diffusion length of the depletionregion diffuse into the depletion region and then are swept across the

junction by the electric field.

The negative current flowing in a reverse-biased diode depends on

the rate at which minority carriers are supplied from the quasi-neutral regions.Electron-hole pair generation within the depletion region alsocontributes negative diode current.

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Varactor Diode

Voltage-controlled capacitanceUsed in oscillators and detectors

(e.g. FM demodulation circuits in your radios)

Response changes by tailoring doping profile:

2

1

mn b iV V

for

V C

r

nr j

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Tunnel Diode (contd)

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Optoelectronic Diodes

L N P L

LkT V q

G LW LqA I I e I I

)()1(

A0

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1ln0A Ln L p L

LW L

qkT

I ocGV V

pn

nn

p

p

n p

Open Circuit Voltage, VOC

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p-i-n Photodiodes

W Wi-region, so most carriers are generatedin the depletion region faster response time (~10 GHzoperation)Operate near avalanche to amplify signal

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Light Emitting Diodes (LEDs)

LEDs are typically made of compound semiconductors(direct bandgap)

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Organic LEDs

Some organic materialsexhibit semiconductingproperties

OLEDs are attractive for low-cost, high-quality flexibledisplays