光檢測器 -...

223

第五章

光檢測器

224

目錄

5-1 pN接面光二極體的原理5-2 拉摩定理和外光電流5-3 吸收係數和光二極體材料5-4 量子效率和響應率5-5 pin光二極體5-6 累崩光二極體 (APD)5-7 累質接面光二極體5-8 光電晶體5-9 光電導檢測器和光電導增益5-10 光檢測器中的雜訊

225

5-1 pN接面光二極體的原理

226

p+SiO2

Electrode

ρnet

–eNa

eNd x

x

E(x)

R

Emax

e–h+

Iph

hυ > Eg

W

En

Depletion region

(a)

(b)

(c)

Antireflectioncoating

Vr

(a) A schematic diagram of a reverse biased pn junctionphotodiode. (b) Net space charge across the diode in thedepletion region. Nd and Na are the donor and acceptorconcentrations in the p and n sides. (c). The field in thedepletion region.

Electrode

Vout

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

227

5-2 拉摩定理和外光電流

一個載子的躍遷時間 (transit time) 是從它的產生點漂移到收集電極所需的時間；電子和電洞的躍遷時間 te 、 th 被標示在圖5.2(b) 的 t 對 x 圖，這裡

和 (1)e

elLt

v−

=h

hlt

v=

228

229

e–h+

iph(t)

Semiconductor

(a)

V

x

(b)

(a) An EHP is photogenerated at x = l. The electron and the hole drift in oppositedirections with drift velocities vh and ve. (b) The electron arrives at time te = (L − l)/ve andthe hole arrives at time th = l/vh. (c) As the electron and hole drift, each generates anexternal photocurrent shown as ie(t) and ih(t). (d) The total photocurrent is the sum of holeand electron photocurrents each lasting a duration th and te respectively.

E

l L − l

t

vevhvh

0 Ll

t

e–h+

th

te

t

0

th

te

iph(t)

i(t)

t

0

th

te

evh/L + eve/Levh/L

ie(t)

ih(t)

(c)

(d)Charge = e

evh/L eve/L


233

5-3 吸收係數和光二極體材料

237

0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8

Wavelength (µm)

In0.53Ga0.47As

Ge

Si

In0.7Ga0.3As0.64P0.36

InPGaAs

a-Si:H

12345 0.9 0.8 0.7

1×103

1×104

1×105

1×106

1×107

1×108

Photon energy (eV)

Absorption coefficient (α) vs. wavelength (λ) for various semiconductors(Data selectively collected and combined from various sources.)

α (m-1)

1.0

Figure 5.3

圖5.3 對於不同的半導體材料，吸收係數與波長的關係。

238

E

CB

VB

k–k

Direct Bandgap Eg Photon

Ec

Ev

(a) GaAs (Direct bandgap)

E

k–k

(b) Si (Indirect bandgap)

VB

CB

Ec

Ev

Indirect Bandgap, Eg

Photon

Phonon

(a) Photon absorption in a direct bandgap semiconductor. (b) Photon absorptionin an indirect bandgap semiconductor (VB, valence band; CB, conduction band)


圖5.4 (a) 光子在直接能隙半導體的吸收，(b) 光子在間接半導體的吸收 (VB，價帶；CB，傳導帶 )

239

5-4 量子效率和響應率

光二極體的響應率 (responsivity)R，定義為在某特定波長，每入射光功率 Po 產生的光電流Iph

(1)o

ph

PI

==)W(

)A(入射光功率

光電流R

242

244

Responsivity (R) vs. wavelength (λ) for an idealphotodiode with QE = 100% (η = 1) and for a typicacommercial Si photodiode.

0 200 400 600 800 1000 12000

0.10.20.30.40.50.60.70.80.9

1

Wavelength (nm)

Si Photodiode

λg

Responsivity (A/W)

Ideal PhotodiodeQE = 100% ( η = 1)


245

5-5 pin光二極體

246

p+

i-Si n+

SiO2Electrode

ρnet

-eNa

eNd

x

x

E(x)

R

Eo

E

e-h+

Iph

hυ > Eg

W

(a)

(b)

(c)

(d)

VrThe schematic structure of an idealized pin photodiode (b) The netspace charge density across the photodiode. (c) The built-in fieldacross the diode. (d) The pin photodiode in photodetection isreverse biased.

Vout

Electrode

© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

247

如同平行板電容器，兩層非常薄的正、負電荷被寬度 W 的 i-Si 所分離， Pin 二極體的接面或空乏層電容 (junction or depletion layer capacitance) 可表示為

(1)WAC rodep

εε=

251

Drift velocity vs. electric field for holes and electrons in Si.

102

103

104

105

107106105104Electric field (V m-1)

Electron

Hole

Drift velocity (m s -1)


圖5.7 在矽中，電洞與電子的漂移速度和電場的關係。

253

hυ > Eg

p+ i-Si

e– E

h+

W

Drift

Diffusion

A reverse biased pin photodiode is illuminated with a shortwavelength photon that is absorbed very near the surface.The photogenerated electron has to diffuse to the depletionregion where it is swept into the i-layer and drifted across.

Vr


圖5.8 一個逆向偏壓的 Pin 光二極體被一個短波長的光照射，且在非常靠近表面被吸收。光產生的電子必須擴散到空乏區才會被電場加速通過 I - 層。

255

5-6 累崩光二極體 (APD)

256

š p+

SiO2Electrode

ρn et

x

x

E(x)

R

Ehυ > Eg

p

Ip h

e– h+

Absorptionregion

Avalancheregion

(a)

(b)

(c)

(a) A schematic illustration of the structure of an avalanche photodiode (APD) biasedfor avalanche gain. (b) The net space charge density across the photodiode. (c) Thefield across the diode and the identification of absorption and multiplication regions.

Electrode


n+

圖5.9 (a) 一個照光且偏壓下的累崩光二極體 (APD)，(b) 整個光二極體的淨空間電荷密度，(c) 整個二極體的電場分佈並標示出吸收和累崩區。

257

h+E

šn+ p

e–

Avalanche region

e–

h+

Ec

Ev

(a) (b)

E

(a) A pictorial view of impact ionization processes releasing EHPs andthe resulting avalanche multiplication. (b) Impact of an energeticconduction electron with crystal vibrations transfers the electron'skinetic energy to a valence electron and thereby excites it to theconduction band.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

圖5.10 (a) 一種圖示的累崩撞擊電離過程和電子–電洞對的釋放，(b) 一個具有晶格振動的高能傳導電子的撞擊，並將電子動能轉移給價電子而將它激發到傳導帶。

260

SiO2

Guard ring

ElectrodeAntireflection coating

nn n+

p+

š

p

SubstrateElectrode

n+

p+

š

p

SubstrateElectrode

Avalanche breakdown

(a) (b)

(a) A Si APD structure without a guard ring. (b) A schematic illustration of thestructure of a more practical Si APD?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

圖5.11 (a) 一個沒有護環的矽製APD結構，(b) 更實際的APD結構。

264

5-7 累質接面光二極體

265

E

N n

Electrode

x

E(x)

R

hυ

Ip h

Absorptionregion

Avalancheregion

InP InGaAs

h+

e–E

InP

P+ n+

Simplified schematic diagram of a separate absorption and multiplication(SAM) APD using a heterostructure based on InGaAs-InP. P and N refer top and n -type wider-bandgap semiconductor.

Vr

Vout


266

InP

InGaAsh+

e–

E

Ec

Ev

Ec

Ev

InP

InGaAs

Ev

Ev InGaAsP grading layer

h+

∆Ev

(a) Energy band diagraSAM heterojunction Athere is a valence band ∆Evfrom InGaAs to InP thahole entry into the InP

(b) An interposing grad(InGaAsP) with an intebandgap breaks ∆Ev and makeasier for the hole to palayer

(a)

(b)


267

P+ְךnP Substrate

P+ְךnP (2-3 µm) Buffer epitaxial layerNְךnP (2-3 µm) Multiplication layer.

Photon

nְךn 0.53Ga0.47As (5-10µm) Absorption lay

Graded nְךnGaAsP (

268

hυ

h+

e–

n+Ec

Ev

10?0 nm

p+

E

Eg1

Eg 2∆Ec

Energy band diagram of a staircase superlattice APD (a) No bias. (b) Withan applied bias.

(a) (b)


圖5.15 一個階梯式超晶格APD(a) 沒有偏壓，(b) 外加偏壓。

269

5-8 光電晶體

270

n

hυ

Base Collector

h+

e–

Emitter

pn+E

e–

SCLSCL Iph

VBE VBC

VCC

The principle of operation of thephotodiode. SCL is the space chargelayer or the depletion region. Theprimary photocurrent acts as a basecurrent and gives rise to a largephotocurrent in the emitter-collectorcircuit.


圖5.16 光電晶體的操作原理。SCL是空間電荷層或空乏區，一次光電流就像是基極電流會在射 - 集電路產生大量的光電流。

271

5-9 光電導檢測器和光電導增益

272

Light

w

d

V Iphoto

A semiconductor slab of length , width w and depth d isilluminated with light of wavelength λ.

n = no + ∆np = po + ∆p


273

Iph

Photoconductore–

h+

Iph Iph Iph Iph

A photoconductor with ohmic contacts (contacts not limiting carrier entry) can exhibit gain. Asthe slow hole drifts through the photoconductors, many fast electrons enter and drift through thephotoconductor because, at any instant, the photoconductor must be neutral. Electrons drift fasterwhich means as one leaves, another must enter.

(a) (b) (c) (d) (e)


280

5-10 光檢測器中的雜訊

282

Vou

Current

Time

Id

Vr

In pn junction and pin devices the main source of noise is shotnoise due to the dark current and photocurrent.

pn

Po

Dark

IlluminatedId + IphId + Ip h + in

R A


297

00.10.20.30.40.50.60.70.8

0.5 1 1.5 2Wavelength(痠)

The responsivity of a commercial Ge pnjunction photodiode

Responsivity(A/W)


298

0

0.1

0.2

0.3

0.4

0.5

0.6

200 400 600 800 1000 1200Wavelength(nm)

A B

The responsivity of two commercial Si pinphotodiodes

Responsivity(A/W)


299

0

0.2

0.4

0.6

0.8

1

800 1000 1200 1400 1600 1800Wavelength(nm)

The responsivity of an InGaAs pinphotodiode

Responsivity(A/W)


300

x

R

E

e–h+

iph

hυ > Eg

W

VrAn infinitesimally short light pulse is absorbed throughoudepletion layer and creates an EHP concentration that decexponentially

Photogenerated electron concentrationexp(−αx) at time t = 0

BA

vde


301

第五章目錄5-1 pN接面光二極體的原理5-2 拉摩定理和外光電流5-3 吸收係數和光二極體材料5-4 量子效率和響應率5-5 pin光二極體5-6 累崩光二極體 (APD)5-7 累質接面光二極體5-8 光電晶體5-9 光電導檢測器和光電導增益5-10 光檢測器中的雜訊

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