Photodetectors Read: Kasip, Chapter 5 Yariv, Chapter 11 Class Handout. ECE 162C Lecture #13 Prof. John Bowers

Transcription

1 Photodetectors Read: Kasip, Chapter 5 Yariv, Chapter 11 Class Handout ECE 162C Lecture #13 Prof. John Bowers

2 Definitions Quantum efficiency η: Ratio of the number of electrons collected to the number of photons incident. Responsivity: current out divided by optical power incident R e e ηλ = η = ηλ = d hν hc 1.24W / A

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4 External Quantum Efficiency and Responsivity Schematic photogeneration profiles Different contributions to the photocurrent I ph. Photogeneration profiles corresponding to short, medium and long wavelengths are also shown.

5 External quantum efficiency (QE) η e of the detector η e = Number of free EHP generated and collected Number of incident photons η e = I P o ph / / e h υ

6 Responsivity R R = Photocurrent (A) Incident Optical Power (W) = I ph P o e R =η = e hυ η e e λ hc

7 Responsivity R Responsivity (R) vs. wavelength (λ) for an ideal photodiode with QE = 100% (η e = 1) and for a typical inexpensive commercial Si photodiode. The exact shape of the responsivity curve depends on the device structure. The line through the origin that is a tangent to the responsivity curve at X, identifies operation at λ 1 with maximum QE

8 pin Photodiode The responsivity of Si, InGaAs and Ge pin type photodiodes. The pn junction GaP detector is used for UV detection. GaP (Thorlabs, FGAP71), Si(E), IR enhanced Si (Hamamatsu S11499), Si(C), conventional Si with UV enhancement, InGaAs (Hamamatsu, G8376), and Ge (Thorlabs, FDG03). The dashed lines represent the responsivity due to QE = 100 %, 75% and 50 %.

9 pin Photodiode E = E o Vr W Vr W C t dep drift ε ε o r A = W W = v d

10 pin Photodiode Width of i-region (Depletion region) t drift = W v d Transit time (Drift time) Drift velocity Drift velocity vs. electric field for holes and electrons in Si.

11 pin Photodiode Speed A reverse biased pin photodiode is illuminated with a short wavelength light pulse that is absorbed very near the surface. The photogenerated electron has to diffuse to the depletion region where it is swept into the i-layer and drifted across. In time t, an electron, on average, diffuses a distance given by = (2D e t) 1/2 Electron diffusion coefficient

12 PIN Impulse Response?

13 PIN Impulse Response? j = v e eσ e v L h eσ h Displacement current flows. That is what is measured in an external circuit, not conduction current.

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15 Avalanche Photodiodes (APDs) α Rate at which electrons multiply β Rate at which holes multiply A large ratio of α/β or β/α results in a large gain bandwidth product and low noise amplification. True for Si Most III-Vs have a small ratio, and limited gain bandwidth product. The noise is larger, but still lower than a PIN receiver.

16 Ionization Coefficients for Semiconductors Ionization Coefficients[cm -1] 10 e 6 10 e 5 10 e I n G a A s e l e c t r o n s h o l e s G a A s e l e c t r o n s h o l e s Si (electrons) e l e c t r o n s E l e c t r i c F i e l d [ k V / c m ] h o l e s I n P F r o m : S i : P. P. W e b b, " M e a s u r e m e n t s o f I o n i z a t i o n C o e f f i c i e n t s i n S i l i c o n a t L o w E l e c t r i c F i e l d s ", G E C a n a d a I n c. I n P : L. W. C o o k, e t. a l., A p p l. P h y s. L e t t. 4 0 ( 7 ), 1 A p r i l I n G a A s : T. P. P e a r s a l l, A p p l. P h y s. L e t t. 3 6 ( 3 ), 1 F e b r u a r y G a A s : H. D. L a w a n d C. A. L e e, S o l i d - S t a t e E l e c t r o n i c s, 2 1,

17 The Avalanche Multiplication Process P N Si Multiplication Layer P N InP Multiplication Layer

18 Avalanche Photodiode Gain or Multiplication M Ionization coefficient ratio α e = Aexp(-B/E) Chyoweth's law

19 Avalanche Photodiode Gain or Multiplication M Electrons only M = exp(α e w) Ionization coefficient M = Electrons and holes 1 k exp[ (1 k) α e w] k k = α h / α e

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

21 Heterojunction Photodiodes: SAM (a) Energy band diagrams for a SAM detector with a step junction between InP and InGaAs. There is a valence band step ΔE v from InGaAs to InP that slows hole entry into the InP layer. (b) An interposing grading layer (InGaAsP) with an intermediate bandgap breaks ΔE v and makes it easier for the hole to pass to the InP layer for a detector with a graded junction between InP and InGaAs. This is the SAGM structure.

22 Heterojunction Photodiodes: SAM Simplified schematic diagram of a more practical mesa-etched SAGM layered APD

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24 SAM APDS: Need for Separate Absorption and Multiplication Regions Small bandgap avalanche regions tend to have large dark current. Absorption Region e- e- e- Multiplication Region Electric Field position

25 Electric Field Simulation for SHIP Electric Field [kv/cm] InGaAs Si 5 V 10 V 23 V Distance into Detector [µm]

26 Gain and Dark Current vs. Bias. 23 µm diameter SHIP Gain Reverse Bias [V] Dark Current [µa] Theoretical Gain Measured Photocurrent Gain

27 SHIP Detector 3-dB Bandwidth versus gain 3 db Bandwidth [GHz] GHz GB Gain

28 Comparison of Acheivable GB Product for SHIP, SL, and InP APDs 800 Gain-Bandwidth-Product (GHz) SL APD SHIP APD 100 InP APD Multiplication Layer Thickness [µm]

29 Staircase APD: Use of bandgap engineering to increase the ratio of ionization coefficients.