Lecture 12. Semiconductor Detectors - Photodetectors

Transcription

1 Lecture 12 Semiconductor Detectors - Photodetectors Principle of the pn junction photodiode Absorption coefficient and photodiode materials Properties of semiconductor detectors The pin photodiodes Avalanche photodiodes

2 Photodiode (a) Electrode SiO 2 p + V r I ph R V out hυ > E g h+ E e n Antireflection coating W Electrode Depletion region (b) ρ net en d x (c) en a E(x) x E max (a) A schematic diagram of a reverse biased pn junction photodiode. (b) Net space charge across the diode in the depletion region. N d and N a are the donor and acceptor concentrations in the p and n sides. (c). The field in the depletion region.

3 V i ph (t) (a) v h h + e Semiconductor E v e t e 0 ev h /L Charge = e ev h /L + ev e /L i ph (t) (d) t h (b) l L l 0 l L h + e t e x t e t 0 ev h /L ev e /L i e (t) i(t) (c) t h t h i h (t) t t t (a) An EHP is photogenerated at x = l. The electron and the hole drift in opposite directions with drift velocities v h and v e. (b) The electron arrives at time t e = (L l)/v e and the hole arrives at time t h = l/v h. (c) As the electron and hole drift, each generates an external photocurrent shown as i e (t) and i h (t). (d) The total photocurrent is the sum of hole and electron photocurrents each lasting a duration t h and t e respectively.

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5 Photon energy (ev) Ge In 0.7 Ga 0.3 As 0.64 P α (m -1 ) Si GaAs InP In 0.53 Ga 0.47 As a-si:h Wavelength (µm) Absorption coefficient (α) vs. wavelength (λ) for various semiconductors (Data selectively collected and combined from various sources.) Figure 5.3

6 E E CB E c Direct Bandgap E g Photon E v Photon CB Indirect Bandgap, E g E c VB VB E v Phonon k k k k (a) GaAs (Direct bandgap) (b) Si (Indirect bandgap) (a) Photon absorption in a direct bandgap semiconductor. (b) Photon absorption in an indirect bandgap semiconductor (VB, valence band; CB, conduction band)

7 Responsivity (A/W) Ideal Photodiode QE = 100% ( η = 1) Si Photodiode Wavelength (nm) Responsivity (R) vs. wavelength (λ) for an ideal photodiode with QE = 100% (η = 1) and for a typica commercial Si photodiode. λ g

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9 pin Photodiode SiO 2 Electrode p + Electrode (a) i-si n + ρ net en d (b) x en a E(x) (c) x E o W hυ > E g E (d) h +e I ph R V out V r The schematic structure of an idealized pin photodiode (b) The net space charge density across the photodiode. (c) The built-in field across the diode. (d) The pin photodiode in photodetection is reverse biased.

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12 Drift velocity (m s -1 ) 10 5 Electron 10 4 Hole Electric field (V m -1 ) 10 7 Drift velocity vs. electric field for holes and electrons in Si.

13 hυ > E g p + Diffusion e i-si E h + l Drift W A reverse biased pin photodiode is illuminated with a short wavelength photon 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. V r

14 Avalanche Photodiode (APD) Electrode SiO 2 I ph R E hυ > E g n + p e h + š p + (a) ρ net Electrode (b) x E(x) (c) Avalanche region Absorption region x (a) A schematic illustration of the structure of an avalanche photodiode (APD) biased for avalanche gain. (b) The net space charge density across the photodiode. (c) The field across the diode and the identification of absorption and multiplication regions.

15 E h + E e E c e E v h + n + p š Avalanche region (a) (b) (a) A pictorial view of impact ionization processes releasing EHPs and the resulting avalanche multiplication. (b) Impact of an energetic conduction electron with crystal vibrations transfers the electron's kinetic energy to a valence electron and thereby excites it to the conduction band.

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17 Antireflection coating Electrode SiO 2 n + p Guard ring n n + p n (a) š p + Avalanche breakdown (b) š p + Substrate Electrode Substrate Electrode (a) A Si APD structure without a guard ring. (b) A schematic illustration of the structure of a more practical Si APD

18 Heterojunction Photodiode V r I ph Electrode InP InP InGaAs R V out hυ EE e h + P + N n n + E(x) Avalanche region Absorption region x 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.

19 E c E (a) InP E v E v e E c InGaAs (a) Energy band diagram for a SAM heterojunction APD where there is a valence band step E v from InGaAs to InP that slows hole entry into the InP layer. h + E v InP (b) E v InGaAsP grading layer InGaAs h + E v (b) An interposing grading layer (InGaAsP) with an intermediate bandgap breaks E v and makes it easier for the hole to pass to the InP layer

20 Photon Electrode Electrode n In 0.53 Ga 0.47 As (5-10µm) Absorption lay Graded n InGaAsP (<1 µm) N InP (2-3 µm) Multiplication layer. P + InP (2-3 µm) Buffer epitaxial layer P + InP Substrate Simplified schematic diagram of a more practical mesa-etched SAGM layered APD.

21 Superlattice APDs nm hυ e E E c E g2 E c p + E g 1 n + h + E v (a) (b) Energy band diagram of a staircase superlattice APD (a) No bias. (b) With an applied bias.

22 hυ Phototransistors Emitter Base Collector e n + p e h + E n The principle of operation of the photodiode. SCL is the space charge layer or the depletion region. The primary photocurrent acts as a base current and gives rise to a large photocurrent in the emitter-collector circuit. SCL SCL I ph V BE V BC V CC

23 Light n = n o + n p = p o + p l V I photo w d A semiconductor slab of length l, width w and depth d is illuminated with light of wavelength λ.

24 (a) (b) (c) (d) (e) h+ e Photoconductor I ph I ph I ph I ph I ph A photoconductor with ohmic contacts (contacts not limiting carrier entry) can exhibit gain. As the slow hole drifts through the photoconductors, many fast electrons enter and drift through the photoconductor because, at any instant, the photoconductor must be neutral. Electrons drift faster which means as one leaves, another must enter.

25 Current I d + I ph Illuminated P o I d + I ph + i n I d Dark n p Time R A V ou In pn junction and pin devices the main source of noise is shot noise due to the dark current and photocurrent. V r

26 Responsivity(A/W) Wavelength(µm) The responsivity of a commercial Ge pn junction photodiode

27 Responsivity(A/W) A B Wavelength(nm) The responsivity of two commercial Si pin photodiodes

28 Responsivity(A/W) Wavelength(nm) The responsivity of an InGaAs pin photodiode

29 Photogenerated electron concentration exp( αx) at time t = 0 v de A W B x hυ > E g E h + e i ph R V r An infinitesimally short light pulse is absorbed throughout the depletion layer and creates an EHP concentration that decays exponentially