Chapter 4 Photodetectors
Types of photodetectors: Photoconductos Photovoltaic Photodiodes Avalanche photodiodes (APDs) Resonant-cavity photodiodes MSM detectors In telecom we mainly use PINs and APDs.
General concepts Basic principles for photodetection: The photoelectric effect in semiconductors: The absorption of a photon, with appropriate energy greater than the bandgap hν>e g, results the excitation of a valence band electron to the conduction band. This is accompanied by the excitation of a hole from the conduction band to the valence band. The resulting electron-hole pairs are free electron-hole pairs that can be transported to generate electric current (photocurrent). Relation to photocurrent material was discussed last time.
Basic operation of a PN photodetector: Absorption takes place in 3 places: (1) within depletion region, (2) away from it, and (3) near it. Those from category (1) generate photocurrent (as done in class) Those from category (2) recombine and are not useful Those from category (3) diffuse (if they don t recombine) to the depletion region and then generate photocurrent. Diffusion is a slow process, however. Bottom line: We want to confine the absorption to the depletion region A PIN diode has this advantage
V r (a) Electrode SiO 2 p + 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. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
1 10 8 5 4 3 2 1 Photon energy (ev) 0.9 0.8 0.7 1 10 7 Ge In 0.7 Ga 0.3 As 0.64 P 0.36 1 10 6 α (m -1 ) Si GaAs InP In 0.53 Ga 0.47 As 1 10 5 a-si:h 1 10 4 1 10 3 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Wavelength (µm) Absorption coefficient (α) vs. wavelength (λ) for various semiconductors (Data selectively collected and combined from various sources.) Figure 5.3
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) 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Responsivity (A/W) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Ideal Photodiode QE = 100% ( η = 1) Si Photodiode 0 200 400 600 800 1000 1200 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 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Quantum efficiency η, definition. Examples of QE for different detectors Responsivity: definition: R=η q P/hν = ηλ (in microns) /1.24 units of Amps per Watt Speed: carrier transport time τ tr ; RC-time constant τ RC f = 1/2π(τ tr + τ RC ) If the photodetector has gain, there is also a buildup time, τ b, associated with the gain that affects the bandwidth. τ b Tradeoff between responsivity and bandwidth
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. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Drift velocity (m s -1 ) 10 5 Electron 10 4 Hole 10 3 10 2 10 4 10 5 10 6 Electric field (V m -1 ) 10 7 Drift velocity vs. electric field for holes and electrons in Si. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Operation of PIN diodes Properties; 1. Controllable increase in width of the depletion region (good) 2. Reduced junction capacitance (reduced RC constant, good for speed) 3. Diffusion current is reduced due the increase of the depletion region width 4. Higher carrier transport time (bad for speed). 5. Next step: reduce absorption in doped layers in a PIN by using higher bandgap material such as InP (in the case of InGaAs absorber) 6. This is what PIN heterostructures do 7. Next thing is to have gain: impact ionization
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. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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. 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Modes of Operation of an APD Recall that APDs are photodetectors that have optoelectronic gain Linear mode (sub-breakdown): photocurrent optical power e.g., optical communication Geiger mode (post-breakdown): Ideally, each detected photon results in breakdown e.g., photon counting, coincidence counting
Current Photodetectors Needs for Long-haul Optical Communication High quantum efficiency at 1.55 µm (beyond 80%). High speed: 10 Gbps (OC 192) and beyond. Internal gain: ~10-50 (preferred over external EDFA amplification). Wavelength selectivity (WDM). Low noise (excess noise, dark current). Compactness, reduced cost, OEIC (solid-state).
Thin Multiplication- & Absorption-Region Benefits: High speed (up to 40 GHz +) APDs Low multiplication noise (factor of ~2) similar mechanism as noise suppression in superlattice MQW APDs). Higher optimal gain values: better SNR and BER Breakdown characteristics (Geiger mode) not so good Challenges: Quantum efficiency must be enhanced by employing new structures: Waveguide structures (lateral absorption) Resonant-cavity structures (vertical absorption) Detection efficiency is low 1.55µm and beyond Dark current is always a problem
Edge-coupled waveguide APD s: Idea: Reduce absorption-region (~0.8 µm or less) width without killing quantum efficiency. High gain-bandwidth (> 12 GHz at gain of 10) Reduce charge-space effects Challenge remains: coupling efficiency (QE ~25%) [Kinsey et al, 00] light 10 µm n: InAlAs InP buffer InP Substrate InGaAs cap p: InAlAs InGaAs 800 nm absorption p: charge InAlAs 400 nm multiplication 800nm
... Resonant cavity photodiodes Increase: Quantum efficiency Increase: Bandwidth Wavelength selectivity Drawbacks: Increased fab. complexity p Selectivity may not be desirable in some i applications n Input light p: InP InGaAs absorption layer n: InP } InP/InGaAsP Bragg reflector n- contact [Unlu et al, 1995]