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Photodetectors Photodetectors • Convert light signals to a voltage or current. • The absorption of photons creates electron hole pairs. • Electrons in the CB and holes in the VB. • A p + n type junction describes a heavily doped p-type material(acceptors) that is much greater than a lightly doped n-type material (donor) that it is embedded into. • Illumination window with an annular electrode for photon passage. • Anti-reflection coating ( Si 3 N 4 ) reduces reflections. Vr (a) SiO 2 R Vout Electrode p+ Iph Photodetectors h" > E + g h e– n E + Antireflection Electrode • The side is on the order of less than a coating p W Depletion region micron thick (formed by planar diffusion ! (b) net into n-type epitaxial layer). eNd x • A space charge distribution occurs about the junction within the depletion layer. –eNa E (x) (c) • The depletion region extends x predominantly into the lightly doped n region ( up to 3 microns max) E max (a) A schematic diagram of a reverse biased pn junction photodiode. (b) Net space charge across the diode in the depletion region. Nd and Na 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) Photodetectors Short wavelengths (ex. UV) are absorbed at the surface, and longer wavelengths (IR) will penetrate into the depletion layer. What would be a fundamental criteria for a photodiode with a wide spectral response? Thin p-layer and thick n layer. What does thickness of depletion layer determine (along with reverse bias)? Diode capacitance. What does capacitance dictate? Response time. Vr (a) SiO 2 R Vout Electrode p+ Iph Photodetectors h" > E + g h e– n • Assume a reverse bias condition E (Vr) applied to the device. Antireflection Electrode coating W Depletion region (b) !net • The depletion layer presents a high resistance under this condition and a eNd x voltage of Vr + Vo is developed across W. (Vo is a built in voltage). –eNa • An E field develops in the depletion E (x) region and can be determined by (c) integration of the net space charge x density pnet. E max • The field is not uniform. It is maximum at the junction and tapers (a) A schematic diagram of a reverse biased pn junction photodiode. (b) Net space charge across the diode in the into the n region. depletion region. Nd and Na 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) Photodetectors Regions outside the depletion region (neutral regions) hold majority carriers. These neutral regions can be considered resistive extensions of electrodes to the depletion region. When a photon with an energy greater than bandgap ( E g ) is incident, the photon is absorbed and generates a free EHP in the depletion layer. Photodetectors • The E field separates the electron and hole and V causes them to drift in i (t) opposite directions until ph they reach neutral regions. • The drifting carriers 0 ev /L ev /L + ev /L Semiconductor h h e generate a photocurrent in iph(t) Charge = e the external circuit E developing an electrical (a) h+ e– te (d) v h ve signal. th • The photocurrent exist for l L ! l a time frame equal to the t time it takes for the 0 l L 0 evh/L eve/L x i(t) electron and hole to cross h+ e– the depletion layer and (b) (c) arrive at the neutral region. te te ie(t) • The magnitude of the t th h ih(t) photocurrent is dependant on the number of EHPs t t t generated and the drift velocities of the carriers (a) An EHP is photogenerated at x = l. The electron and the hole drift in opposite while moving across the directions with drift velocities vh and ve. (b) The electron arrives at time te = (L ! l)/ve and depletion layer. the hole arrives at time th = l/vh. (c) As the electron and hole drift, each generates an external photocurrent shown as ie(t) and ih(t). (d) The total photocurrent is the sum of hole and electron photocurrents each lasting a duration th and te respectively. © 1999 S.O. Kasap, Optoelectronics (Prentice Hall) Photodetectors • Note: the absorption of photons occurs over a distance that is dependant on wavelength. Remembering that the distribution of the field is not uniform tells us that determining the time dependence of the photocurrent signal is difficult. • The resultant photocurrent is a result of electron flow only not hole migration. • Integrating the hole current to calculate the Q charge will show that the total photogenerated electrons is eN (electrons) and not 2 eN (electrons and holes). SiO2 Electrode Electrode p+ PIN Photodetectors i-Si n+ (a) • Lower doping levels cause depletion region ! net to become thicker which in turn reduces eNd diode capacitance. (b) x • PIN photodiode implements this concept by –eNa insertion of a thick, high Z low doped n-type E (x) layer (middle layer) between the p and n x (c) layers of the original model. E o W • The middle layer is called the intrinsic layer or I-layer. E h" > Eg e– (d) h+ • A moderate quantity of reverse bias can extend the depletion layer to the bottom of I R V p h out the I-layer. Vr Result: 1. Faster response time. The schematic structure of an idealized pin photodiode (b) The net 2. Improved (wider) spectral response. 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) PIN Photodetectors p+ i-Si Diffusion – h! > Eg e E h+ Drift l W Vr 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. © 1999 S.O. Kasap, Optoelectronics (Prentice Hall) Responsivity (A/W) 1 -1 0.9 Drift velocity (m s ) 0.8 Ideal Photodiode 105 0.7 QE = 100% ( " = 1) Electron 0.6 0.5 104 ! Hole 0.4 g 0.3 Si Photodiode 103 0.2 0.1 0 102 0 200 400 600 800 1000 1200 104 105 106 107 Wavelength (nm) Electric field (V m -1) Responsivity (R) vs. wavelength (!) for an ideal Drift velocity vs. electric field for holes and electrons in Si. photodiode with QE = 100% (" = 1) and for a typical commercial Si photodiode. © 1999 S.O. Kasap, Optoelectronics (Prentice Hall) © 1999 S.O. Kasap, Optoelectronics (Prentice Hall) Silicon Photodetectors -- Interdigitated Lateral Trench Interdigitated electrodes are often used to increase the active region area while optimizing the electric fields in the carrier collection region. Electrode can either be p+/n+ or just metal. Finger space = 3.3 µm Trench depth = 8 µm Finger size = 0.35 µm For λ=845 nm, BW=1.5 GHz, Responsivity = 0.47 A/W at 5V Silicon Photodetectors -- Resonant-cavity-enhanced Why? High Speed Uses three pair of quarter wavelength SiO2 and polysilicon at bottom (LPCVD). SiO2 Side-wall to prevent defects at the edge of poly. Two pairs of ZnSe-MgF on top (evaporated). Silicon Photodetectors -- Schottky Barrier 300,000 PtSi/p-Si Schottky barrier IR detector focal plane arrays have been developed and used on Air Force B-52 Silicon Photodetectors -- Schottky Barrier • High dark current, has to operate at low temperature (40 ~ 80 K). • Low quantum efficiency (QE). 2 h) ' q* 2 & 1 1 # QE C ( B ) 1.24C $ ! = 1 = 1($ ' ! h) % ( (C " High λC gives high QE. To expand the spectrum, need to decrease the barrier height. Avalanche Photodiodes R Ip h Electrode SiO2 E h" > Eg e– h+ (a) n+ p s p+ ! n et Electrode (b) x E (x) (c) x Absorption region Avalanche region (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 E e– E h+ c e– Ev h+ n+ p s 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) Electrode Antireflection coating SiO2 + + n Guard ring n n p p n s Avalanche breakdown s (a) (b) p+ p+ Substrate Substrate Electrode Electrode (a) A Si APD structure without a guard ring. (b) A schematic illustration of the structure of a more practical Si APD © 1999 S.O. Kasap, Optoelectronics (Prentice Hall) Vr Ip h Ec E Electrode R Vout InP InP InGaAs (a) InP e– (a) Energy band diagram for a – SAM heterojunction APD where EE e there is a valence band step !Ev h! Ev !E Ec from InGaAs to InP that slows + v h InGaAs hole entry into the InP layer. h+ Ev P+ N n n+ Avalanche InP E (x) Absorption region E region v InGaAsP grading layer (b) An interposing grading layer (b) (InGaAsP) with an intermediate x InGaAs bandgap breaks !Ev and makes it h+ E easier for the hole to pass to the InP v layer Simplified schematic diagram of a separate absorption and multiplication (SAM) APD using a heterostructure based on InGaAs-InP.
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