Application of photodiodes A brief overview 4/11/2006 BAE 5413

Diode devices Check valve behavior When forward biased Diffusion at the PN junction of P into N and N into P causes a depleted non-conductive region Depletion is enhanced by reverse bias Depletion is broken down by forward bias When forward biased High current flow junction voltage When reverse biased Very low current flow unless above peak inverse voltage (PIV) (damaging to rectifying diodes, OK for zeners) A semiconductor diode's current-voltage, or I-V, characteristic curve is ascribed to the behavior of the so-called depletion layer or depletion zone which exists at the p-n junction between the differing semiconductors. When a p-n junction is first created, conduction band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (places for electrons in which no electron is present) with which the electrons "recombine". When a mobile electron recombines with a hole, the hole vanishes and the electron is no longer mobile. Thus, two charge carriers have vanished. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator. However, the depletion width cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a 'built-in' potential across the depletion zone. If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator preventing a significant electric current. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed resulting in substantial electric current through the p-n junction. For silicon diodes, the built-in potential is approximately 0.6 V. Thus, if an external current is passed through the diode, about 0.6 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be 'turned on'. Wikipedia 4/11/2006 BAE 5413

Conductivity of semi-conductor is increased. Quantum devices Absorption of a photon of sufficient energy elevates an electron into the conduction band and leaves a hole in the valence band. Conductivity of semi-conductor is increased. Current flow in the semi-conductor is induced. 4/11/2006 BAE 5413

Photodiode structure 4/11/2006 BAE 5413 Absorbtion in the depletion layer causses current to flow across the photodiode and if the diode is reverse biased considerable current flow will be induced Absorbtion of photodiodes in the depletion layer causes current to flow across the diode and if the diode is reverse biased considerable current will flow. 4/11/2006 BAE 5413

Photodiode fundamentals Based on PN or PIN junction diode photon absorption in the depletion region induces current flow Depletion layer must be exposed optically to source light and thick enough to interact with the light Spectral sensitivity Material Band gap (eV) Spectral sensitivity silicon (Si) 1.12 250 to 1100 nm indium arsenide (InGaAs) ~0.35 1000 to 2200 nm Germanium (Ge) .67 900 to 1600 nm 4/11/2006 BAE 5413

Photodiode characteristics Circuit model I0 Dark current (thermal) Ip Photon flux related current Noise characterization Shot noise (signal current related) q = 1.602 x 10–19 coulombs I = bias (or signal) current (A) is = noise current (A rms) Johnson noise (Temperature related) k = Boltzman’s constant = 1.38 x 10–23 J/K T = temperature (°K) B = noise bandwidth (Hz) R = feedback resistor (W) eOUT = noise voltage (Vrms) 4/11/2006 BAE 5413

Photodiode current/voltage characteristics 4/11/2006 BAE 5413

Trans-impedance amplifier function Current to voltage converter (amplifier) Does not bias the photodiode with a voltage as current flows from the photodiode (V1 = 0) Circuit analysis Note: current to voltage conversion 4/11/2006 BAE 5413

Diode operating modes Photovoltaic mode Photoconductive mode Photodiode has no bias voltage Lower noise Lower bandwidth Logarithmic output with light intensity Photoconductive mode Higher bandwidth Higher noise Linear output with light intensity 4/11/2006 BAE 5413

For the photovoltaic mode I = thermal component + photon flux related current where I = photodiode current V = photodiode voltage I0 = reverse saturation current of diode e = electron charge k = Boltzman's constant T = temperature (K) n = frequency of light h = Plank’s constant P = optical power h = probability that hv will elevate an electron across the band gap 4/11/2006 BAE 5413

Burr-Brown recommendations (TI) Circuit Optimization Burr-Brown recommendations (TI) Photodiode capacitance should be as low as possible. Photodiode active area should be as small as possible so that CJ is small and RJ is high. Photodiode shunt resistance (RJ ) should be as high as possible. For highest sensitivity use the photodiode in a “photovoltaic mode”. Use as large a feedback resistor as possible (consistent with bandwidth requirements) to minimize noise. Shield the photodetector circuit in a metal housing. A small capacitor across Rf is frequently required to suppress oscillation or gain peaking. A low bias current op amp is needed to achieve highest sensitivity 4/11/2006 BAE 5413