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

4/11/2006BAE Diode devices Check valve behavior –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)

4/11/2006BAE 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.

4/11/2006BAE Photodiode structure 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

4/11/2006BAE 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 MaterialBand gap (eV) Spectral sensitivity silicon (Si) to 1100 nm indium arsenide (InGaAs)~ to 2200 nm Germanium (Ge) to 1600 nm

4/11/2006BAE Photodiode characteristics Circuit model –I 0 Dark current (thermal) –I p Photon flux related current Noise characterization –Shot noise (signal current related) –q = x 10–19 coulombs –I = bias (or signal) current (A) –i s = 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) –e OUT = noise voltage (Vrms)

4/11/2006BAE Photodiode current/voltage characteristics

4/11/2006BAE Trans-impedance amplifier function Current to voltage converter (amplifier) Does not bias the photodiode with a voltage as current flows from the photodiode ( V 1 = 0) Circuit analysis –Note: current to voltage conversion

4/11/2006BAE Diode operating modes Photovoltaic 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

4/11/2006BAE For the photovoltaic mode I = thermal component + photon flux related current where I = photodiode current V = photodiode voltage I 0 = reverse saturation current of diode e = electron charge k = Boltzman's constant T = temperature (K) = frequency of light h = Plank’s constant P = optical power  = probability that hv will elevate an electron across the band gap

4/11/2006BAE 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 C J is small and R J is high. Photodiode shunt resistance (R J ) 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 R f is frequently required to suppress oscillation or gain peaking. A low bias current op amp is needed to achieve highest sensitivity