1. 3/26/2003BAE 54131 of 10 Application of photodiodes A brief overview

2. 3/26/2003BAE 54132 of 10 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.

3. 3/26/2003BAE 54133 of 10 Photodiode structure

4. 3/26/2003BAE 54134 of 10 Photodiode fundamentals Based on PN or PIN junction diode –photon absorption in the depletion region induces current flow Spectral sensitivity MaterialBand gap (eV) Spectral sensitivity silicon (Si)1.12250 to 1100 nm indium arsenide (InGaAs)~0.351000 to 2200 nm Germanium (Ge).67900 to 1600 nm

5. 3/26/2003BAE 54135 of 10 Photodiode characteristics Circuit model –I 0 Dark current (thermal) –I p Photon flux related current Noise characterization –Shot noise (signal current related) –q = 1.602 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)

6. 3/26/2003BAE 54136 of 10 Photodiode current/voltage characteristics

7. 3/26/2003BAE 54137 of 10 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

8. 3/26/2003BAE 54138 of 10 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

9. 3/26/2003BAE 54139 of 10 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

10. 3/26/2003BAE 541310 of 10 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 RF is frequently required to suppress oscillation or gain peaking. A low bias current op amp is needed to achieve highest sensitivity