A silicon photodiode with responsivity is illuminated with of optical power

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Semiconductor devices and analogue circuits · Sensors, detectors and experimental instrumentation

Electronics for Measurement, Ch The junction: an element whose current is exponential in its voltage

Problem

A silicon photodiode with responsivity

\mathcal{R} = 0.40\,\mathrm{A\,W^{-1}}

is illuminated with

5.0\,\mu\mathrm{W}

of optical power. Reverse-biased, it behaves as a current source and feeds its current into a

100\,\mathrm{k}\Omega

resistor to ground; dark current is negligible here. ((a)) Find the photodiode current and the voltage this produces across the

100\,\mathrm{k}\Omega

resistor with nothing else connected. ((b)) An oscilloscope's

\times 10

probe, of input resistance

10\,\mathrm{M}\Omega

, is now connected across the resistor to observe this voltage. Treating the photodiode as an ideal current source (so the resistor alone sets the Thevenin resistance the probe sees), find the voltage the probe actually reads and the percentage by which loading has reduced the reading. ((c)) Elsewhere on the same board, this same

2.0\,\mu\mathrm{A}

current happens to forward-bias a silicon protection diode with

I_S = 1.0\times 10^{-13}\,\mathrm{A}

and

n = 1.0

T = 298\,\mathrm{K}

(

V_T = 25.7\,\mathrm{mV}

). Someone designing the board uses the constant-drop model and assumes

0.60\,\mathrm{V}

across this diode. Find the diode's actual forward voltage from the diode equation, and state the model's error in volts and as a percentage of the true drop.

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