Page 54 - Photonics Essentials an introduction with experiments

Photodiodes

48 Photonic Devices

c. The photovoltage response cannot be larger than V Bi , the built-in
voltage, which is always less than the (band gap energy)/q.

This means that the photovoltaic mode can be used to advantage in
situations where circuit simplicity or battery-free operation is an ad-
vantage, and where accurate conversion of photon flux to an electrical
signal is not an important requirement. Examples of such an applica-
tion are optical burglar alarms or solar cells.
An application example where use of the photovoltaic mode will
lead to erroneous results is its use in the measurement of the spectral
line shape of a light-emitting diode or a laser. The lineshape is charac-
terized by its full width at half maximum (FWHM). The nonlinear re-
sponse of the photovoltaic mode will cause the line shape to appear
broader than it actually is. The amount of the error will depend on the
details of the photodiode I–V characteristic.

3.4 Photodiode Properties

There are four important components of the performance of a photodi-
ode detector:

1. Spectral response: What is the range of optical wavelengths that
the photodiode can convert to electrical current?
2. Quantum efficiency: What is the ratio of the number of electrons
created to the number of incident photons?
3. Response time: What is the shortest optical pulse that the photodi-
ode can detect?
4. Noise: What are the sources of noise, generated by the photodiode,
that limit the minimum detectable signal?

We will consider the first two properties in this chapter. The speed of
response is covered in Chapter 4. The noise generated by the photodi-
ode needs to be considered relative to the amplification system that
follows the photodetector.

3.4.1 Spectral Response
The spectral response of the photodiode is directly related to the op-
tical absorption of the semiconductor materials used to make the
photodiode. Optical absorption occurs in an ideal semiconductor
when an incident photon has enough energy to break a bond in the
valence band, thereby promoting an electron into the conduction
band. Only those photons that have an energy greater than the band

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