Page 77 - Photonics Essentials an introduction with experiments

Electrical Response Time of Diodes

Electrical Response Time of Diodes 71

This measurement of this capacitance in forward bias is not so easy as
the measurement of capacitance in reverse bias. A typical electronic
capacitance meter can make an accurate measurement of capacitance
under the condition that the conductance parallel to the capacitance
of the diode is small (R diode C diode > 1/measurement frequency). This is
true for diodes in reverse bias, but it is no longer true in forward bias
when the diode begins to conduct strongly. The capacitance can still
be measured, but a technique different from lock-in detection is re-
quired. An example of such a technique would be the use of a resist-
ance–capacitance bridge to determine the diode capacitance in for-
ward bias.
For devices that operate in forward bias, the product of the diffu-
sion capacitance and series resistance of the device will limit the elec-
trical bandwidth of the junction. Examples of such devices are LEDs,
bipolar transistors, and lasers. Since excess charge is necessary for
the diode to operate, efforts to improve the response time concentrate
on reducing the area of the device and reducing the series resistance.

4.7 Measurement of Diode Capacitance and
Carrier Concentration

A capacitance meter usually looks like any other meter. You connect
the leads to your device and it reads the capacitance. This is simple,
but how does it work?
A capacitance meter is secretly a lock-in amplifier. The design and
operation of the lock-in is described in Chapter 10, Section 10.7. Your
capacitor is put in series with a load resistor that is hidden inside the
meter. The capacitance–voltage (CV) meter generates a small ac test
signal that serves also as the reference for the lock-in. As you remem-
ber from your elementary circuit classes, the ac current going through
the resistor is in phase with the driving voltage, whereas the ac cur-
rent in and out of the capacitor is 90° out of phase with the voltage
across it. Your capacitor is probably not perfect. That is, it has some
shunt resistance, too. The lock-in compares the current that is in
phase with the driving voltage to the current that is 90° out of phase
with the driving voltage. The in-phase part of the current gives the re-
sistance, the out-of-phase component at +90° gives the capacitance.
(The out-of-phase component at –90° gives the inductance.)
The Boonton CV meter shown in Fig 4.6 has its own programmable
internal dc bias supply.
The diode is connected to the measurement terminals, and an ini-
tial measurement of the capacitance will be displayed on the screen.
The accuracy of such a measurement will be affected by the stray ca-

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.