SC5-1
RECOMBINATION TIME IN SEMICONDUCTOR DIODES
Objective
The purpose of this experiment is to measure the recombination times and the junction
capacitance of typical diodes.
Introduction
If a square wave is applied to a diode, neither the current through the diode nor the
voltage across it is a square wave. This is the result of a series resistance in the diode, junction
capacitance and a finite recombination time for the minority charge carriers. Hence it is possible
to determine these three parameters from the switching characteristics of the diode. It is
necessary to know these parameters for the diodes whenever they are used in high frequency
circuits. Some diodes are manufactured to exaggerate some of the parameters so that the diodes
may be used in special applications. e.g. Varactor diodes have a junction capacitance which
varies over a considerable range with an applied D.C. voltage -- they may be used to voltage tune
FM receivers. Step-recovery diodes have a long recombination time but small junction
capacitance, thus giving rise to a sharp step in their switching characteristics -- this property
enables the generation of frequencies above 1 GHz from low frequency sine waves.
The circuit being used to observe the diodes
is shown in Diagram 1 and the expected waveforms
are shown in Diagram 2.
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Explanation of the turn-off transient
Once the diode is turned on and has reached a steady state condition, the forward current
is If = Vf/R. Now the input voltage is suddenly reversed. In particular the current applied to the
diode is suddenly reversed to Ir. You will note that although the applied current reverses at the
same time as the input voltage Vi, the diode voltage does not. This occurs because the minority
carriers that were established in the junction region with If must first be swept out (recombined
with opposite polarity charge), much as one would first have to discharge a capacitor to zero
charge and voltage before one could recharge it with the opposite polarity. Thus we speak of a
"diffusion" capacitance Cd being in parallel with the ideal diode. The effective circuit is shown
below.
This effect gives rise to the storage delay time ts as shown on the waveforms. The storage time
occurs whenever a diode is switched from forward conduction to reverse bias, and is a
consequence of the storage of excess minority carriers in the neutral regions of the diode. The
storage delay time can be reduced by removing the stored carriers faster, which is effected
eitherby reducing the lifetime τ or by increasing the reverse current Ir.
The storage delay time can be used to estimate the lifetime of the carriers in the
semiconductor of which a diode is fabricated. Diagram 4 shows ts/τ as a function of Ir/If.
Once the minority carriers have been cleared
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out of the junction region, the reverse current charges up the junction capacitance Cj (a real
capacitance) to the negative voltage Vr. As this occurs, the diode junction becomes reverse
biased and reduces the current to the typical reverse leakage current Io, unless Vr is sufficient to
cause the diode to enter the avalanche region.
The time taken for the above to happen is called the recovery time tr. This time can be
used to estimate the junction capacitance:
tr ~ 3RCj where R ~ Vf/If and Vf = Vi - If X 100Ω
The factor '3' comes from using the full tr rather than measuring the time for a (1/e) change in the
current.
Procedure
1.
(i)Familiarize yourself with the apparatus to be used. The function generator and the
oscilliscope are a single unit. There should be a “TEE” connected to the output of the function
generator portion of the unit, with one end of the “TEE” connected directly to CH1 of the
oscilliscope (to give you the input signal) and tthe other end going first to the diode holder and
the output going to CH2 of the oscilliscope.
(ii)Disconnect the diode holder from the function generator output. Turn the power on
and allow a few minutes for the oscilliscope to warm up. On the function generator settings
select a square wave. In the Range(Hz) section select 100K. Adjust the Frequency to the 0.5
position. Set the TIME/DIV knob to the 2µS position, and then set the VOLTS/DIV for CH1 to
the 2V position. On your oscilliscope you should see a single square wave. (If you do not, first
check that you have CH1 selected as your Vertical Mode and that the DC channel (just above the
CH1 input) has been selected. After that try adjusting the amplitude of the square wave, and
make sure that the ‘OFFSET’ knob is pushed in.)
(iii)Adjust the horizontal position of the square wave until it is centred on the oscilliscope
tube. Measure the frequency of the signal (you should get a value close to 50KHz). Adjust the
amplitude of your signal until it is 10 Volts Peak to Peak. Now attach the diode holder to the
other end of the “TEE” from your function generator output. The wave form on CH1 should
now look like the graph of Vi vs t in Diagram 2. Switch the Vertical MODE to CH2. The
waveform on CH2 should look similar to the Id vs t graph in Diagram 2. (Note: Your
oscilliscope is actually showing you the voltage across the 100Ω resistor and not the current
through the diode which you will have to calculate yourself).
2.
Pull out the OFFSET knob on the Function Generator. This will allow you to add a DC
voltage bias to your square wave signal. Observe the effect on Vi and Id as you vary the
OFFSET knob. Varying the OFFSET knob changes the (Ir/If) ratio and hence the storage time.
3.
Select D.C. settings for both CH1 and CH2. For each of the two diodes provided
measure ts, Ir and If. Do this using 5 different ratios of (Ir/If). The ratios must be in the range of
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0.01 and 6.0 in order that you can use the graph provided on page SC5 - 2 of (ts/τ) versus (Ir/If).
Using the graph and your measurements calculate the recombination time τ for the material from
which the diodes are made: one is germanium(the siver “top-hat”), and the other is silicon(the
small back diode with the silver stripe).
4.
Using a very short ts time, measure tr, If , and Vi for each diode. Calculate R and
estimate the junction capacitance for each diode.