Drexel University
ECE-E302, Electronic Devices
Lab IV: Switching Application of a Diode
Switching Applications of a Diode
Objective
To determine the carrier lifetime in a p-n junction and to examine the switching capabilities of various types of diodes.
Introduction
In the previous experiment, the forward biased and reverse biased characteristics of a diode were examined. As seen from this experiment, the current through the diode increases exponentially with the forward voltage across the diode. With the diode reverse-biased, the current through the diode is extremely small. This would suggest that a diode could be used as a switch. Under forward bias, a large amount of current can flow through the diode, but with a relatively small voltage across the diode – a short circuit. Under reverse-bias, almost no current flows through the diode, but a voltage potential still appears – an open circuit. While a diode is not a true switch, it is often used in switching applications since the switching times attainable with diodes is considerably smaller than with mechanical switches.
Theory
In a switching application, a diode is switched between a forward-bias state and a reverse-bias state. Figure 1 shows the experimental setup.
Figure 1. Circuit used and input waveform
In this setup, the diode is driven by a square wave generator which switches between +E and -E. When e(t) is positive, the diode is forward biased; when e(t) is negative, the diode is reverse biased. With e(t) constant at +E, the diode is forward biased, and almost all of E will appear across the resistor if E is much larger than Vo, the turn-on voltage of the diode. Thus, the steady-state current i = If ≈ E/R flows. When e(t) is switched to -E, the diode is now reverse biased. One would expect that almost no current would flow, as seen in the PN Junction lab. However, the stored minority carriers and excess majority carriers in the diode cannot be removed instantaneously. The excess majority carriers flow out of the system while the stored minority carriers flow back across the junction or recombine. Since the excess carriers (both minority and majority) are given by the relation Q = If tp in a p+-n junction, to remove these a reverse current Ir must flow for some finite time. Thus, an initial reverse current i = –Ir ≈ –E/R must flow. As the charge is redistributed, more and more of e(t) will appear across the diode, since the diode is reverse-biased. This will decrease the voltage across the resistor, and thereby the current becomes smaller as time proceeds. Eventually, the current will reach the low value of the reverse saturation current of the diode, as almost all of e(t) appears across the diode. Figure 2 shows the current as a function of time.
Figure 2. Variation of current with time
The time tsd is the storage delay time. It is the time it takes for the stored charge to become zero while the redistribution of charge takes place when switching from +E to –E. This delay time is an important factor in rating diodes for switching applications. It is desirable to have tsd small compared to the switching times necessary in a specific application. This storage delay time is a direct function of the carrier lifetime. For a p+–n junction the relationship is given by
where erf is the error function, a standard tabulated function. When both the forward and reverse currents flow for time long compared to p, it is possible to derive a simpler expression using the charge control equations, that is accurate enough for most purpose,:
Commercial diodes are accurately modeled as p+–n junctions, so the above equations will hold. If the forward abd reverse currents flow for short times, then
(3)
where Qp(0+) is the stored charge in the n-region in forward bias just as the voltage is reversed, and is given by
(4)
When If = Ir
(5)
where t is the time the diode is in forward bias, i.e., t = 1/2f where f is the frequency of the square wave generator.
Note that if one simply sets the charge removed by the reverse current in a time tsd equal to the stored excess charge, one finds:
(6)
and
which gives a good estimate if If < Ir.
Procedure
In this experiment, you will be testing various different types of diodes, and comparing their switching capabilities.
1. Setup the circuit as shown in Figure 1, with the value and power rating (in Watts) of R chosen so that the current through the diode will not exceed the rating of the diode, and the resistor will not overheat.
2. Set the function generator to produce a square-wave output, 10 V peak to peak amplitude, centered on 0 V.
3. Start with a 5 kHz square wave. Since we are interested in the current through the circuit, observe the voltage across the resistor with the oscilloscope. (The current is then the voltage divided by the resistance value.)
4. Measure tsd and record this value.
5. Increase the frequency of the square wave input, until tsd goes to zero or no longer decreases. Note this frequency.
6. Replace the diode with another type. Repeat Steps 1-5.
7. Repeat Step 6 for each diode to be examined.
Report
Determine the carrier lifetime for each measurement of tsd taken. For each diode, indicate the maximum frequency. Plot the maximum frequency versus lifetime. Discuss the results. Which of the diodes was the fastest? Which was the slowest? What makes one diode faster than another in the same switching application?
4-4