Characteristic Curves of a Junction Diode

Transcript

0:00Introduction

Hello everyone, here is a small puzzle before we start. Two students study the same diode, which carries eight milliamperes at zero point seven zero volts. One says its resistance there is eighty seven point five ohms. The other says it is just five ohms. Both read the same forward characteristic, so who is right? We will settle it by the end. This experiment is part of experimental skills in JEE, and questions on it do come up. So, let's begin.

0:31The two circuits

Look at the figure: forward bias on the left, reverse bias on the right. The diode is the triangle with a bar across its tip, on the far right of each circuit. The long box marked divider, with the arrow touching it, is a potential divider, and it lets us raise the voltage from zero. So let me ask you, why a potential divider, and not simply a rheostat in series with the diode? Yes, exactly, the divider changes the voltage across the diode smoothly, right from zero. We need that to trace the part of the curve below the knee, which we will meet in a minute. Okay? Now take forward bias, where the p side of the diode goes to the positive terminal, the plus sign on screen. The voltmeter, the circle marked capital V, has a small range, zero to one volt. Right? For reverse bias, the diode is simply turned round, so in the right hand circuit its triangle points up. And now the voltmeter has a larger range, zero to thirty volts in the figure. Now look at the meters at the top: m A, a milliammeter, on the left, and mu A, with the Greek letter mu, a microammeter, on the right. So why do we change the meter? Very good if you said the size of the current. In forward bias, the current is carried by the majority carriers, and it is of the order of milliamperes, so a milliammeter does the job. With me? In reverse bias, only the few minority carriers carry the current, so it is of the order of microamperes, and we need a microammeter. Clear?

2:06Reading the characteristic

Now let us read the curve itself. Capital V, in volts, runs across, and capital I runs up and down. But see the units: milliamperes above the axis, and microamperes below it, so the two halves are drawn on very different scales, okay? On the right, in forward bias, the current stays almost zero up to the knee. The knee, also called the threshold voltage, is about zero point seven volt for silicon, the S i on screen. For germanium, the G e, it is about zero point three volt, right? Beyond the knee, the current rises steeply, and the dark blue curve shoots upwards. Got it? Now look at the left side, the orange curve, in reverse bias. Suppose we keep raising the reverse voltage. Does the current keep growing, like a wire obeying Ohm's law? No, it hardly grows at all, and that surprises many students. The current stays tiny and almost constant, and we call it the reverse saturation current, okay? It stays so right up to breakdown, at minus capital V with the subscript b r, the dashed mark on the far left. Beyond that, the curve labelled breakdown plunges steeply down. Clear?

3:21Two kinds of resistance

Now, for a diode we define two kinds of resistance. The static, or d c, resistance is simply capital V over capital I, the voltage over the current at that point. Okay? The dynamic resistance is small r with the subscript d. It equals delta V over delta I, where capital delta, the little triangle, means a small change. So it is a small change in voltage, over the change in current it produces, at the working point. Right? Now look at the curve again. In which bias would you expect these two resistances to be very large? In forward bias, beyond the knee, a large current flows for a small voltage, so both of them are small. With me? And in reverse bias the current is tiny, so both are very large. Remember both, because our puzzle depends on them. Got it?

4:13Protecting the diode, and the effect of heat

Now, a practical point: beyond the knee the current rises steeply, so we put a resistance in series with the diode, to limit the forward current to its rated value. Okay? Here is one from the course. Suppose a silicon diode is to carry twenty milliamperes from a five volt supply. What series resistance, capital R, do we need? Start with this: how much of the five volts is left for the resistor? The silicon diode takes about zero point seven volt, so the resistor gets five minus zero point seven, which is four point three volts. Divide by the current, zero point zero two zero ampere, and that gives two hundred and fifteen ohms, the omega sign on screen. Right? Now suppose we repeat the experiment with the diode warmed. What happens to that tiny reverse current? Think about where it came from, the minority carriers. In a warmer diode, more minority carriers are generated by the heat. Clear? So the reverse saturation current rises rapidly with temperature, and the knee voltage falls slightly. Okay?

5:22Example 1: static resistance, 8 mA at 0.70 V

Now the examples, the easier one first. On the forward characteristic of a diode, the current is eight milliamperes at zero point seven zero volts. Find the static resistance of the diode at zero point seven zero volts. Pause the video and try it first. Done? Okay, let's check. First, put the current in amperes, because eight milliamperes is eight times ten to the power minus three ampere. With me? The static resistance is simply capital V over capital I, the voltage over the current. Zero point seven zero over eight times ten to the power minus three gives eighty seven point five ohms. And that is the first student's answer from our puzzle, right?

6:09Example 2: dynamic resistance from 0.70 V to 0.75 V

Now the harder one, for the same diode. On the forward characteristic, the current rises from eight to eighteen milliamperes, as the voltage goes from zero point seven zero to zero point seven five volts. Find the dynamic resistance. Pause the video and try it first. Done? Okay, let's check. First, the change in voltage, delta V, is zero point seven five minus zero point seven zero, which gives zero point zero five volt. Now, some students simply divide zero point seven five volts by eighteen milliamperes. Is that right? That is the trap, so be careful here, because that would be the static resistance at zero point seven five volts. The dynamic resistance needs the changes. So delta I is eighteen minus eight, which gives ten milliamperes, that is, ten times ten to the power minus three ampere. Okay? So small r with the subscript d, delta V over delta I, is zero point zero five over ten times ten to the power minus three, which gives five ohms. Clear? Now let us go back to our puzzle. The static resistance is eighty seven point five ohms, and the dynamic resistance is five ohms. So both students were right; they simply answered different questions. If you got both on your own, very well done!

7:38Quick revision: meters, knee voltage, static vs dynamic

So, quick revision. First, forward bias uses a zero to one volt voltmeter and a milliammeter, and the knee is about zero point seven volt for silicon, zero point three for germanium. Second, reverse bias uses a larger range voltmeter and a microammeter, and the current is tiny and almost constant. Third, the static resistance is capital V over capital I, the dynamic resistance is delta V over delta I, and a series resistance protects the diode. Now go and try the practice problems on this page, and all the best!