A photo diode is a PN junction constructed so that it can be exposed to light. When reverse biased, it behaves as a photo conductive device because its resistance changes with light intensity. In this case, the change in resistance manifests itself as a change in reverse leakage current. Recall that reverse leakage current in a conventional diode is due to thermally generated minority carriers that are swept through the depletion region by the barrier voltage. In the photo diode additional minority carriers are generated by light energy so the greater the light intensity, the greater the reverse current and the smaller the effective resistance.
Figure 18-26 shows a family of characteristic curves for a typical photo diode. For a fixed value of reverse bias (along a vertical line), ihe magnit ude of the reverse current increases with increasing light intensity. Along a line of constant light intensity, there is relatively little change in reverse current with increasing reverse voltage.
Example 18-11
The photo diode whose characteristics are shown in Figure IR-20 lias a reverse bias of 2 V. Find its resistance when the light intensity is SOOO II11/m~ and when it is 20,000 II11/m2
Solution.
Constructing a vertical line through the 2- V reverse-bias voltage in
Figure I X-26, we lind that it jntcrsccts the 5(}O()-IIll/Ill~ characteristic at approximately II( = I IIlA. It intersects the 20,(}()()-IIll/m! characteristic at approximately II( = 3.9 mA. Thus,
In the preceding example, it is worth noting that a fourfold increase in light intensity resulted in a very nearly fourfold decrease in resistance, indicating that the photodiode has good linearity. This property is apparent in the way the characteristic curves are nearly equally spaced, implying that the change in reverse current is proportional to the change in light intensity. But, from an applications standpoint, perhaps the most desirable feature of a photodiodc is its very fast response time, on the order of nanoseconds. The ability to change resistance very quickly when
When a small forward-biasing voltage is connected across the photodiode, it is found that reverse current continues to flow. This phenomenon can be seen in .he forward-bias characteristics of Figure IR-26 and is attributable to the excess carriers produced by the light energy. If the forward bias is made sufficiently large, the reverse current must eventually become O. As can be seen in the forward characteristics, the greater the light intensity, the greater the value of forward bias required to reduce the current to O.The implication of this result is that lightenergy creates an internal voltage across the terminals of the photodiode, positive on the anode side and negative on the cathode side, since an external voltage of equal . magnitude and opposite polarity is necessary to stop the /low of current. When the terminals are left open and the photodiode ‘is exposed to light, that internal voltage appears across the terminals and can be measured. The value increases with increasing light intensity and is identified as the open-circuit output voltage in Figure 18-26. When operated in this manner, as a voltage generator, the photodiode is said to be a photoooltaic device, rather than a photoconductive device
When the photodiode terminals are open-circuited, Ix = 0 in equation 18-26. Also, letting V (}C be the open-circuit output voltage, we substitute – V x = V’I(‘ in equation 18-25 and obtain
Example 18-12
A photodiode operated in the photovoltaic mode under moderate lijht intensity has an open-circuit output voltage 0[004 V. Assuming that the component of reverse current produced by light energy is directly proportional to light intensity, find the output voltage when the intensity is doubled. Assume that 1) = 1 and VT = 0.026 V
Solution.
From equation 18-29, Voc = 0.4 = (1)(0.026)ln(Ipll,). By the linearity assumption, doubling the light intensity doubles Ip, so we must find V’I< from
Phototransistors
Like a photodiode, a phototransistor has a reverse-biased PN junction that is exposed to light. In this case, the junction is the collector-base junction of a bipolar transistor. Figure 18-27(a) shows the currents that flow in an NPN transistor when the base is open. Recall from Chapter 2 that reverse leakage current luw flows across the reverse-biased collector-base junction due to thermally generated minority carriers. Also recall that the collector current in this situation i
Equation 18-30b shows that the collector current in a phototransistor is directly proportional to I,,, which, as in the photodiode, is proportional to light intensity. The advantage of the phototransistor is that it provides current gain and is therefore more sensitive to light than the photodiode. It is used in many of the same kinds of applications as photodiodes, but it has a slower response time, on the order of microseconds, compared to the nanosecond response of a photodiode. The phototransistor is constructed with a lens that focuses incident light on the collectorbase junction. Some are constructed with no externally accessible base terminal, and others have a base connection that can be used for external bias purposes. Figure IR-28 shows the schematic symbol and a typical set of, output (collector) characteristics for a phototrnnsistor. Notice the similarity of these characteristics to those of a conventional transistor. Lig,ht intensity in milliwatts per cubic centimeter serves as the control parameter instead of base current.
Example 18- 13
The photo trannsistor whose characteristics arc shown in Figure IX-2S is to be used in the detector circuit shown in Figure IK-21)(a). When the light intensity falls below a certain level, the collector voltage rises far enough to supply the 100 p.,A of gate current that is necessary to turn on the SCR. Find the value of Rc that should be used if the SCR must lire when the light intensity falls to 10 mW/cm’.
(Example 18-13)
Solution.
To fire the SCR, the collector voltage must rise to VCE = (100 /LA)(lOO kfl) = 10 V. Therefore, the de load line for the phototransistor must intersect Ole IO-mW/cIl12 characteristic at VCE = 10 V. As shown in Figure lR-29(b), the load line is drawn between that point and the point where it must intersect the Vu-axis at V( c = 15 V. (Use the output characteristic in Figure I X-‘2X to construct an accurate plot of the load line.) The load line is then extended to its intersection with the I(-axis, where I(“(JUI) “‘=’ 4.4 mA. Thus,
A pltoto darlington is a photo transistor packaged with another transistor connected in a Darlington configuration, as studied in Chapter II. Figure IX-3~ shows the conliguruion. With its large current gain, the photo darlington can produce greater output current than either the photodiode or phototransistor and it is therefore a more light-sensitive device. However, the photodarlington has a slower response time than either of the other two devices.
Figure IH-3J shows typical manufacturer’s specifications for photo diodcs, photo transisrors, and photo darling tons. Compare the response limes and sensitivities for these devices. Note, for example, that the response times of the photo diodes are all J ns, compared to rise and fall times on the order of 2 ~s for the photo transistors and 15}-ts for the photo darlingtons. On the other hand, the light currents range from only 2 }-tA at 5 rnW/cm” for a photo diode to 25 mA at 0.5 mWfcm2 for a photo darlington. Note also the very small dark currents of each, in comparison to the values of light-generated current.
