This page contains a Flash digital edition of a book.
characterisation  industry


up to 50 A; two units can be combined using the TSP- Link bus to pulse up to 100 A. It can capture transient behavior such as changing thermal effects with one- microsecond per point (1 MHz) sampling. The width of


a sourced pulse can be programmed from 100 µs to DC and duty cycles from 1 percent to 100 percent are also programmable.


The Model 2651A provides a digitizing measurement mode that uses 18-bit A/Ds for characterising transient behavior precisely. A separate integrating measurement mode, based on 22-bit A/Ds, provides the maximum measurement accuracy and repeatability.


For applications like studying the thermal impedance of power diodes and LEDs, characterising the slope of the measured voltage at the top of the pulse is important. This capability is also useful for characterizing pulse amplitude flatness. The Model 2651A’s high speed A/Ds simplify digitizing the top of the pulse accurately when the measurements are made synchronously with the source.


What are the key areas of power semiconductors that this testing platform addresses?


Q A


Perhaps the most significant area is the enhanced efficiency of new materials and the testing challenges that come along with that greater efficiency. ‘More efficient’ means that when the semiconductor is ‘on’, it’s really on and when it’s ‘off,’ it’s really off. Because it is designed to source and measure pulses of up to 50 A and measure voltages down to a microvolt, the Model 2651A offers the developers of new materials the ability to characterize the resistance from drain to source when the device is on (RDSon


) with high accuracy. At the same


time, manufacturers of these new materials are striving to minimize leakage current from drain to source when the device is ‘off’(IDSoff


); with its one-picoamp current


measurement resolution, the Model 2651A makes it possible to characterise this parameter with high confidence.


What are the key areas of LED brightness that this testing platform addresses?


Q A


One of the methods HBLED manufacturers use to control the brightness of the devices they produce is known as pulse width modulation. In this technique, the current through the HBLED is pulsed at a constant frequency with a constant pulse level, but the width of the pulse is varied. This changes the amount of time the device is in the ‘on’ state, as well as the perceived level of brightness. In this drive scheme, the HBLED is actually flashing, but the frequency of the flashing is so high that the human eye can’t distinguish it from a constant light level.


Although it’s possible to control the brightness of a HBLED simply by lowering the forward drive current, the pulse width modulation technique is preferable for several reasons, the most important of which is to maintain the consistency of the colour of the light as the device’s brightness is reduced. In a HBLED, the colour of the light it emits is related to the forward voltage at which it operates. Although the forward voltage will remain relatively constant as the forward current is changed, it actually does vary by as much as tens to even hundreds of millivolts. This occurs especially at lower current levels. This slight variation in forward voltage equates to a slight variation in light colour, which is undesirable for the end user. If heating effects are ignored, in the pulse width modulation technique, the LED is pulsed using exactly the same current level on every pulse, so the forward voltage is the same for every pulse; therefore, the colour of the light emitted won’t vary.


Fortunately for HBLED device developers, the Model 2651A is capable of outputting a pulse width modulated waveform with up to 100 percent duty cycle from 020 A, 50 percent duty cycle from 20–30 A, and 35 percent duty cycle from 30–50 A. Its advanced trigger model allows for precision pulse widths and duty cycles and tight synchronization with other instruments. These synchronization features can be used to combine two Model 2651As to achieve a pulsed width modulation waveform with pulse current levels twice as high as a single Model 2651A allows with the same duty cycle.


Keithley has a long history of test and measurement in the semiconductor and related industries. What are the key issues that Keithley sees facing the industry over the next five to ten years as more advanced and multiple device requirements are needed to meet roadmap intentions?


Q A


Obviously, the demand for higher efficiency devices won’t be going away. That means that not only will current manufacturers be experimenting with new materials – new companies will also enter this segment of the market. Typically, when that happens, to meet the new manpower demands, less experienced people are going to be chasing more complicated technologies. That obliges Keithley and other test vendors to keep producing products that are as simple as possible to use, so someone doesn’t have be a test expert to start using them effectively. It also means we have to stay on top of providing applications support to get these new users up to speed quickly so they can find the products they need to do their jobs more efficiently. High accuracy products alone aren’t enough—we have to continue making those products easy to use.


© 2011 Angel Business Communications. Permission required.


October 2011 www.compoundsemiconductor.net 29


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200