The Hewlett Packard 4145A Semiconductor Parameter Analyzer is an extremely useful tool for characterizing semiconductor devices. It has up to four probes that can serve as voltage sources, current sources, voltage monitors, and current monitors. These probes are designed to be programmed through a menu-driven user-interface. Possible measurements include, I-V curve of a simple two terminal device, Id-Vds-Vg graphs for MOSFETs, switching characteristics of logical gates, etc.
Programming the HP4145 to do a particular measurement requires stepping through a short sequence of menu screens to
- define the connections
- define the variables or parameters to be measured
- set up the graphic display of the measured data
- take the data.
- When you first approach the HP4145 you should see the MENU page. If the menu page is not displayed, simply push the menu key, and the menu page should appear. The menu page has a number of different choices that you can make using the soft keys. You can play with these things later. For now, simply go to the next page in the normal sequence by pushing the NEXT key
- Push NEXT to go to the CHANNEL DEFINITION page.
- This is where the operation of the each of the SMUs (source monitor units - the probes) is defined. A SMU can operate as either a combination voltage source/ammeter or current source/voltmeter. Depending on the type of measurement, you can use 2, 3, or 4 SMUs, and if that isn't sufficient, there are two more dedicated voltage sources (Vs 1 and Vs 2) and two dedicated voltmeters (Vm 1 and Vm 2) that can also be used.
- Variable names - The names are simply labels that you attach to the particular voltage and current. For instance, if you are measuring a diode, you might label the voltage for one of the SMUs as Vdiode and the current as Idiode. But these are simply names for the variables, the actual names used aren't terribly important.
- SMU Mode - The entries under the SOURCE columns define how the SMUs will operate. There are three possible MODEs: V (for voltage source), I (for current source), or COM (for the common terminal). There must always be one common terminal, and all voltages will be defined with respect to the common terminal.
- SMU Function - Once the MODE of the SMU has been defined, then the FCTN further defines how the SMU will be used. The SMU source will either be CONST (constant), meaning that the voltage or current is fixed at some value, or the SMU can function as a variable source, in which case the voltage or current can be stepped through a sequence of values. There are two choices for the variable, VAR 1 and VAR 2, so that you can change voltages or currents on two SMUs in any given measurement. An obvious example of when you would need two SMUs would be when measuring iD vs. vDS curves for a MOSFET. In that case you would set the gate voltage at some voltage, and then step the drain voltage through a range of values, measuring the current at each point. Then you would change the gate voltage to some new value and measure ID vs. VDS for the new drain voltage. When using both VAR 1 and VAR 2, the 4145 will start by going to the first value of VAR 2, and then step all the of prescribed values for VAR 1. Then VAR 2 will be incremented to the requested value, and VAR 1 will again be stepped through all of its prescribed values. In programming parlance, VAR 1 and VAR 2 work like nested DO-loops, with VAR 1 being the inner loop and VAR 2 the outer loop. A final note: If your measurement requires changing only one variable, you must use VAR 1.
- Your own functions - Notice at the bottom of the channel definition page, there are spaces for entering user-defined functions. You won't always want to measure a simple voltage or a current. For instance, in characterizing MOSFETs, it is sometimes useful to measure the square-root of the drain current.
- Canned programs - The 4145 has several common measurement setups stored in memory. These can be accessed using the soft keys. The three canned setups are: 1) IC vs. VCE for BJTs (B-Tr VCE-IC soft key), 2) ID vs VDS for FETs (FET VDS-ID soft key), and 3) I-V for diodes (or any two-terminal device) (DIODE VF-IF soft key). Simply punch the appropriate soft key and the setup is done.
- Once you are finished with the channel definitions, you will need to tell the 4145 the range of voltages and currents that you want to measure. This is done on the SOURCE SETUP page. Push the NEXT key to go there, and you should see a page similar to the one shown below.
- Variable Names - The variable names show up in the first row. These were determined previously on the channel definition page.
- Sweep mode - The sweep mode determines how the measurement points will be spaced. You can have either a normal linear spacing or a variety of logarithmic spacings. Linear spacing is most common. Notice that these are entered using soft keys.
- Setting the measurement range - You must use the entry keys to enter the next bits of information. For VAR1, you need to select a starting point, a stoppint point, and the size of the step. The 4145 then calculates the number of steps for VAR 1. Curiously, the procedure for VAR 2 is slightly different. For VAR 2, you must enter at starting point, the step size, and the number of steps. The 4145 will determine the stopping point for VAR 2. The maximum number of points that can be measured is 1024. If you attempt to exceed that number, the 4145 will simply measure the first 1024 points and then stop.Subtle point: When entering values like the start and stop points you need not enter the standard units (V or A). However, if you want to enter the value in mV or µV or something like that, you must include the unit prefix. For example, each of the following is a viable way for entering 20 millivolts: 0.02, 20m, 20000u.
- Compliance - Next, enter a compliance limit for VAR1. The compliance is a way of protecting the device being measured. If the SMU is used as a voltage source, the compliance is the maximum current that will be allowed to flow through that terminal of your device. If the SMU is a current source, the compliance will be the maximum voltage that will be applied to that terminal of your device.
- Constant sources - Finally, enter values for any constant sources that may be used in your measurement.
- Now move on to the MEAS & DISP SETUP page by pressing the NEXT key. This page is used to set up the graphics display for the data to be collected. It is important to remember that entries here affect only how the data will be plotted - it is independent of the information entered on the SOURCE SETUP page. This means that you could set up the on-screen graph so that none of the collected data would appear! That data is not lost (it's stored in the machine's memory). All you would need to do in such a case is to redefine the graph.*
- You should see the word GRAPHICS following DISPLAY MODE: You can choose other ways to look at the data using the soft keys. GRAPHICS mode is the most commonly used display mode. Sometimes LIST mode is useful. LIST mode displays the raw data in one long list.
- Enter the names of the things you wish to have plotted. You have considerable flexibility here. Remember that for a given SMU, the 4145 is setting one parameter (voltage or current) and measuring the other. So for each SMU, you have both voltage and current data that can be used in constructing a plot. You can also use any user-defined functions. Finally, note that you use double-y-axis graphs, so if appropriate you can plot two data sets on a single graph. The tutorials provide several examples of how to set up graphs.
- Enter MIN and MAX values for the x and y-axes of the graph. You will have to use your best judgment on these values. If you don't get a nice graph after the measurement, you can always come back to this page and adjust all these settings to give you a better graph.
- Finally, you are ready to take some data. Push the NEXT button again to get to the GRAPHICS PLOT page. This is where all the action occurs. You should see an empty graph, set up according to the entries from the Meas & Disp Setup Page. To measure data, you need to push one of the measurement keys. SINGLE will cause the 4145 to sweep through the full range of VAR1 and VAR2 points, measuring the current at each point, and plotting the results. Each time you press the SINGLE key, the previous data will be erased and new data plotted in its place. REPEAT has the same effect as continually pushing the SINGLE key - the 4145 will repeatedly sweep through the measurement cycle until you push the STOP key. APPEND is very similar to SINGLE, except that the old data is not erased. You will get two or more plots, with each new one being superimposed over all the previous plots. This is very handy for making comparisons between different devices or for measurements taken under different conditions (eg. with lights on and lights off).
- Now that you have some data, there are a number of fun things you can do with it. Many of these things are accomplished using the soft keys associated with the graphics display page. There are five levels of soft keys that go with this page, so you will need to push the EXTN soft key a number of times to find all of the available operations.
- Auto Scale - It is quite possible that the graphing range you chose on Meas & Disp Setup Page wasn't appropriate, so that the graph is mostly (or even completely) empty. AUTO SCALE will automatically resize the graph so the data very nearly fills up the whole graph. This is very handy. The only drawback is that the resulting scale is usually an awkward value like 3.29mA/div. If you want a more readable graph, you can use AUTO SCALE to get a rough idea of the range of the data, then go back to the MEAS & DISP SETUP page and pick more sensible ranges.
- Commnt - You can enter a comment that will appear below the graph. Comments must be less than 30 characters long.
- Grtcl tick - You can turn off the grid lines and leave just tick marks.
- Markers - Pressing the MARKER key turns on a marker that can be moved along the data by turning the marker dial (the steering wheel located in the center of the front panel). As the marker is moved along the graph, the numerical x and y-values are displayed above the graph. If you have data plotted using the y2-axis, you have two markers that move together as you turn the wheel. Usually the marker jumps from point to point. If you think you need to interpolate between two points, you should press the INTERPOLATE soft key. If your measurement used a VAR2 so that there are family of curves plotted, the MARKER SKIP soft key causes the marker to move to the next VAR2 curve without changing the VAR1 value. (This might be useful when measuring transistors later.)
- Cursors - There are two cursors, a short one and a long one. The cursors differ from the marker in that the cursors can be moved to any point with the boundaries of the graph whereas the marker only moves along the data points. Cursors are moved using the cursor control keys. Using the soft keys which have sets of vertical or horizontal arrows on them, you can "zoom in" and "zoom out" around the cursors. Arrows pointing out from the center zoom in (make it so you look more closely at a smaller area, like increasing the magnification on a microscope) by factor of 2. Arrows pointing toward the center zoom out, or decrease the magnification by a factor of 2. MOVE WINDOW moves the graph so that the cursor is at the center of the plot area. Finally CURSOR (with an arrow and a dot) (which is on the next set of soft keys) moves the cursor to the marker, if the marker is already turned on.
- Lines - You can use lines to help evaluate the graphical data. The LINE ON soft key enables these capabilities. There are two lines you can use (LINE1 and LINE2). (Note that you don't need to push LINE ON prior to implementing a line. If you simply push LINE1 or LINE2, the line features come on automatically.) Typically you use the lines to find the slope of a curve over some portion of it. Each line has two cursors that are used to move the lines. One cursor is fixed and the other is movable and these can be interchanged using the CHANGE POINT soft key. Probably the easiest way to manipulate lines is to use a marker and the CURSOR (with right arrow) key. First move the marker to a point on the curve where you want a line to pass through and hit the CURSOR(with and arrow and a dot) soft key. This moves the movable cursor to that point. Then move the marker to another point on the curve where you want the line to pass through. Press CHANGE POINT and then CURSOR . The line should be pass through the interesting part of the curve. Values for the slope (GRAD), the inverse of the slope (1/GRAD), and the x and y-intercepts for the line are given below the graph.
- Integration time - Finally, if you are performing measurements at very low voltage or current levels or are in an electrically noisy environment, you can alter the integration time of the measurement to try to lessen the effects of noise. Integration simply means that a voltage or current is measured over some period of time, and the average value is recorded. Increasing the integration time should help average out some of the random noise. You have three choices for integration time: "short", "medium", and "long". (If you need to know exactly what these mean, you are welcome to check out the manual.) These can be selected using the buttons in the upper right-hand corner. of the front panel. The 4145 starts up with the integration set to "short". The is a trade-off for using more integration is longer total measurement time.
- Plotting graphs - The 4145 will plot your graphs on an HP plotter. This is rather useful since it is generally much easier to show off your data on a piece of paper than it is to lug the 4145 around. Start the process by inserting a sheet of paper into the tray of the plotter, making sure that the paper is against the guides in back and on the left-hand side. Then flip the small lever on the right to pinch the paper between the rollers. To plot the entire graph, push the PLOT button (on the right-hand side of the front panel). Near the bottom of the display will appear the word PLOT, followed by 4 numbers. The numbers correspond to xmin, ymin, xmax, ymax respectively, for the plot on the paper. As such, the numbers are not particularly meaningful. A minimal amount of experimentation has shown that it is relatively convenient to plot one graph per half page of paper. To plot on the lower half of the page use the numbers 200, 200, 7200, 5000, and to plot on the upper half of the page use 200, 5200, 7200, 10000. Note that to change these numbers you must use the EDIT keys and there must be commas between numbers. If you want different size plots, you can experiment on your own.
- Saving programs and data - You probably noticed that the 4145 comes equipped with a disk drive. You guessed it - you can save your data and your measurement setups.
- Before doing anything, you need to initialize a disk. Go back to the MENU page (the very first one). Push the CAT (catalog) soft key. Then press EXTN to find INIT DISC. Insert your blank diskette and then press the INIT DISC soft key. You should see a message "Initialize OK?" on the screen. Push the EXEC soft key and the process will start. The data will be saved in the 4145's own format, so the disks can't be read in a PC.
- Once the disk is initialized, you can save your programs and data. Once saved, programs and data can be read back into the 4145 at a later time. These actions are done with the User File keys, GET and SAVE (RESAVE). To save your data, push the SAVE button. Then enter a file name which must be prefixed with a capital D (for data). (e.g. Ddiode). To save the program, do the same thing, except the file name must prefixed with a P (for program, obviously).
- You can view the saved files on a disk by going back to the MENU page and pushing the CAT soft key again. The programs and data files stored on the disk will be listed. To load a file (data or program) into the 4145, press the GET key and enter the appropriate file name, again using the D or P prefix.
*DISPLAY MODE: GRAPHICS
|X AXIS||Y1 AXIS||Y2 AXIS|