Lab 7b
Collect Analog Data
In this exercise you will use your analog nanoammeter to record the current generated by a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET).
Reading Assignment
Building Scientific Apparatus. Section 6.3.2 (Field-effect Transistors and The Art of Electronics).
Supplies and Materials
Student Workstation. Card Edge Connector. FEEM Demo Apparatus (Optional). Analog nanoammeter from Lab 6; MOSFET: 2N7000; Resistors @ 1/4 W: 10 kΩ, 180 kΩ; Potentiometer, trim: 10 kΩ, 100 kΩ; Hookup wire, tinned copper (AWG 22); Wire cutter; Digital multimeter: (4 1/2 digit); Screwdriver (miniature), Xcelite R3323. MOSFET socket with 18" leads.
I. Introduction
The metal-oxide-semiconductor Field-effect transistor (MOSFET) is a building block of modern semiconductor devices. The first practical MOSFET was made in 1962 by Steven Hofstein and Fredric Heiman at the RCA research laboratory in Princeton, New Jersey. A MOSFET is a three terminal device. The insulated gate controls the flow of current between the drain and the source. See Figure 1
Figure 1. An N-Type MOSFET
In the absence of a gate voltage the source and drain regions of the MOSFET are electrically isolated from each other and no current will flow between them. Notice that the metal gate film and the channel region of the semiconductor are separated by a thin, insulating layer of silicon dioxide. The silicon dioxide layer provides the MOSFET with a very high input impedance. MOSFETs are fabricated on either a P-type or an N-type substrate.
II. A MOSFET Current Source
Consider a current source using an N-Type MOSFET. See Figure 2.
Figure 2. An N-Type MOSFET Current Source
When a potential difference, V, is applied between the gate and the source a current flows in the channel region between the source and the drain, resulting in a flow of current, I, in the external circuit. The body of the MOSFET is internally connected to the source (S).
III. Prepare the Card Edge Connector
Insert your nanoammeter into the socket on the Card Edge Connector. The nanoammeter components must face the row of pins on the Card Edge Connector. Turn the DC power supply ON or OFF by plugging its AC power cord into or out of a wall receptacle. If the card edge connector is not mounted on your breadboard see the Card Edge Connector instructions on the resource page.
power supply is turned ON.
IV. Construct a MOSFET Current Source
Construct a MOSFET current source. See Figure 3.
Figure 3. A MOSFET Current Source
As the potential difference between the gate and the source, ΔV, is increased a current will be measured by your nanoammeter. Set potentiometer A and potentiometer B to their minimum resistance.
V. Collect Analog Data
Copy the table from Figure 4 in your lab notebook. Set the output voltage of the nanoammeter to each target voltage in the first column of the table with potentiometer A (course control), and potentiometer B (fine control). Record the actual op-amp output of the nanoammeter in the second column of the table. Record the voltage across the potentiometers, ΔV, in the last column of the table. When you are finished set ΔV = 0.
Figure 4. Experimental Data
VI. Calculate an Input Current
Use the slope, m, and the intercept, b, from lab 6 to calculate an input current to the nanoammeter, I (nA), that would produce each output voltage in the second column of the table, where:
I = m(Op-Amp) + b
Enter the calculated current in the third column of the table.
VII. Collect Analog Data at 78K (Optional)
If a MOSFET is cooled in liquid nitrogen the current will be dominated by tunneling because the ordinary current (determined by the number of charge carriers) will decrease with temperature but a tunneling current will be independent of temperature. A convenient approach to cool the MOSFET is to insert it into a socket with 18" leads so that the socket can be immersed in a styrofoam cup filled with liquid nitrogen. When violent bubbling subsides thermal equilibrium will be reached and analog data can be collected at 78K.
VIII. Enter Your Data in a Spreadsheet File
Open Lab7Data.txt located in the CEsoftware folder. Enter the data from the table in your lab notebook under the headings I(na) and V. Save the file in your folder in the CEsoftware folder.
M Lenzlinger and E. H. Snow. J. Appl. Phys. 40, 278-283 (1968).
Fowler-Nordheim Tunneling into Thermally Grown SIO2.pdf
Z. A. Weinberg. J. Appl. Phys. 53, 5052-5056 (1982).
On Tunneling in Metal-Oxide-Silicon Structures.pdf
X. Homework Assignment
Update your lab notebook to include this exercise and questions raised in class.
Read the next laboratory exercise.
Complete GTutorial Exercise 10.
