Lab-14 MOSFET

Objectives

This lab will introduce you to MOSFETs (metal-oxide-semiconductor field effect transistors). You will build a MOSFET inverter and determine the voltage transfer characteristic of this device (\(V_{out}\) vs. \(V_{in}\)). From this transfer characteristic you will learn how to extract information about the MOSFET such as threshold voltage (\(V_T\)).

In the cutoff and linear regions the MOSFET approximates the operation of a switch. In between these two regions lies the saturation region. In saturation the MOSFET makes a good amplifier.

Components Required:

One CD4007

One 10K ohm resistor

One Breadboard

One Bag of wires

Four pairs of banana to alligator

Note: The lab measurements can be done as a group. However, the plots should be done individually. All results should be submitted to Blackboard individually.

MOSFET characteristics:

The MOSFETs that you will be using for this lab are in a 14-pin package as shown in Figure 14-1. Study this diagram and become familiar with the layout of the individual MOSFETs within the CD4007 chip. Notice there are three n-channel devices and three p-channel devices. Although some devices share pins, all terminals for each device are available at an external pin on the chip.

MOSFETs are actually 4-terminal devices: gate, drain, source, and substrate. In the CD4007 the substrates for all n-channel devices are connected to pin 7. Likewise, the substrates for all p-channel devices are connected to pin 14. For proper function of the MOSFETs you must attach pin 7 to the lowest potential in your circuit (usually ground) and you must attach pin 14 to the highest potential in your circuit (\(V_{DD}\)).

MOSFETs are susceptible to electrostatic discharge (ESD). You have no doubt experienced large ESDs if you have ever scuffed your feet across a carpet and touched a metallic object. Even very small ESDs can damage a MOSFET by blowing-out the gate oxide. This is mainly because the gate oxide is very thin (<< 100 nm). These MOSFETs have been protected from minor forms of ESD by two clamping diodes attached to each gate. One diode prevents \(V_{GS}\) from exceeding \(V_{DD}\) (pin 14) + 0.7 V. The other prevents \(V_{GS}\) from becoming more negative than \(V_{SS}\) ( pin 7) - 0.7 V.

Concept: The DC voltage transfer characteristic of a MOSFET inverter

Build the simple MOSFET inverter shown in Figure 14-2. Use the rightmost MOSFET in the chip: gate connected to pin 6, source to pin 7, drain to pin 8. Don’t forget to connect pins 7 and 14 as described above!

  1. Measure the voltage transfer characteristic for the MOSFET inverter by varying \(V_{in}\). Use the table below to guide you in selecting the appropriate data points.

Notice that the circuit is an inverter: when the input voltage is “high,” the output voltage is “low” and vice versa.

    1. When the gate reaches the threshold voltage, the MOSFET begins to conduct current through the drain (\(I_D\)) and starts to invert. Based on the measurements in part (a), what is the approximate Threshold voltage (\(V_T\)) for this MOSFET?

  1. Plot the square-root of Drain current against Input voltage (\(V_{GS}\)) . The active region now becomes a straight line. Extrapolate this straight line to the Input voltage axis . The intercept gives us the accurate Threshold voltage \(V_T\).

  2. Plot the voltage transfer characteristic (i.e., \(V_{out}\) vs. \(V_{in}\)) in your lab report. Identify the three regions of MOSFET operation on your plot:

Cut-off:\(V_{GS}~ \lt~ V_T\) Saturation:\(V_{GS}~ \gt~ V_T\) and \(V_{DS}~\gt~V_{GS}-V_T\) Linear: \(V_{GS}~gt~V_T\) and \(V_{DS} ~\lt~V_{GS}-V_T\)

  1. Submit all results into Blackboard  Brightspace.