EE 2212

EXPERIMENT 5

22 October and 5 November

MOSFET I-V Characteristics and MOSFET Circuits

Note 1: Because of the inter-related nature of the MOS topics, this laboratory set of experiments will extend over two weeks. Recall that there is no laboratory on 29 October, Fall Recess.

(May be a bit early for this but Thanksgiving recess is coming in a month!)

Note 2: The report will be due on 12 November and will be valued at 40 Points. All rubric values will be multiplied by two compared to the 20 Point rubric.

Note 3: There is a six (6) page limit exclusive of the cover page.

Note 4: You will have enough class backround through this Wednesday’s class to proceed through the NMOS resistively-loaded amplifier circuit.

Note 5: There is a noon seminar by Bruce Howell, Cirrus Design, which means that I will only be in the lab until 11; Karin will assist at noon. I am arranging the lab work such that the morning lab can complete the work during the second week.

PURPOSE

1. To measure the I-V characteristics of an N-channel MOSFET on the CD 4007 array

2. Measure and simulate the transfer characteristics (input/output characteristics) of:

Ø NMOS Inverter with a Resistive Load

Ø NMOS Inverter with an Active Load

Ø CMOS Inverter

Ø CMOS Inverter effective output (Thevanin) resistance.

COMPONENTS

Ø CD4007 MOSFET array

Ø 0.01mF capacitor

Ø 3.3 kW and 100 W resistors

PRELAB

Prepare a detailed circuit diagrams in your notebook of how you will connect an NMOS for measuring the I-V curves and how you will connect the inverter circuits. Study the material in Chapter 4. A complete manufacturer’s data sheet has been posted as a pdf file on the class WEB page.

The device you will use throughout this experiment is a CD4007B Transistor array. It contains three N-channel and three P-channel devices connected as shown. Detailed schematic diagrams and pinouts are available on the data sheet and also given below. Please use care when working with these chips. They are very susceptible to excessive voltage and ESD (Electro-Static Damage). Do not exceed the experiment settings in an attempt to make your experiment work. The pin configuration is given in Figure 5.1. Note that you will be using the CD4007B which have a lower maximum voltage rating than the CD4007UB. The diagrams are the same for both the “B” and “UB” suffix devices. Avoid exposing the chip to ESD (electrostatic damage). This time of the year often has low relative humidities which make ESD more of an issue. Do not exceed the V_DD maximums!!!

Study the I-V curves provided in the data sheets so that you have some idea of what to expect. Also study the chip circuit diagram. You should be able to identify the operation and function of all of the individual devices. Observe the input protection circuitry that we discussed in last Friday’s class.

Figure 5.1 Pin Configuration of CD4007.

Warning: Pin 14 should always be connected to the most positive dc voltage in the circuit. Pin 7 will always be connected to the most negative dc voltage in the circuit

(or else )!!!

PROCEDURE

I-V Characteristic of an N-channel MOSFET

Ø Connect the circuit shown in Figure 5.2. Use the NMOS connected to pins 6, 7, and 8 and 14. Remember to connect pin 14 also to the +VDS supply. Pin 7 is shown connected to ground. Use the built-in mA meter on the power supply to measure I_D . I discourage use of the digital multimeter to measure current because of the hassle in replacing the internal fuse. Use the voltage readout on the power supply as you sweep V_DD from 0 to 10 volts for each value of VGS from 0 to 6 volts in 1-volt increments. Note that V_T is in the 1 to 2 volt range. Refer to the data sheets where similar curves are illustrated.

Ø Another approach to measure current is to measure the voltage drop across a 100 Ω resistor connected from Pins 8 and 14.

Figure 5.2 I_D-V_DS As A Function of V_GS Characteristic Measurement For an NMOS

Ø Note that you should keep below I_D = 10mA; since this is the maximum rated value for this chip, consequently you may not be able to use all values of V_GS depending upon your chip. Plot data as you proceed.

Ø Using a spread sheet and extracting graphs from the spread sheet is a good way to display and understand the data.

Ø The CD 4007 is unforgiving for ESD and over voltage and over current.

Ø Plot your data and use a linear regression (least squares fit) to extract values for VTO, LAMBDA, and KP and develop a SPICE model that compares well with your measured curves. The objective is to obtain I_D versus V_DS for several different values of V_GS. Look at Figures 5 and 8 on the CD 4007 data sheet as a guide as to what to expect. You will have to assume W/L=1 because you do not know the actual values of W and L and then adjust KP accordingly. This model development from your parameter extractions should be included in your report. Develop a Shichman-Hodges model equation for your NMOS.

Ø Observe that SPICE syntax for K_n^’ (for an NMOS) and K_p^’ (for a PMOS) is KP independent whether you are modeling an NMOS or PMOS. Refer to Table 4.2 from the text; also on the EE 2212 WEB page.

INVERTER CIRCUITS

Refer to the three circuit diagrams in Figures 5.3, 5.4, and 5.5. All will operate with a VDD = +8 volt power supply. Remember Pin 14 should always be connected to the most positive dc voltage in the circuit. Pin 7 will always be connected to the most negative dc voltage in the circuit.

You will need to arrange for an offset voltage from the signal generator so that V_in does not go below zero volts.

To standardize on the SPICE simulations, use VTO = 2 volts for the NMOS and -2 volts for the PMOS; λ= 0.02 volts^-1, and KP = 50E-3. The default KP does not have enough gain. Use default values for all other SPICE model parameters.

1. Set up the NMOS Inverter with a Resistive Load as shown in Figure 5.3. Use the NMOS FET connected to Pins 6, 7, and 8. Plot the transfer characteristic. Identify the saturation, ohmic, and cutoff regions of operation. Connect the input and output to the horizontal and vertical inputs (respectively) of your oscilloscope set to the x-y mode. This arrangement allows you to display the transfer characteristic of the circuit. Suggest a Q-point to obtain the largest small-signal voltage gain. Verify your experimental results with a load line and SPICE simulation. You will need your model parameters as obtained earlier in this experiment. Observe that you will need to provide a dc offset from the signal generator.

2. Set up the actively-loaded NMOS Inverter as shown in Figure 5.4. Use M1 Pins 6, 7, and 8 and M2 Pins 3, 4, and 5. Connect the input and output to the horizontal and vertical inputs (respectively) of your oscilloscope set to the x-y mode. This arrangement allows you to display the transfer characteristic of the circuit. Plot the transfer characteristic. Identify the saturation, ohmic, and cutoff regions of operation for each FET. Suggest a Q-point to obtain the largest small-signal voltage gain. Verify your experimental results with a “load line” which consists of the M2 characteristic and SPICE simulation. Compare your results with the resistively-loaded circuit. Note that this circuit is different than the depletion mode inverter circuit discussed and SPICE demonstrated in class. Observe that you will need to provide a dc offset from the signal generator.

Figure 5.3 NMOS Inverter Resistive Load Figure 5.4 NMOS Inverter Active Load Figure 5.5 CMOS Inverter

3. Connect the CMOS inverter circuit of Figure 5.5. with the pins shown. You can also use the CMOS inverter FETS connected using Pins 9, 10, 11, and 12. Connect the input and output to the horizontal and vertical inputs (respectively) of your oscilloscope set to the x-y mode. This arrangement allows you to display the transfer characteristic of the circuit. Before connecting your function generator to the circuit input, adjust it for a 0-8 V triangular waveform at a frequency of 1 kHz. You will need to use the dc-offset control on your function generator to do this. That is an 8 volt peak-to-peak triangular wave added to a 4 volt dc offset. Observe and sketch the transfer characteristic, recording all critical values of voltage. Your report should include a PSPICE simulation of this circuit using your parameter extraction NMOS and PMOS models. Compare to the 4007 curves on the data sheets.

4. Refer to Figure 5.6. Measure the pulse response of the CMOS inverter with a capacitor C_o that has been added from the output to ground to “slow down” the output waveform so that measurements can be more easily made. Since the input of a CMOS gate is primarily capacitive, this also will provide the output behavior when a CMOS gate is driving many other CMOS gates (a capacitive load). Adjust your function generator to 0 to 8V square wave at a frequency of about 10 kHz, then connect it to the input. Measure the rise and fall times of V_out(t). You should be able to compute the effective value of the CMOS inverter output resistance, R_eq, from the rise and fall time measurements. Refer to what you did in Experiment 1 for a basic RC network. Compare your measured results with a PSPICE simulation.

A few more items to think about. Compare the static power dissipation of the four circuits CMOS Inverter, NMOS Inverter with a resistive load, NMOS inverter with an active load, and the NMOS NOR gate with a resistive load) when operated as switches/inverters as opposed to amplifier operation.

Now to assist with your mathematics skills:

How I feel about some aspects of WINDOWS 8 and now WINDOWS 10 is coming!