NMOS I-V Characteristics and at DC

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Morgan State University Department of Electrical and Computer Engineering EEGR 317 Microelectronics Credits – 4 LAB 2 : NMOS I-V Characteristics and at DC Lab Due: February 15, 2017 Objective: To study NMOS transistor I-V curves and DC biasing of an NMOS transistor by • Simulating a transistor to investigate the drain current vs gate-to-source voltage and drain-to-source voltage. • Implementing a circuit and taking measurements of the ID vs VGS and ID vs VDS curves. • Extracting values of kn , Vtn and λn . • Completing the DC analysis of three circuits: (1) an NMOS biased in the saturation region, (2) an NMOS biased in the triode region • Simulating the circuits to compare the results with the paper analysis. • Implementing the circuits in an experimental setting, taking measurements, and comparing their performance to theoretical and simulated results. • Qualitatively seeing the impact of transistor-to-transistor variations. Materials: 1. Laboratory setup, including breadboard 2. 1 enhancement-type NMOS transistor (e.g., 2N7000) 3. Several wires and resistors of varying sizes Part 1:Simulation::Resistive Circuit Network Many times circuit designs consist of complex circuit schematics that may contain hundreds of circuit elements. The knowledge of the voltage and current results for these elements must be determined. Conventional hand analysis is impractical for these types of circuits, so the electrical properties of the elements must be determined using simulation software. 1 Consider the circuit in Figure 1. Enter the circuit into your simulator’s schematic editor, applying DC voltage supplies to the gate and drain of the transistor. ID vs VGS While setting VDS to a constant value of 5 V, sweep the gate voltage from 0 V to 5 V in increments of 0.1 V. Plot a curve of ID vs. VGS . At what value of VGS does the current turn on? ID vs VDS For three values of VGS (2.5 V, 3.0 V, and 3.5 V), sweep the drain voltage from 0 V to 5 V in increments of 0.1 V. Plot the curves for ID vs. VDS onto a single graph, clearly indicating the value of VGS next to each curve. Part 2:Measurement Assemble the circuit from Figure 1, using a power supply to generate the DC voltages. ID vs VGS While setting VDS to a constant value of 5 V, sweep the gate voltage from 1.0 V to 3.5 V in increments of 0.25 V (note, we have reduced the number of data points with respect to the simulations), and measure the drain current using the power supply. (Note: Not all power supplies allow you to measure current accurately; if this is the case for your lab setup, you may place a small resistor in series with the drain and measure the voltage drop across the resistor.) Plot a curve of ID vs. VGS . At what value of VGS does the NMOS turn on? ID vs VDS For three values of VGS (2.5V, 3.0V, and 3.5V), sweep the drain voltage from 0V to 3.5V in increments of 0.5V, and measure the drain current using the power supply. Plot the curves for ID vs. VDS onto a single graph, clearly indicating the value of VGS next to each curve. Part 3:Post-Measurement Exercise Simulation vs. Measurement What are the main differences between your simulated and measured curves? Can you explain the differences? Parameter Extraction (1) Threshold voltage, Vtn From the measured ID vs. VGS curve, at what value of VGS does the NMOS turn on? Set this as the threshold voltage Vtn of your transistor. (2) MOSFET transconductance parameter, kn Based on the value of drain current ID at VGS = 3.0 V, and using the saturation model for the transistor, i.e., ID = (1/2)kn (VGS − Vtn )2 , extract the value of kn = µn COX (W /L). Using your extracted values of Vtn and kn, plot a curve of ID vs VGS , using the saturation model, and compare with your simulated and measured curves. Are there any differences? Can you explain the differences? (3) Early voltage, VA Based on your measured ID vs VDS curves for a saturated transistor, extract the Early voltage VA . Does VA change signi icantly for each value of VGS ? What is the average value of VA ? Based on your average value of VA , calculate ln = 1/VA . 2 Part 4:NMOS in Saturation Mode Consider the circuit shown below. Design the circuit in the igure such that ID = 1mA, VG = 0 V, and VD = +5V . Use supplies of V+ = −V− = 15V . Calculations • Based on the speci ications, calculate VOV · Set kn = 1.0mA/V 2 • From the datasheet, ind the threshold voltage Vtn of the transistor (Note: The datasheet may use a different symbol, e.g., VT H . . . ). What is VGS ? (Note: The datasheet indicates a range of values of Vtn that you will ind among the batch of transistors. Use the ”nominal” value in your calculations, but remember: Your actual transistor has a value of Vtn that falls somewhere in that range, which will slightly affect your measurement results!) What is VS ? • You now have enough in formation to calculate RS . Is the calculated value available in your kit? Can you achieve this value by combining several resistors? Comment. • You also have enough information to calculate RD · Is the calculated value available in your kit? Can you achieve this value by combining several resistors? Comment. • What values of R1 and R2 do you need to use? Is the problem completely speci ied? Simulations • Simulate your circuit using the values of RS , RD , R1 , and R2 based on your preceding calculations. • Report the values of VS , VD , VG , and ID . How closely do they match your calculations? (Remember: The simulator has its own, more complex model of the real transistor, so there should be some small variations.) Prototyping and Measurement • Assemble the circuit onto a breadboard. • Using a digital multimeter, measure VG , VS , and VD · • Using a digital multimeter, measure all resistors to three signi icant digits. Post-Measurement Exercise • What are the measured values of VGS and VDS How do they compare to your pre-lab calculations? Explain any discrepancies. • Based on the measured values of VD and VS and your measured resistor values, what is the measured value of ID based on your lab measurements? 3 Part 5:NMOS in Triode Mode: Design the circuit in the igure such that ID = 1mA, VD = +2V , and VDS = 0.5V . Use supplies of V+ = −V− = 15V . Note that you must use the triode model. Calculations • Based on the speci ications, calculate VOV and VGS (kn = 1.0mA/V 2 ). What is VG ? • You now have enough information to calculate RS . Is the calculated value available in your kit? Can you achieve this value by combining several resistors? Comment. • You also have enough information to calculate RD . Is the calculated value available in your kit? Can you achieve this value by combining several resistors? Comment. • What values of R1 and R2 do you need to use? Is the problem completely speci ied? Simulation • Simulate your circuit using the values of RS , RD , R1 , and R2 based on your preceding calculations. • Report the values of VS , VD , VG , and ID . How closely do they match your calculations? Prototyping and Measurement • Assemble the circuit onto a breadboard. • Using a digital multimeter, measure VG , VS , and VD · • Using a digital multimeter, measure all resistors to three signi icant digits. Post-Measurement Exericise • What are the measured values of VGS and VDS How do they compare to your pre-lab calculations? Explain any discrepancies. Is the transistor in the triode operating region? • Based on the measured values of VD and VS and your measured resistor values, what is the measured value of ID based on your lab measurements? 4
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