UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE. Department of Electrical and Computer Engineering

Transcription

1 UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE Department of Electrical and Computer Engineering Experiment No. 8 - MOSFET Basics Overview: The purpose of this lab is to familiarize the student with the basic regions of operation of a MOSFET transistor. The MOSFET device is a four terminal device with connections for the drain, gate, source, and body as shown in the symbol in Figure 1. However, a more common symbol used to identify the MOSFET is shown in Figure 2 and is what will be used in this experiment. The MOSFET devices used in this lab have the body internally connected to the source such that the body terminal is not externally accessible. Shown in these figures are N-channel MOSFETs. D D G G S B Figure 1. MOSFET 4-Terminal Symbol S Figure 2. MOSFET 3-Terminal Symbol To study the MOSFET we connect two external voltage sources to the device as shown in Figure 3. These provide a drain-source voltage V and a gate-source voltage V GS. VGS M1 V Figure 3. MOSFET with V GS and V Connected The voltage V may cause a drain-source current, I, to flow provided a path exists from the drain to the source inside the device. This internal current path can be

2 controlled by the gate-source voltage V GS. For a sufficiently high V GS, an internal current path, called the channel, is established between the drain and the source. The higher the V GS value the easier it is to flow for the drain-source current I. Note that the only DC current in the MOSFET is I since the gate is internally insulated from the channel. By operating the MOSFET in particular bias regions, based on the V GS, V, and I values, it can be used to perform a variety of functions. The two regions that the MOSFET device can operate in are the ohmic (linear) and saturation (active) regions. Both of these regions will be explored in this experiment through the exploration of a few of the MOSFET device implementations. These regions can be graphically represented. Shown in Figure 4 is the MOSFET I vs. V curve for constant values of V GS. Notice that the ohmic region exists where V is very small and the curve is nearly linear, hence another name for the region is the linear region. As V increases, the curve begins to flatten. When V is equal to the saturation voltage, V AT, the device enters the saturation region. This voltage, V AT, is determined by the voltage V GS of the MOSFET along with a physical parameter of the MOSFET called the threshold voltage V T. Figure 4. Typical MOSFET I vs. V Curve For a MOSFET, V AT = V GS V T, and for V values equal to or greater than V AT, the device is in the saturation region of operation. The current flowing through the device in the ohmic region of operation is given by the following equation: W L (1) I = µ ocox ( VGS VT ) V V 2 Given the V GS and V values as well as the physical MOSFET sizing W/L and the physical parameter μ o C ox, the current I can be found for the ohmic region. The W/L 2

3 parameter is the width of the device divided by its length and the fabrication process of the device determines the physical parameter μ o C ox. In this region of operation, the MOSFET acts as a voltage controlled resistor where the value of drain-source resistance can be found by taking the partial derivative of I with respect to V as shown below: V (2) µ C ( V V ) V = µ C [ V V V ] o ox W L GS T V 2 o ox W L GS T = 1 r ohmic The device can also be operated in the saturation region. This region is primarily used for amplification of an input signal. However, amplification will be studied in subsequent experiments; in this experiment we are limiting the discussion to DC bias conditions in the saturation region of operation. The current I in the saturation region of operation can be found from the following equation (for simplicity, λ is assumed to be zero): 1 2 µ W I = C V V L (3) ( ) 2 o ox GS T In this region, the device acts as a voltage-controlled current source, hence its use as an amplifier. The current source created is not ideal and is shunted by a small signal equivalent resistance referred to as r ds. The small signal resistance r ds will be studied in more detail in the subsequent experiments covering amplification using MOSFETs. For this experiment, the concern is with find the current I based on the values of V GS and V. In the saturation region, the MOSFET can be connected to act similar to a diode. The MOSFET is commonly called diode-connected when configured as in Figure 5. M1 V=VGS 0 0 Figure 5. Diode-Connected MOSFET The voltage across the device can be set to provide a reference voltage that may be needed in a particular application. The equation to find the voltage across the device can be found by solving Equation 3 for V GS since the V GS of the device is equal to V. This equation for V MOSDIODE is shown below: 3

4 (4) V MOSDIODE = V T + I 1 µ oc 2 ox W L By diode-connecting the device, the saturation region is virtually guaranteed. Knowing the I, W/L, and μ o C ox the voltage V MOSDIODE can be found. 4

5 Pre-Lab - MOSFET 1. For the MOSFET shown in Figure 6, solve for the drain-source current I, indicate the region of operation, and, if necessary, solve for the drain-source resistance r ohmic for the following conditions. μ o C ox (W/L) = 650μA/V 2 ; V T = 1.19V a) V GS = 1.19V & b) V GS = 2.4V 5

6 2. For the MOSFET shown in Figure 7, solve for the drain-source current I, indicate the region of operation, and, if necessary, solve for the drain-source resistance r ohmic for the following conditions. μ o C ox (W/L) = 650μA/V 2 ; V T = 1.19V; V GS = 2V ; V = 5V. VGS M1 V Figure 7. Problem 2 3. Desiring a MOSFET resistor with a resistance of 100kΩ, find the value of V GS needed to create this resistor given μ o C ox (W/L) = 650μA/V 2, V T = 1.19V, and V =0V. 6

7 4. For the MOSFET shown in Figure 8, solve for V =V MOSDIODE and the value of R assuming that V T = 1.19v, and μocox(w/l) = 650μA/V 2. (Hint: Write a KVL equation at the drain of the MOSFET and use Equations 3 and 4.) Figure 8. Problem 4 *Note: The values given for the μ o C ox (W/L) and V T were found using measured data on the CD4007UBE chip [1]. (INSTRUCTOR S SIGNATURE DATE ) 7

8 Lab Session - MOSFET 1. Observe the schematic shown in Figure 9. Notice that the numbers correspond to the pin connections on the CD4007UBE chip. 8 - A + VGS 6 V Figure 9. MOSFET Connections MOSFET Curves 2. Before connecting the circuit shown in Figure 9 do the following: a. Prepare the power supply for V GS to ensure a DC voltage of +2V (before connecting to the circuit. b. Prepare the power supply for V to ensure a DC voltage of 0V (before connecting to the circuit. 3. Measure the drain current I as V is varied from 0V to +5V with V GS = 2V. Take data points in 0.25V increments in order to have a sufficient number of values since these will be used to plot I vs. V. 4. Repeat the entire Step 3 for V GS = 1.9V and V GS = 2.4V. 5. With V = 5V, determine the value of V GS at which the current I becomes negligible; assume for the purposes here that this means 1μA. This value of V GS is close to the threshold voltage, V T, for the MOSFET we are working with. 6. Now observe the schematic shown in Figure 10. Again, note the pin connections to the CD4007UBE chip. 8

9 8 + VGS R 0 0 Figure 10. MOSFET Connections MOSFET Resistor 7. Connect the circuit as shown in Figure 10 with the ohmmeter across the drain and source. Vary V GS and notice the change in resistance values. Record the values of resistance for V GS = 1.9V, 2V, and 2.4V. 8. Prepare a DC current supply to a value of 100μA before connecting it to the circuit as shown in Figure 11. R Vdd 5V Ids V 7 - Figure 11. MOSFET Connections Diode Connected MOSFET 9. Measure and record to voltage across the MOSFET as shown. 10. Now vary the DC current supply I to the following values: 200μA, 250 μa, and 300μA. Use your equations from the Prelab to solve for the circuit values. Record the values of V for each condition. 0 9

10 Lab Session MOSFET (Data Sheet) INSTRUCTOR'S INITIALS DATE: 10

11 Post Lab - MOSFET 1. Using the data collected in Steps 3 and 4 plot a family of curves using Excel or similar spreadsheet tool with the curves for the three values of V GS overlaid on the same graph. There should be one curve for each of the three values of V GS (1.5V, 2V, and 2.5V). Label each curve with the appropriate V GS values and label approximately where the ohmic and saturation regions exist. 2. Using Equation 2 for the drain-source resistance r ohmic for a MOSFET in the ohmic region of operation, find the resistance of the MOSFET for the following conditions given: μ o C ox W/L = 650μA/V 2, V = 0V, and V T = 1.19V. Compare these with the values obtained in Step 7. Complete a table of results compiling the measured and calculated values. a) V GS = 1.5V b) V GS = 2V c) V GS = 3V d) V GS = 4V e) V GS = 5V 3. Using Equation 4 for the voltage of a diode-connected MOSFET, calculate the voltages for the following conditions given: μocox W/L = 650μA/V2 and V T = 1.45V. Compare these values with those obtained in Step 10. Complete a table of results compiling the measured and calculated values. a) I =200μA b) I =250μA c) I =300μA 4. Why would it be necessary to create a resistor from a MOSFET? Name some advantages of doing so. 5. Why would a MOSFET be connected like a diode? Would the voltage set by the MOSFET be helpful in circuit design? Reference: [1] The values given for μ o C ox (W/L) and V T were obtained from measured data on the CD4007UBE chip; courtesy of Daniel Evans, PhD candidate under Dr. David M. Binkley, Associate Professor at UNC Charlotte, and Clark Hopper M.S, R.A. and Harold Hearne M.S., R. A. both also at UNC Charlotte. 11