The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

Transcription

1 The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) OBJECTIVES The objective of this lab is to help you assemble and test a common source amplifier circuit that uses a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). You will measure and plot the characteristic curves for an n-channel enhancement type MOSFET to observe how the device is controlled. The transfer curve will be measured in order to find the threshold voltage, find the operational point and determine the gain from the slope of the linear region of the transfer curve. INTRODUCTION MOSFETs work on an entirely different principle from the bipolar junction transistors (BJTs) although they may have similar functions. The bias conditions and the characteristics are also different. A MOSFET has 3 terminals called source, gate, and drain as shown below. Two general types of MOSFETs are enhancement type and depletion type, with two different semiconductor substrates: the n-channel and the p-channel. Figure 1. MOSFET p-channel 1 The MOSFET is a layered device. The base layer is semiconductor substrate, then insulating SiO 2 oxide, then metal (typically in modern devices, this is actually semiconductor as well, 1 Introductory Electronics for Scientist and Engineers, Simpson. p.270 1

2 doped to behave as a metal), as shown above. MOSFETs are also referred to as Insulated Gate FETs (IGFETs). P-channel MOSFETs are built on n-type substrate, and have source and drain contacts connected to p-type regions where the substrate was p-doped. The p-channel enhancement MOSFET operates as follows: if there is a negative voltage with respect to the source applied to the gate, there is an induced charge separation created in the oxide layer, with positive charge accumulating in the oxide closer to the gate, and negative charge accumulating in the oxide farther from the gate (Figure 1). This induces positive charge carriers to accumulate at the oxide-substrate barrier, creating an effective n-channel. This allows conduction across the drain-source. So, the applied voltage enhances conductivity across the drain and source with an induced p-channel. Similarly, in an n-channel enhancement MOSFET, the current is formed by the flow of electrons. A positive voltage applied to the gate attracts mobile electrons from the n-doped source and drain regions, inducing a conducting channel between drain and source (Figure 2). The enhancement-type MOSFET is off when V GS = 0. Depletion-type MOSFETs, on the other hand, are on when V GS = 0. A depletion-type MOSFET induces a depletion regime, and pinches off the already existing conducting channel at some cutoff voltage. This on/off behavior makes a MOSFET an excellent switch, which is demonstrated later in the characteristic curves. Figure 2. MOSFET n-channel schematic 2 2 Introductory Electronics for Scientists and Engineers, Simpson, p.270 2

3 This is why the MOSFET is voltage-controlled, unlike the bipolar junction transistor, which is current-controlled. However, unlike the Junction FET (JFET), V GS is not required to be kept reverse-biased; the MOSFET gate can be biased either in the reverse or forward direction due to the insulating oxide barrier. It is required that the channel-substrate be kept reverse-biased at all times, however. This is accomplished in the device used here by internal shorting of the source to the substrate for an n-channel, and shorting of the gate to the source in a p-channel MOSFET. MOSFET N-CHANNEL COMMON SOURCE CHARACTERISTIC CURVES a) b) Figure 3. a) Typical characteristic curve for a ZVN2106A MOSFET for varying V GS. b) Transfer curve for a ZVN2106A MOSFET. 3 The transistor you will be testing is a ZVN2106A n-channel MOSFET (an excellent substitute is the 2N7000). For a given value of gate-source voltage V GS, the current is nearly constant over a wide range of drain-source voltages V DS. The MOSFET controls the drain current by controlling the population of charge carriers from the n-channel. When the gate is made more positive, it accumulates the majority carriers to a larger zone around the gate and this increases the current flow for a given value of V DS. Therefore, modulating the gate voltage modulates the current flow through the device. This is evident in the characteristic curves for the device (Figure 3a). The transfer curve for the MOSFET is useful for visualizing the gain from the device and identifying the region of linearity (Figure 3b). The gain is proportional to the slope of the transfer curve. The gate voltage at which the current turns on is called the threshold voltage, V T. For the ZVN2106A, the datasheet gives V T ~ 2 V. The operational point of the transistor should be chosen in the middle part of the linear region on the transfer curve. 3 ZVN2016A Datasheet, Diodes Inc. 3

4 LIST OF COMPONENTS NEEDED 1. Resistors: a. R 1 = 10 kω (used for protection in case the MOSFET is forward-biased accidentally) b. R 2 = 100 Ω (serves as a current sensing resistor) 2. N-channel enhancement MOSFET ZVN2106A (or equivalent, 2N7000); case type and terminals order are shown below variable power sources 4. One multimeter. 5. One small breadboard (also known as protoboard ): solder-less board used to build a temporary prototype of an electronic circuit. Front Connections on the Back 6. Jumper wires for connecting the individual components on the breadboard. Please note: you should not try to put more than one wire or component terminal in each hole! 4

5 PROCEDURE AND DATA ANALYSIS R2 100 Ω R1 G D ZVN2106A + VGS kω S - VDS Figure 4: Common source circuit with an n-channel MOSFET. VGS j i h 10 kω - + g f D G S ZVN2106A e d 100 Ω c b a VDS - + Figure 5: Suggested arrangement of the components on the breadboard 5

6 Table 1: Listed and Measured Values for R 1 and R 2 RESISTOR LISTED VALUE MEASURED VALUE R 1 10 kω R Ω 1. Measure and record the values of R 1 and R 2 listed above in the Table Connect the components on the breadboard as shown in Figure 5. Your transistor should be connected to pins f10-12, with drain in 10, gate in 11, and source in 12. The common indicated in the schematic, Figure 4, is just using the terminals of the power supply as a reference. 3. Before connecting the two variable sources, set V GS and V DS at 0V. 4. Use your voltmeter to measure the potential between the drain and the source, V DS, a second to measure V GS, and a third to read the voltage across R 2, V R2. 5. Measure the V R2 for all V DS in Table Slowly increase V GS until it becomes 2.0 V, and repeat for all V DS. Continue for all combinations in Table Compute the drain current I D by applying Ohm s Law to R 2. The current through R 2 is the same as I D for the transistor. Table 2: Characteristic Curves Data for ZVN2106A V DS (V) V G = 0V V G = 2.0 V V G = 3.0V V G = 4.0V V R2 I D V R2 I D V R2 I D V R2 I D 8. Plot the characteristic curves for your transistor on one graph, using the data in Table 2. 6

7 9. Now, we focus on the transfer curve. Reduce V GS to 0 V. Slowly, increase V DS to 4 V. Varying V GS, we can find the threshold voltage, VT, which we will define as where I D is 10 μa. 10. To put together the data for showing the transfer curve of the transistor, vary V GS and record the V R2 values in Table 3. Calculate I D as before. Table 3: Transfer Curve Data for ZVN2106A V GS (V) V R2 (V) I D (ma) Draw the transfer curve. 12. To determine the operational point Q, we need to draw the load line on the results of our first plot (characteristic curves). The intersection of the load line with the transfer curve gives Q. The load line is defined by two points: the voltage applied by the drain/source V DS, 4 V, at I D = 0, and the current given by V DS /R L = 4 V/100Ω. The intersections of the load line with the characteristic curves give graphical solutions for the operating point Q. Choose a Q point that falls in the middle of the linear region in your transfer curve. 13. Find the transfer conductance, g m, at this point. Figure 6 will help you understand how the MOSFET would amplify a sinusoidal signal applied to the gate. The operating point of an amplifier is within the saturation regime. g m = "I D "V GS 14. What is the approximate drain-source resistance for this device at the V GS = 4 V and V DS = 5 V? Determine this using Ohm s Law. Compare and contrast the results you have found here to the datasheet for the transistor! you are measuring. Datasheets are easily located online through electronics vendors, and the datasheet for the transistors studied here is linked on the Physics website. 7

8 ID Q IDQ Id VGSQ VT VGS Vgs Figure 6. Amplification of a sinusoidal signal input at the gate, shown using the transfer characteristics. 8