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Exercise 3 Analyses of Parameters and Characteristics of MOS transistor I. Theoretical settings a) б) Circuits for input and output capacity determination of MOS transistors The transistors input (gate) is feet by voltage generator where the output is short connected. The chosen dc regime UDS=UGS=0 (Ids=0), wipes out the influence of the static parameters over the measurement. On the same way you can determine the input capacity (Cin). The DC regime is the same one but the signal goes to the transistor’s output (drain) where the input is short connected. The Cin and Cout can be easily determinate by the formulas given below: Cin I1 I 2 2fUm Cout I1 I 2 2fUm 1. Simulative determination of Cin and Cout (1) (2) The shown transistor is nmos4 from library PRIMLIB (for 0.35m CMOS technology) with parameters: Chanel width: w=100m Chanel length: l=2m Number of gates: NG=1 The voltage source vsin used for the simulations you can find in library analogLib. Set the values as shown below: II. Simulations 1. Simulator choice In Virtuoso Schematic Composer go to Tools Analog Environment to start ADE. In order to work with simulator SpectreS from ADE go to Setup Simulator/Directory/Host. In the drop-down menu you choose the name of the desired simulator and give the directory path where you will keep all the simulating results. 2. AC analysis settings 1) From ADE choose Analyses Choose ac. 2) For the analysis set the following parameters: Sweep Variable frequency Sweep Range from 10Hz to 500MHz Sweep Type Automatic 3) Press OK 4) From ADE choose Outputs Save all AC currents 5) Simulation results visualization Go to Results Direct Plot AC Magnitude. Select the transistor’s gate and drain currents and press Esc. b) a) Drain and Gate currents amplitude In order to calculate the current for any frequency Results Display Window, go to Markers Vertical Markers. There you type the desired frequency (for example 10MHz) and press consecutively the buttons Apply and Display Intercept Data. Cin can be calculated using the formula Cin I1 I 2 2fUm as was shown before. For Cout use the scheme shown below and follow the steps that we just passed for Cin If frequency is 10MHz then the values for drain and gate current are shown in the next figure: Cout capacity can be calculated using formula Cout I1 I 2 as was shown 2fUm before. If a transistor has more than one gates (5 and 10 for example) and we use the formula for Cout calculation than the graphical result is shown in the next diagram: 2.6 2.2 2.2 1.8 Кe Кo Кч _Cout 2.6 Кн _Cout 1.4 1.8 1.4 1 1 1 5 9 13 17 Odd number of gaits а) 21 25 2 6 10 14 18 Even number of gaits 22 b) To make the fixed characteristics we use coefficients for odd and even numbers - Ko and Ke. They are defined with the help of the formulas below: Кo Кe Co C1 C o1 C1 Where Co is the transistor’s capacity with odd numbers of gates, Co+1 is the transistor’s capacity with even number of gates and C1 is transistor’s capacity with one gate. The calculated results can be compared with those from the simulation. 3. MOS transistor’s output characteristic Draw the circuit of nmos4 shown below in Virtuoso Schematic Composer. 1) Variable initiation In window Edit Object Properties that comes from source Vgs type the name of the variable in box DC voltage. The same variable name you should type in ADE, menu Variables Edit as well. 2) DC analysis In ADE go to Analyses Choose dc. Check Vds as a source and set its range to be from 0 to 2.5V. For the simulation we need the transistor’s drain current Id. In order to visualize that current and any other current you may need, it is necessary previously to save them. In ADE window go to Outputs Save all DC currents. 3) Parametric Analysis In ADE go to Tools Paramertric analysis and set Vgs (gate voltage) as a parameter of Vgate variable. Set values for example: Vgate = 0, 0.5V, 1.5V, 2V, 2.5V and 3V. 4) Start simulation From Parametric Analysis window go to Analysis Start 5) Results visualization When the simulation is over, choose drain current Id. Use command Results Direct Plot dc. Id can be graphically drawn if it is checked before the simulation. NMOS transistor’s output characteristics (w=100m, l=2m, number of gates=1) 4. Conductivity gm In order to calculate the conductivity we use the Calculator tool. To start it go to menu Tools in ADE. Press button op in order to open window for dc parameters. Select the transistor in the circuit and press button list. From the list choose gm and in Calculator press button plot. The gm results are shown below: gm dependence on Vgate gm and Id dependence on Vgate In order to visualize gm as a function of Id, follow the steps described below: 1) Choose Id from list from parameter’s window. 2) In the window for results visualization choose Axes X Axis gm dependence on Id 3) Press button independent variable and choose Id. That way you draw the Id on X-axis 5. Transit frequency ft The frequency ft can be found with the following formula: ft gm 2Cgs In Calculator type that formula in order to draw ft as a function of Id. The results are shown below: Ft dependence on Vgate ft dependence on Id ft and Id dependence on Vgate 6. Defining Vth Follow the same steps to define the dependence between Vth and Vgate Vth dependence on Vgate

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