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Diodes Fundamental

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Logic gates with diodes
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  CS 849C Lecture 6 1 Diodes (1) Symbol and I-V Characteristic of Diode A diode is a two-terminal active electronic device containing an anode and a cathode, and it is characterized by the ability to pass an electric current more easily from cathode to anode than from anode to cathode. Symbol of the diode is shown in figure 1-(a). V D  is the bias voltage across the diode, and i D  is the current through the diode. The diode I-V characteristic is shown in figure 1-(b). Here the dashed line indicates the ideal I-V characteristic and the solid line indicates a realistic I-V characteristic. Below forward voltage, current through the diode can be taken as zero, above forward voltage, current increases rapidly with the increase of voltage. (2) Logical Implementation for Diodes Given the basic I-V characteristic of diode, we can discuss a couple of structures that can implement logic functions. The first one is an “OR” gate. V Forward i D V D i D + v D -- (b) I-V Characteristic (a) Symbol Figure 1 Symbol and I-V characteristic of Diode   In Figure 2-(a), A and B are the two inputs, and Y is the output. Resistor R is connected to ground. When A and B are both low, Y is low. If either A or B is high,  because resistor R is much greater than the internal resistor of A and B, voltage across R is high, so output Y is high. The truth table is listed in figure 2-(b).  Next we introduce an “AND” gate which is shown in figure 3. Again, A and B are the inputs and Y is the output. The resistor is connected to V DD . Here if A and B are both high, no current flows through resistor R, and the output Y is high. If A or B is low, the respective diode is turned on and there is a large voltage drop across R, so output Y is low. Let us discuss this structure in more detail. (a) Schematic Y A B R 1 1 1 1 0 1 1 1 0 0 0 0 Y B A (b) Truth Table Y A B R +V DD (a) Schematic 1 1 1 0 0 1 0 1 0 0 0 0 Y B A (b) Truth Table Figure 2 Schematic and truth table for “OR” gate Figure 3 Schematic and truth table for “AND” gate    In order to get the characteristic of transform function of V OUT  over V IN , we set input A to V DD  and change the input voltage V IN  on input B. When V IN  is equal to V DD , there is no current through resistor R, and output Y is equal to V DD . Now we reduce V IN . Nothing happens until V IN  reaches V DD -V F  (where V F  is the forward voltage of diodes). Then when V IN  continues to decrease, V OUT  decreases accordingly. At last when V IN  is zero, the curve intercepts Y axis at V F . (3) Disadvantages of Diode Circuits Diode circuits have several disadvantages as listed below. (1) We cannot implement inverting logic with diodes. The only option is to use other logic devices for inversion. (2) The I-V characteristic has low small signal gain in the transition region. (3) There is no isolation between input and output in these gates. 2 Resonant Tunneling Diode Resonant tunneling diode is a device that has two tunneling Junctions. Its I-V characteristic shows negative differential resistance characteristic (NDR). (1) Structure of RTD Figure 5 shows the structure of RTD. Here the gray part is GaAs that has small  band gap, and the white part is AlGaAs that has large band gap. Y A B R +V DD V IN V OUT V IN V DD V DD -V F V F V DD V OUT <1   Figure 4 Detailed analysis of “AND” gate (a) Schematic (b) Voltage Transfer Curve for V in  vs V out      (2) Energy Band Diagram of RTD We show the energy band diagram of RTD in figure 6. From the figure we can see that RTD has thin well region and a thin barrier region. According to quantum mechanics, electrons can tunnel from outside into the well through barrier under suitable conditions. Without any voltage bias, the electron energy level in the well is higher than fermi levels of both sides. So, no electron in the conduction band can tunnel to the well, and there is no current. GaAs AlGaAs Large Energy Gap Small Energy Gap Thin Thin Thin Barrier Barrier Fermi Level Well Conduction Band Fermi Level Conduction Band Figure 5 RTD Structure Figure 6 Energy band diagram of RTD

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