# ELC terminal of the p-type is called the

ELC 111.2 Laboratory Activity 1: Discrete GatesPre-laboratory Report; Submitted: 21 January 2018Mark Jumel P. Cablay, John Marco MinaBS Electronics EngineeringElectronics, Computer, and Communications EngineeringAteneo de Manila UniversityPhilippinesIntroduction This laboratory activity aims to connect the discrete electronic elements such as the diodes and transistors to its equivalent logic gates. This laboratory discusses the reaction of the electronic components when wired in a way such that they perform a specific logic gate. This can be done using diodes and transistors (which is basically two diodes placed back-to-back) because of their unique trait such as biasing.1 The objective of this activity is to see if the results of the discrete components is the same as the equivalent logic gates by using the truth table. This objective is used in order to learn more about the logic gates and some of the boolean algebra while using the truth table.Theoretical BackgroundDiodesA semiconductor diode is an electronic device formed by joining together a p-type and an n- type material end-to-end. The terminal of the n-type material is called the cathode while the terminal of the p-type is called the anode 1. When lead is added to each end, the diode becomes a two-terminal device. The schematic diagram of a diode is illustrated in Fig. 1.Fig. 1: Schematic diagram of Diode 2Three options become available then: No bias, Forward bias, or Reversed bias. The term ‘bias’ refers to the application of an external voltage across the two terminals of the diode to get a response 1. When there is no applied bias, the diode has zero current and zero voltage. When the positive potential of the voltage supply is connected to the anode and the negative potential of the supply to the cathode, the result is a Forward bias, where the voltage is greater than 0. In this case, there is a thinner depletion region which results to more current flowing through the diode.The reverse bias condition is basically the opposite of the forward bias: the positive potential is connected to the cathode while the negative potential to the anode, wherein the voltage across the diode is less than zero. In this case, there would be a thicker depletion region which results in less current flowing through the diode 1. The overall current through a reverse biased diode can be considered as zero.Thus, a diode that is forward biased will permit a current ID to flow in a single direction same as that of the direction of the arrow on the diode. This current increases until the voltage in the diode reaches its “turn-on voltage.” The turn-on voltage of a diode is the minimum amount of voltage needed to turn the diode on. The turn-on voltage, for different types of diode varies. Some values of diodes’ turn- on voltages are given in Table I 1.Table I. Diodes and their Turn-on voltagesDiode MaterialTurn-on Voltage (V)Germanium (Ge)0.3Silicon0.7Gallium Arsenide (GaAs)1.2If the diode receives enough voltage based on the values given in Table I, then the diode will operate and current can flow through it. Otherwise, the diode will not work. Bipolar Junction Transistors (BJTs)Bipolar Junction Transistors are three-terminal electronic devices which can act as either an insulator or a conductor when you apply a small signal voltage 3. This ability of transistors to change between these two states makes it have two specific functions: amplifier and switch. This experiment will mainly discuss the switch function. BJTs are mainly constructed by combining two PN junctions, which will result in either of the two types of transistors: NPN transistor and PNP transistor. They are named so because of how the p-type and n-type semiconductor materials are ordered or arranged from each other. 4 Using a transistor as a solid state switch, it can be used to “turn on” or “turn off” a DC output. Transistor switches are used for high power output devices, such as motors and lamps because they require more power than that supplied of a regular logic gate. If the used solid state switch is a BJT, the biasing is ordered in a way where the transistor can operate at both sides of the current-voltage characteristic curve 3. This situation is illustrated in Fig.2.Fig.2. Operating regions of a transistor switch 3. The regions where the transistor switch operates are called the saturation region and cut-off region. The cut-off region is the area where the transistor is switched “fully off.” In this case, both the base-emitter and base-collector junctions are reversed biased, and the input and base are connected to ground. The collector voltage (VCE) is at its maximum, which results to the formation of thick depletion layer. Moreover, the voltage across the base-emitter terminals (VBE) is less than 0.7 V (turn on voltage of a Si diode), and no current flows through the collector (IC). Thus, the transistor functions as an “open switch.” 3 On the other hand, the saturation region is the area where the transistor is switched “fully on.” In this case, both the base-emitter and base-collector junctions are forward biased, and the input and base are connected to the common collector voltage (VCC). There is no voltage across the collector-emitter terminals, which results to the formation of very thin depletion layer. Furthermore, the voltage across the base-emitter terminals is greater than 0.7 V, and the current that flows through the collector is at its maximum. Thus, the transistor works as a “closed switch” 3. The operation of transistor switch at saturation and cut-off regions are illustrated in Fig. 3 and Fig. 4.