In this chapter, you will learn about: The space-charge region boundaries represent an a step junction, the abrupt depletion layer approximation applies, no carriers exist in the space-charge region, in the bulk of the diode outside the depletion region, the semiconductor is neutral.
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 Lecture No: 7 IV Characteristics of PN Junctions Kwangwoon University Semiconductor device lab Semiconductor Devices PN Junction Ideal IV Characteristics: Assumptions 1) The spacecharge region boundaries represent an a step junction 2) The abrupt depletion layer approximation applies abrupt boundaries & neutral outside of the depletion region 5) No carriers exist in the spacecharge region 6) In the bulk of the diode outside the depletion region, the semiconductor is neutral 7) Diode operation is considered at a temperature at which all impurity atoms are ionized 8) Perfect ohmic contacts are made to the ends of the p and n regions Qualitative Description of Current Flow Equilibrium Reverse bias Forward bias v CurrentVoltage Relationship Quantitative Approach Kwangwoon University Semiconductor device lab Semiconductor Devices CurrentVoltage Characteristics THE IDEAL DIODE Positive voltage yields finite current Negative voltage yields zero current REAL DIODE VoltageCurrent Characteristics of a PN Junction BuiltinPotential Vbi Na Nd Vt ln (Vbi : built in potential barrier ) ni Boundary Conditions: If forward bias is applied to the PN junction np Pn eVa n po exp( ) kT eVa Pno exp( ) kT Zener Effect • Zener Break Down: VD 28 Example 2.13 Zener diode A 1N754A Zener diode has a dc power dissipation rating of 500 mW and a nominal Zener voltage of 6.8 V. What is the value of IZM for the device? I ZM = PD (max) VZ 500mW = = 73.5mA 6.8V 29 Metal Contacts • • No rectifying action The current can flow in both direction The difference of carrier concentrations of the two materials at the contact n A barrier potential exists n rectifying action occurs n Mostly used in switching circuits (turn on/off switches) n Semiconductor Devices Metal Contacts I-V Characteristics Semiconductor Devices LED • Light emitting diode, made from GaAs – VF=1.6 V – IF >= mA Fig 2.3537 Light emitting diodes LED symbol 33 Table 2.4 Common LEDs Elements Color Emitted Forward voltage (VF) GaAs 1.5 V @ IF = 20 mA Infrared (invisible) AlGaAs 1.8 V @ IF = 20 mA Red GaP 2.4 V @ IF = 20 mA Green AlGaInP 2.0 V @ IF = 20 mA Amber (yellow) AlGaInN 3.6 V @ IF = 20 mA Blue 34 Fig 2.38 A LED needs a currentlimiting resistor I RS I RS 5V LED Driving circuit LED Current sinking circuit 35 Fig 2.39 Multicolor LED 36 Fig 2.43 Common diodes Rectifier Zener LED Schematic symbol Bias for normal operation Switched back and forth between forward and reverse Reverse Forward Normal VF Si: VF = 0.7 V Ge: VF = 0.3 V VF = 0.7 V (not normally operated) 1.2V VF Normal VR Equal to applied voltage Equal to VZ Equal to applied voltage Primary factors to consider for device substitution I0 and VRRM ratings PD(max) and VZ ratings 4.3V VF(min), IF(max), and VBR 37 ... Lecture No: 7 IV Characteristics of PN Junctions Kwangwoon University Semiconductor device lab... VZ VR VF 19mA IZT (20mA) 21mA ZZ IZ VZ IZ IR 27 Fig 2.32 Zener equivalent circuits IR VZ ZZ Ideal: ZZ = Prac.: ZZ > 28 Example 2.13 Zener diode A 1N754A Zener diode has a dc power dissipation rating of 500 mW and a nominal Zener ... No carriers exist in the spacecharge region 6) In the bulk of the diode outside the depletion region, the semiconductor is neutral 7) Diode operation is considered at a temperature at which all impurity atoms are ionized 8) Perfect ohmic contacts are made to the ends of the p and n regions