EMT111 CHAPTER 1 Introduction to Semiconductor By Pn EMT111 CHAPTER 1 Introduction to Semiconductor By Pn. ‘Aini Syuhada Md Zain

Introduction to Semiconductor - Chapter Outline : 1.8 Voltage Current Characteristic of a Diode 1.9 Diode Models 1.10 Testing a Diode

1.8 Voltage-Current Characteristic of a Diode ( V-I Characteristic for forward bias) -When a forward bias voltage is applied – current called forward current, -In this case with the voltage applied is less than the barrier potential so the diode for all practical purposes is still in a non-conducting state. Current is very small. -Increase forward bias voltage – current also increase Fig 1-26a measurements with meters FIGURE 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

1.8 Voltage-Current Characteristic of a Diode ( V-I Characteristic for forward bias) -With the applied voltage exceeding the barrier potential (0.7V), forward current begins increasing rapidly. -But the voltage across the diode increase only above 0.7 V. Fig. 1-26b measurements with meters FIGURE 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

dynamic resistance r’d decreases as you move up the curve 1.8 Voltage-Current Characteristic of a Diode ( V-I Characteristic for forward bias) -Plot the result of measurement in Figure 1-26, you get the V-I characteristic curve for a forward bias diode Increase to the right increase upward dynamic resistance r’d decreases as you move up the curve zero bias Fig. 1-26b measurements with meters

1.8 Voltage-Current Characteristic of a Diode ( V-I Characteristic for Reverse bias) Breakdown voltage not a normal operation of pn junction devices the value can be vary for typical Si Fig. 1-26b measurements with meters Reverse Current

1.8 Voltage-Current Characteristic of a Diode ( Complete V-I Characteristic curve) Combine-Forward bias & Reverse bias  Complete V-I characteristic curve Fig. 1-26b measurements with meters

1.8 Voltage-Current Characteristic of a Diode ( Temperature effect on the diode V-I Characteristic) Forward biased dioed : for a given value of For a given Barrier potential decrease as T increase Reverse current breakdown – small & can be neglected Fig. 1-26b measurements with meters

1.9 Diode Models ( Diode structure and symbol) Directional of current cathode anod Fig. 1-33 ideal diode curve

1.9 Diode Models DIODE MODEL The Ideal Diode Model The Practical Diode Model DIODE MODEL Fig. 1-33 ideal diode curve The Complete Diode Model

1.9 Diode Models ( The ideal Diode model) Ideal model of diode- simple switch: Closed (on) switch -> FB Open (off) switch -> RB Assume Forward current, by Ohm’s law (1-2) Fig. 1-33 ideal diode curve

1.9 Diode Models ( The Practical Diode model) Adds the barrier potential to the ideal switch model ‘ is neglected From figure (c): The forward current [by applying Kirchhoff’s voltage low to figure (a)] Ohm’s Law Equivalent to close switch in series with a small equivalent voltage source equal to the barrier potential 0.7V Represent by produced across the pn junction Same as ideal diode model Fig. 1-33 ideal diode curve (1-3)

1.9 Diode Models ( The Complete Diode model) Complete model of diode consists: Barrier potential Dynamic resistance, Internal reverse resistance, The forward voltage: The forward current: acts as closed switch in series with barrier potential and small acts as open switch in parallel with the large (1-4) Fig. 1-33 ideal diode curve (1-5)

1.9 Diode Models ( Example) (1) Determine the forward voltage and forward current [forward bias] for each of the diode model also find the voltage across the limiting resistor in each cases. Assumed rd’ = 10 at the determined value of forward current. 1.0kΩ Fig. 1-33 ideal diode curve 1.0kΩ 5V 10V

1.9 Diode Models ( Example) Ideal Model: Practical Model: (c) Complete model: Fig. 1-33 ideal diode curve

1.9 Diode Models ( Typical Diodes) Diodes come in a variety of sizes and shapes. The design and structure is determined by what type of circuit they will be used in. Fig. 1-33 ideal diode curve

1.10 Testing A Diodes ( By Digital multimeter) Testing a diode is quite simple, particularly if the multimeter used has a diode check function. With the diode check function a specific known voltage is applied from the meter across the diode. With the diode check function a good diode will show approximately .7 V or .3 V when forward biased. Fig 1-38 DMM check w/electrode labels When checking in reverse bias the full applied testing voltage will be seen on the display. K A A K

1.10 Testing A Diodes ( By Digital multimeter) Defective Diode Fig 1-38 DMM check w/electrode labels

1.10 Testing A Diodes ( By Analog multimeter – ohm function ) Select OHMs range Good diode: Forward-bias: get low resistance reading (10 to 100 ohm) Reverse-bias: get high reading (0 or infinity) Fig 1-38 DMM check w/electrode labels

Summary Diodes, transistors, and integrated circuits are all made of semiconductor material. P-materials are doped with trivalent impurities N-materials are doped with pentavalent impurities P and N type materials are joined together to form a PN junction. A diode is nothing more than a PN junction. At the junction a depletion region is formed. This creates barrier which requires approximately .3 V for a Germanium and .7 V for Silicon for conduction to take place.

Summary A diode conducts when forward biased and does not conduct when reverse biased When reversed biased a diode can only withstand so much applied voltage. The voltage at which avalanche current occurs is called reverse breakdown voltage. There are three ways of analyzing a diode. These are ideal, practical, and complex. Typically we use a practical diode model.