Lecture 2 Instructor: Rashedul Islam Course: Electronics I

Outline: Contact Potentials Current-Voltage Characteristics of a Diode Simplified AC & DC Diode Model Dynamic Resistance 2

Contact Potentials: The potential difference that exists across the space between two electrically connected materials is known as contact potential. In P-N Junction, Contact Potentials can be defined as the Potential difference of the Potentials of P-type material and N- type material in Depletion Region. Thus, ∆V= V p – V n in Volts 3

Why we need Contact Potentials? Contact Potential is significant because it indicates the state of equilibrium inside a P-N Junction. As we have already know that inside a “Depletion Region” in equilibrium no current will flow. This phenomenon can also be explained on the basis of Contact Potential. 4

Current-Voltage Characteristics of PN Junction Diode : Diode: In electronics, a Diode is a type of two-terminal electronic component which has nonlinear current–voltage characteristics. Example: P-N Junction. 5

Current-Voltage Characteristics of PN Junction Diode Continues... Since the diode is a two-terminal device, the application of a voltage across its terminals leaves three possibilities: 1.No Bias (V D = 0 V), 2.Forward Bias (V D > 0 V), and 3.Reverse Bias (V D < 0 V). Here. V D = Diode Voltage We will learn about I D = Diode Current 6

Current-Voltage Characteristics of PN Junction Diode Continues... No Applied Bias (V D = 0 V): In the absence of an applied bias voltage, the net flow of charge in any one direction for a semiconductor diode is zero. Thus, V D = 0 V I D = 0 mA 7

Current-Voltage Characteristics of PN Junction Diode Continues... Reverse Bias (V D < 0 V): When we apply reverse bias to a P-N Junction Diode it results the process of widening the Depletion Region. The current that exists under reverse-bias conditions is called the reverse saturation current and is represented by I s. The term saturation comes from the fact that it reaches its maximum level quickly and does not change significantly with increase in the reverse-bias potential. 8

Current-Voltage Characteristics of PN Junction Diode Continues... Forward Bias (VD > 0 V): When we apply Forward bias to a P-N Junction Diode it results the process of narrowing the Depletion Region and thus creates Diode Current I D. It can be demonstrated through the use of solid-state physics that the general characteristics of a semiconductor diode can be defined by the following equation for the forward- and reverse-bias regions: This equation is called Shockley Ideal Diode Equation Here, I s = reverse saturation current k =11,600/η with η =1 for Ge and η =2 for Si for relatively low levels of diode current and η =1 for Ge and Si for higher levels of diode current T K = T C + 273° 9

Current-Voltage Characteristics of PN Junction Diode Continues... For positive values of V D the first term of the equation above will grow very quickly and overpower the effect of the second term. The result is that for positive values of V D, I D will be positive and grow as the function y = e^x. At V D = 0 V, becomes I D =Is(1-1)= 0 mA. For negative values of V D the first term will quickly drop off below I s, resulting in I D = - I s, 10

Current-Voltage Characteristics of PN Junction Diode Continues... 11

Simplified DC Diode Model 12

Simplified AC Diode Model 13

Simplified AC Diode Model Contd… inside the semiconductor. But charges can not move instantaneously and that eventually creates a Diffusion Capacitance (C D ). As, the forward bias resistance is also a function of frequency, the R f is replaced by dynamic resistance r d. So, the resulting AC Diode Model becomes as follows: 14

Dynamic Resistance: At the operating point, Q, the static (DC) current and voltage are I and V. Consequently, the total-variable resistance is R = V/I. Because V and I are constant, R is also the static resistance of the diode at Q. For incremental (small-signal) variations around Q, a change in v will produce a corresponding change in i. The resulting resistance is the incremental resistance: Where, r is calculated at operating point Q. This incremental resistance is sometimes referred to as the Dynamic Resistance because it involves changes in v and i. 15