CSE251 Lecture3 Semiconductor Diodes

I-V Characteristics of a Real Diode p-n Junction Diode Characteristics I-V Characteristics of a Real Diode Diode Equation: ID is the total diode current Is reverse saturation current - VD applied voltage across the diode - n an ideality factor, value between 1&2. - VT thermal voltage: k = 1.38 x 10-23 J/K q = 1.6x10-19 C Note: From now on ID will stand for the total Diode Current not the diffusion current. 2

p-n Junction Diode Characteristics I-V Characteristics With Zero Voltage: Forward-Biased: Under forward-biased condition, VD > 0. When VD >> nVT, then Reversed-Biased: Under reverse-biased condition, VD < 0. When VD << nVT, then 3

p-n Junction Diode Characteristics I-V Characteristics Cut-in voltage When VD < VZK, the diode enters the breakdown region, the reverse current increases sharply. VZK is known as the zener knee voltage. 4

DC Analysis of Diode Circuits Let’s consider the very simple diode circuit: + VR - Applying KVL: (1) Diode Equation: (2) Assuming n, VT, and Is are known, we have two equations with two unknown quantities, VD and ID. Substituting (2) into (1), we get, There is no simple analytical solution for this equation. So how do we solve such a circuit problem? 5

DC Analysis of Diode Circuits 1. Graphical Analysis (2) The I-V characteristics following (2) for the forward biased diode: Rearranging the terms in (1): This is an equation of a straight line (y = mx + c) and is plotted on the right. This line is called the load line. Now we plot both these curves in the same graph which is shown in the next page. 6

DC Analysis of Diode Circuits 1. Graphical Analysis The point where the two curves intersect is the simultaneous solution of the two equations (1) and (2). 7

DC Analysis of Diode Circuits 1. Graphical Analysis The graphical solution is not practical except for very simple circuits. Nevertheless, it provides some insight about qualitative understanding of these circuits. For example what happens when: (a) VDD increases? (b) R increases? 2. Simulation Package: PSpice, ADS 3. Numerical Analysis: MATLAB, Mathematica, Mathcad 8

DC Analysis of Diode Circuits 2. Approximate Analysis: Ideal Diode Model It is the simplest diode model. In this model, diode is assumed to be shorted with zero voltage drop across the diode when forward biased and open circuit with zero diode current when reverse biased. So, VD = 0 (Forward biased) and ID = 0 (Reversed biased) 9

DC Analysis of Diode Circuits 2. Approximate Analysis: Constant Voltage Drop (CVD) Model This is probably the most commonly used diode mode for hand calculations. In this model, a constant voltage of VD is assumed across the diode when forward biased and open circuit with zero diode current when reverse biased. VD = 0.7 for silicon. 10

DC Analysis of Diode Circuits 2. Approximate Analysis: CVD Model Approximate model of the diode forward characteristics and its equivalent-circuit representation. 11

DC Analysis of Diode Circuits 2. Approximate Analysis: CVD Model 12

DC Analysis of Diode Circuits 2. Approximate Analysis: CVD Model 13

DC Analysis of Diode Circuits 2. Approximate Analysis: CVD Model 14

DC Analysis of Diode Circuits 2. Approximate Analysis: Piecewise Linear (PWL) Model This is “battery plus internal diode resistance model”. This model is one step better than the constant voltage drop model as it incorporates a slope to the interpolative line and better approximates the real diode characteristics. The finite slope means that the diode has a non-zero internal resistance and is labeled as rD. 15

DC Analysis of Diode Circuits 2. Approximate Analysis: Piecewise Linear (PWL) Model Approximate PWL model of the diode forward characteristics and its equivalent-circuit representation. 16

DC Analysis of Diode Circuits 2. Approximate Analysis: Piecewise Linear (PWL) Model 17

DC Analysis of Diode Circuits 2. Approximate Analysis: Piecewise Linear (PWL) Model 18

DC Analysis of Diode Circuits 2. Approximate Analysis: Piecewise Linear (PWL) Model 19