ELEC 3908, Physical Devices – Lecture 1

Review of Diodes

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Lecture Outline

•  The first lecture is a review of the basic theory of diodes

•  The simple ideal diode equation is reviewed, and plots of the I-V characteristic on linear and log scales are shown

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Diode - Structure and Symbol

•  Diode or pn-junction is composed of two semiconducting materials, one p-type (anode) one n-type (cathode)

•  Diode voltage VD is defined as the potential of the p-type relative to the n-type –  Positive VD (raise p with respect to n) is forward

bias –  Negative VD (raise n with respect to p) is reverse

bias

•  Positive diode current ID flows from p to n

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Simple Ideal Diode Equation

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Simple Ideal Equation - Characteristics (Linear)

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Simple Ideal Equation - Reverse Bias (Linear)

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Linear and Log Plots (not in notes explicitly)

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Function on Log Axis vs Log(Function) on Linear Axis

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Slope of Log(Function) on Linear Axis Plot

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Slope of Function on Log Axis Plot

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Zero on Linear vs. Log Ordinate

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Simple Ideal Equation - ln(|ID|) Plot

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Simple Ideal Equation - 3kT/q Boundaries

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Simple Ideal Equation - Projection of VDS > 3kT/q

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Example 1.1: Error in VD ≈0.7 V

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Example 1.1: Solution

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Measured Diode Characteristics

•  If real diodes are measured in the lab we find that they do not follow the simple ideal diode eq’n

•  To investigate reasons for device nonideality, measure I-V characteristics for several commercial diodes

•  Diode observations: –  Slope is not kT/q as predicted by

the simple ideal equation, and varies between devices

–  Intercept (IS) also varies –  The current rolls off at high VD

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Conclusions From Diode Measurements

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Lecture Summary

•  The ideal diode equation was reviewed, and plotted on linear and log axes

•  The use of log axes results in a plot with a number of useful features, including –  Linear regions where the dependence is exponential –  A dip to –∞ where the quantity goes to 0 – this will be

particularly useful in lecture 6, which discusses doping profiles

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Lecture Summary – cont.

•  In practice, measured diode characteristics are not well modeled by the simple ideal diode equation

•  In order to develop an improved diode model, the physical structure and behavior of the diode has to be understood

•  Next lecture begins this study with an in depth examination of semiconducting material