ELEC 3908, Physical Devices – Lecture 1
Review of Diodes
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
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐2 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
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐3 Simple Ideal Diode Equation
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐4 Simple Ideal Equation - Characteristics (Linear)
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐5 Simple Ideal Equation - Reverse Bias (Linear)
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐6 Linear and Log Plots (not in notes explicitly)
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐7 Function on Log Axis vs Log(Function) on Linear Axis
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐8 Slope of Log(Function) on Linear Axis Plot
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐9 Slope of Function on Log Axis Plot
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐10 Zero on Linear vs. Log Ordinate
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐11 Simple Ideal Equation - ln(|ID|) Plot
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐12 Simple Ideal Equation - 3kT/q Boundaries
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐13 Simple Ideal Equation - Projection of VDS > 3kT/q
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐14 Example 1.1: Error in VD ≈0.7 V
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐15 Example 1.1: Solution
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐16 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
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐17 Conclusions From Diode Measurements
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐18 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
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐19 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
ELEC 3908, Physical Electronics: Review of Diodes 1-­‐20