Lecture 17 - pn junction small signal response

Lecture 17 - pn junction small signal response -...

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1 Simplifications of “one-sided” diodes: • IF one side of the diode is heavily doped (N A >>N D or N D >>N A ) then we call the doping level on the lightly doped side N B (background doping) and we find bi 2 ln ln 2 G AD B ii E NN N kT kT V qn q n ⎛⎞ =≅ + ⎜⎟ ⎝⎠ () 00 22 1 SS bi A bi A B KK WV V V V qN N q N εε + =− EE360 – Lecture 17 •Fo r p + n (heavy p-doping) r n + p (heavy n-doping) 2 P 0 P i D n D Iq A L N 2 N 0 N i A Dn A L N Small Signal and Transient pn Junction Characteristics Objectives : Understand the physics and models of semiconductor devices including diodes, bipolar junction transistors, and field-effect transistors. Analyze various transistors, and field effect transistors. Analyze various device structures and calculate their model parameters. Questions to be answered : • What are the capacitance and conductance of a diode in reverse bias? • How can we use the reverse bias C-V relationship to profile doping? • What are capacitance and conductance of a diode in forward bias? EE360 – Lecture 17 • How do the capacitance and conductance (or admittance) vary with frequency? What happens when we abruptly switch from reverse to forward bias?
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2 Small Signal Response • Place the diode in a circuit with – d.c. bias V A , and – a.c. bias v a • What is the – direct current, I , and – alternating current i? From Pierret, p. 302 EE360 – Lecture 17 alternating current, i ? • What is an equivalent circuit for the diode? Reverse Bias Response to an a.c. signal • a.c. signal causes depletion width to increase and decrease • additional charge is added and subtracted at the depletion region edges From Pierret, pg. 304. EE360 – Lecture 17 • diode behaves like a parallel plate capacitor Alternating charge build up on plates W Equivalent circuit for reverse biased diode.
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3 Depletion (Junction) Capacitance • The capacitance associated with the depletion region is just that of a parallel plate capacitor: A s 0 ε κ = W Plates with area, A • For a given material, the depletion-region capacitance is completely determined by the depletion width. Thus, W J () junction. step a for 2 2 / 1 0 0 + = A bi D A D A s s J V V N N N N q A C Remember N B is the EE360 – Lecture 17 • In reverse bias, the depletion capacitance dominates the total capacitance. junction. step sided - one a for 2 2 / 1 0 0 = A bi B s s J V V qN A C Remember, is the doping level of the lightly doped side (the “background” doping) Dopant Profiling using the Depletion Capacitance • We can measure capacitance to determine doping levels • For a one-sided step-junction we find tha For a one sided step junction we find that • For an arbitrary one-sided junction we can extract the doping profile from: A bi S B J
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Lecture 17 - pn junction small signal response -...

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