Diode 12q3a

Electronic Circuits EE 359-Lecture 2

Prof. Uf Tureli Dept. of ECE Stevens Institute of Technology 1

Course Outline  Syllabus

and announcements http://koala.ece.stevens-tech.edu/~utureli/EE359  Review of Diodes 

HW 5th Ed.: 2.100,3.18,3.19,3.26

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Basic Semiconductors Diode Structure

Symbol

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Diodes

Fig. 3.1 The ideal diode: (a) diode circuit symbol; (b) i-v characteristic; (c) equivalent circuit in the reverse direction; (d) equivalent circuit in the forward direction. 4

Diode Circuits

During positive cycle, diode allow current to through and output voltage is positive 5

Diode Characterization  Diodes

have a nonlinear response to voltage  We model different regions of operation Forward Bias







i  I s e v / nVT  1



i  I s e v / nVT  1

Reverse Bias i  I s Breakdown

Fig. 3.7 The i-v characteristic of a silicon junction diode.

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Diode Regions  Diodes

have negligible current when biased in reverse direction  Diodes have a 0.7V drop in the forward direction Fig. 3.8 The diode i-v relationship with some scales expanded and others compressed in order to reveal details. 7

Forward-Bias Region  Terminal

voltage v is positive

i  I s (e v / nVT  1)

23

VT 

kT q

k=Boltzmann’s constant= 1.38 x10 T=absolute temparature in K=273+ Celcius q=magnitude of electronic charge= 1.6 x10 19 C i 1 <2, depending on material, assume n=1 i  I S e v / nVT  v  nVT ln IS EXAMPLE: Diode with n=1 displays forward voltage of 0.7V at 1mA. Find I S SOLUTION: i  I S e v / nV  I S  ie  v / nV I S  10 3 e 700 / 25  6.9 x10 16 A  10 15 A For n=1: I S  10 3 e 700 / 50  8.3 x10 10 A  10 9 A For n=2: T

T

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Reverse-Bias Region  Terminal

voltage v is negative

i  I s v is negative and a few times larger than

VT

Current in the reverse direction is constant and equal to and called saturation current. I S  Typically, real life diodes exhibit much larger reverse 14 15 I  10  10 A currents S 

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Break Down Region  Knee

of diode I-v characteristic, the voltage is less than Zener voltage.

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Physical Structure on PN Junction  We can

simplify Diode physics by modeling it as a 2D PN junction  PN junction:P and N regions by different dopings: n  p  ni  Diffusion & Drift

Fig. 3.10 Simplified physical structure of the junction diode. ( 11

Reverse-Bias PN Junction Fig. 3.13 The pn junction excited by a constant-current source I in the reverse direction. To avoid breakdown, I is kept smaller than Is. Note that the depletion layer widens and the barrier voltage increases by Vr volts, which appears between the terminals as a reverse voltage.

 Reverse

Bias: Drift current Is, indep. of voltage,

IS  ID  I 12

Depletion Capacitance  Charge

stored in the PN junction depends on the voltage across junction.

Fig. 3.14 The charge stored on either side of the depletion layer as a function of the reverse voltage Vr. 13

When the pn junction is excited by a constantcurrent source supplying a current I in the forward direction: The depletion layer narrows and the barrier voltage decreases by V volts, which appears as an external voltage in the forward direction.

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Fig. 3.17 Minority-carrier distribution in a forward-biased pn junction. It is assumed that the p region is more heavily doped than the n region; NA  ND. 15

Modeling 

Approximate the diode forward characteristic with two straight lines.

Fig. 3.20 16

Simplified Piecewise Linear Approximation  Diode

Is nonlinear

Fig. 3.21 Piecewise-linear model of the diode forward characteristic and its equivalent circuit representation. 17

Constant Voltage Drop Model 



Development of the constant-voltagedrop model of the diode forward characteristics. A vertical straight line (b) is used to approximate the fastrising exponential

Fig. 3.23 Development of the constant-voltage-drop model of the diode forward characteristics. A vertical straight line (b) is used to approximate the fast-rising exponential.

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Fig. 3.24 The constant-voltage-drop model of the diode forward characteristic and its equivalent circuit representation. 19

Fig.3.25 Development of the diode small-signal model. Note that the numerical values shown are for a diode with n = 2.

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Fig. 3.26 Equivalent circuit model for the diode for small changes around bias point Q. The incremental resistance rd is the inverse of the slope of the tangent at Q, and VD0 is the 21 intercept of the tangent on the vD axis (see Fig. 3.25).

Fig. 3.27 The analysis of the circuit in (a), which contains both dc and signal quantities, can be performed by replacing the diode with the model of Fig. 3.26, as shown in (b). This allows separating the dc analysis [the circuit in (c)] from the signal analysis [the circuit in (d)].

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Fig. 3.30 Circuit symbol for a zener diode.

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Fig. 3.31 The diode i-v characteristic with the breakdown region shown in some detail. 24

Fig. 3.32 Model for the zener diode.

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Fig. 3.36 Block diagram of a dc power supply.

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Fig. 3.37 (a) Half-wave rectifier. (b) Equivalent circuit of the half-wave rectifier with the diode replaced with its battery-plus-resistance model. (c) transfer characteristic of the rectifier circuit. (d) Input and output waveforms, assuming that rD  R. 27

Fig. 3.38 Full-wave rectifier utilizing a transformer with a center-tapped secondary winding. (a) Circuit. (b) Transfer characteristic assuming a constant-voltage-drop model for the diodes. (c) Input and output waveforms. 28

Fig. 3.39 The bridge rectifier: (a) circuit and (b) input and output waveforms. 29

Fig. 3.41 Voltage and current waveforms in the peak rectifier circuit with CR  T. The diode is assumed ideal. 30

Fig. 3.46 A variety of basic limiting circuits.

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Fig. 4.1 A simplified structure of the npn transistor.

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