DIODES

 

            The IDEAL DIODE serves as an electrical one-way valve.  It conducts current much more readily in one direction, called the FORWARD (BIAS) DIRECTION, than it does in the other direction called the REVERSE (BIAS) DIRECTION. 

            IDEAL DIODE acts like a SWITCH that closes to allow current flow in the forward direction but opens to prevent current flow in the reverse direction.

 

 

 

ON
 

OFF
 

(a)                                                                    (b)                                (c)

                                                                          

                                    

 

 

ON
 

OFF
 
 

 

 


                                                                      

 

(d)

 

Example:

Vout
 
                                   Vin

 

 

 

 

The SEMICONDUCTOR DIODE has the circuit symbol

and i-v characteristic as shown:

Cut in
 
                          

A
 

Forward Bias
 

Reverse

Bias
 
                               id

K
 

-Io
 

Reverse Breakdown
 

 

  N
 

  P
 
       

                                               

 

                                                                                                           iD = Io (e VD/hVt – 1)

                                                                                     

 

 

iD             = Io(eVD/hVT-1) where

iD             = current passing through the diode

VD          = voltage across the diode

VT          = kT/e

» 25.3 mV at 20°C

Io             = temperature dependent saturation current or drift current

T          = absolute temperature

k          = Boltzmann’s constant

= 1.38 x 10-23 J/K 

h          = empirical constant

= 1 for Ge

= 2 for Si

Vg        = contact potential or cutin voltage

= 0.2 to 0.3 V for Ge

= 0.6 to 0.7 V for Si

 

Example:                                                                                                              VD

Text Box: VS

R
 

i = iD

 

Figure 1. Example of a Diode Circuit

 

1)         Graphical Method:

VS = VD + iDR

iD = (VS - VD)/R

 

iA
 

VS/R
 

A
 

VS
 

V
 

VA
 

i
 

 

Figure 2. Graphical Method of Solving Diode Circuits

 

 

2)         Numerical Method:

Given   Io = 0.1 mA;     VS = 12V;        R = 1 kW;        T = 20°C

 

VS = VD + iDR

 

iD = (VS - VD)/R

 

Io(e40VD -1) = (12 - VD)/1000

 

VD = 0.292V

 

iD = 11.7 mA

 

 

 

DIODE MODELS

 

1)         Ideal diode

Vg = 0

 

AK    “on”:     zero impedance;           VD ³ 0;            Rf = 0

 

AK          “off”:    infinite impedance;        VD < 0;            Rr = ¥ 

 

 

2)         Piecewise Linear Diode

 

“on”:     AK         VD ³ Vg = 0.1 to 0.2V for Ge

   0.6 to 0.7V for Si

Vg
 

Rf
 

 

 

 

      “off”:    AK          VD < Vg  

 

          

Rr

 
 
 

 

 


Default values if not specified:    Rf = 0;  Rr = ¥; VD = 0.2V for Ge, 0.6V for Si

 

 

ideal
 

i
 

Vz
 

V
 

Io
 

Vg
 

piecewise linear

 
 

semiconductor
 

 

Figure 3. Transfer Function Characteristics

R
 
 


Example:         

 

+

 

Vo

 

 

-

 
 

+

 

Vi

 

 

-

 

 
 

VR
 

D1
 

 

Figure 4. Clipping or Limiting Circuit

 

 

Sol’n 1: ideal diode, V­­g=0

 

            If Vi < VR, D is off: vo=vi                                           If Vi > VR, D is on: vo = vR

 

 

 

 

 

 

 

Sol’n 2: Piecewise linear

 


            If Vi < VR + Vg, D is off

                        Vo = ­­­___Rr__Vi + __R__VR

                                  R + Rr          R + Rr

                        If Rr à ¥

                         Vo @ Vi

                                                                                    Vo = iRr + VR

                                                                                         = Rr(Vi – VR)/(R + Rr) + VR

 

            If Vi > VR + Vg, D is on

                        Vo = ­­­___Rf__Vi + __R__(VR+ Vg)

                                  R + Rf          R + Rf

                        If Rf = 0

                        Vo = VR + Vg

 

 

 

 

 

 

Other Diode Cicuits

 

 

1)

2)

3)

 

 

 

4)

 

i.e.        Vi < VR1< VR2 , Vo = VR1 , D1 on, D2 off

                                     VR1 < Vi < VR2 , Vo = Vi  ,  D1 off, D2 off

                                     Vi  >  VR2 , Vo = VR2 ,          D1 off, D2 on

 

5) Peak Detector

 

 

6)

   Vi = Vp sin (wt)

 

 

   Vo = Vin – Vc @ Vin – Vp

                RC >> T

 

 

 

7)

  

  

    Vo = Vin + Vc @ Vin + Vp

                RC >> T

 

 

 

8) doubler

 

9) triplet

 

10) quadruplet

 

SPECIAL DIODES

 

1) ZENER DIODES

-         diodes that work best in the breakdown region

 

 

2) OPTO-ELECTRONIC DEVICES

            a) LEDs     gallium, arsenic, phosphorous

                           low voltage, long life, fast on-off

                           switching

 

                  b) 7-segment display

                       

                       

 

 

                                       c) photodiode

                       

 

 

 

            d) optocoupler or optoisolator

                       

 

 

 

 

 

 

 

 

            e) varactor (voltage-variable capacitance, varicap, epicap, tuning diode)

                       

 

 

 

                        ® used in AFC, automatic frequency control, circuit of an FM radio.

                                                CR » CC + Co/Ö(1 + 2VR)

 

            f) varistor (transient suppressor)

                        dips – sever voltage drops

                        spikes – short over-voltage of 500V or more

 

            g) Schottky diodes

o       high speed application diodes

o       with very low effective capacitance and low forward voltage drop