PN Junction Diodes

1
PN Junction Diodes

• DIODE
• The term DIODE references the PN junction
  diode. There are other types such as the zener
  diode, light emitting diode (LED). The diode is
  a two electrode device that conducts in one
  direction just as the PN junction.

1. The n type material is the cathode. 2. The p
type material is the anode. 3. The arrow points
to the more negative cathode with respect to the
anode. 4. The difference of potential across the
device ( anode to cathode) exceeds that of the
barrier potential.
2
PN Junction Diodes

• Forward and Reverse Bias
• See Figure 2.2 and 2.3 (Page 25)

Note A junction will not conduct if the arrow
symbol points towards the more positive
potential of the two diode electrodes.
3
PN Junction Diodes

• Testing with a Multi-meter

4
PN Junction Diodes

• THE IDEAL DIODE MODEL
• An ideal diode is an open switch when reverse
  biased or a closed switch when forward biased.
  But, remember we dont live in an ideal world!
  This model is only used in non detailed circuit
  analysis or troubleshooting.
• See graph 2.4 (Page 27)
• IF Diode Forward Current
• VF Forward Voltage
• VR Reverse Voltage
• IR Reverse Current.
• Quadrant 1 is the Forward operating region
• Quadrant 3 is the Reverse operating region
• In Forward bias the closed switch has no
  voltage dropped across it as VF 0 while IF goes
  to maximum dependant on the source. In Reverse
  bias the open switch has all of the voltage
  dropped across it. As VR increases there is no
  increase in IF.
• Remember, these calculations are only used in
  IDEAL examples. We are not including the diodes
  contributions to the circuit here.

5
IF Diode Forward Current VF Forward Voltage
VR Reverse Voltage IR Reverse Current.
6
Practice problem on page 27 Example 2.1
Show circuit on board VD1 V3 12V therefore IT
0 Therefore VR1 VR2 0V
7
(No Transcript)
8
PN Junction Diodes

• THE PRACTICAL DIODE
• There are many real considerations in the
  analysis of the diode. The first is VF or Forward
  Voltage. Others include Peak Reverse Voltage,
  Average Forward Current and Forward Power
  Dissipation.
• FORWARD VOLTAGE (VF)
• A slight voltage developed across the diode
  barrier (Barrier Potential) and is referred to as
  VF.
• KNEE VOLTAGE (VK)
• The point at which VF occurs and IF begins to
  increase suddenly is known as the Knee voltage or
  VK. It can also represent a sudden decrease in
  current. VF for the silicon pn junction diode is
  approximately 0.7V to 1.1 depending on te current
  flow.
• See figure 2.7 a b (Page 30)
• NOTE
• 1. IF 0 until VK is reached.
• 2. Once VS gt VK the diode will be conducting.
• 3. While conducting VF VK

9
PN Junction Diodes
10
PN Junction Diodes
11
PN Junction Diodes

• Effect of VF
• The sum of the device/component voltages must
  equal that of the source. ( Kirchoffs Voltage
  Law)
• therefore Vs VF VR1 (Fig 2.9 Page 31)
• Substituting Vk VF 0.7V
• we get 5V0.7 VR1
• VR1 5 - 0.7 4.3V
• and
• IT VS - 0.7V 5 - 0.7 4.3 mA
• R1 1K?
• NOTE If your measured result are more than /-
  10 (typically), than they are not acceptable.
  This is why the Ideal model is not used in
  mathematical circuit analysis. Read pages 32 and
  33 for details.

12
PN Junction Diodes

• Peak Reverse Voltage (VRRM)
• VRRM is the maximum voltage that a diode can
  handle in a reverse bias mode before the diode is
  forced to conduct. Once the VRRM is exceeded the
  depletion region breaks down and allows
  conduction in the reverse direction. VRRM can
  range from a few volts (Zener diode) to gt 1,000
  V.
• At the point of VR gt VRRM, IR increases quite
  rapidly and will ultimately destroy the
  pn-junction diode. This intense surge of current
  is called AVALANCHE CURRENT

13
PN Junction Diodes

• Peak Reverse Voltage (VRRM)
• When replacing a diode with another type,you must
  ensure that the VRRM is ? that of the original.
  When making a design decision the VRRM must be
  greater than that of the maximum reverse voltage
  (Peak value). A rule of thumb is 20 greater than
  the peak source.
• See figure 2.15 (Page 35)
• VRRM - 1.2 VR (Pk) 1.2 X 50V 60 V
• Look at text book example 2.8 on page 36.
• Look at spec sheet on page 4

14
PN Junction Diodes

• Average Forward Current (Io)
• The maximum allowable amount of dc forward
  current for a diode is the Average Forward
  Current. If this value is exceeded through the
  diode, it may be destroyed.
• You must first determine IF of the circuit that
  the diode is to be placed in. Then add 20 to
  that value. The diode you select will have an Io
  ? that new value. (1.2 X IF Io)
• See example 2.9 - next slide

15
PN Junction Diodes

• Average Forward Current (Io)

16
PN Junction Diodes

• Answer to previous slide

17
PN Junction Diodes

• Forward Power Dissipation (PD(MAX))
• The maximum possible power dissipation of the
  device while being forward biased is the
  (PD(MAX)).
• P IV
• where
• P power dessipated by device
• I the current through the device
• V the voltage drop across the device
• or
• Thus..PF IF X VF (where PF Forward
  Power)
• first IF VS - 0.7 then substitute
• RT
• Then PD(MAX) PF X 1.2 (20 minimum)
• See example 2.10 Page 37 next slide

18
PN Junction Diodes

• Forward Power Dissipation (PD(MAX))

19
PN Junction Diodes

• Answer to previous slide

(minimum)
20
(No Transcript)
21
Answer
Remember When replacing 1 diode with another,
check VRRM, IF and PD(MAX). They must all be at
least 20 greater than their respective values.
VRRM (minimum) 1.2 x Maximum reverse voltage
of circuit. IF (minimum) 1.2 x IF of
circuit. PD(max) (minimum) 1.2 x PF of circuit.
22
PN Junction Diodes

• The complete Diode Model
• To understand the true operating characteristics
  of the diode, we must further investigate two
  items. Bulk Resistance (RB) and Reverse current
  (IR).
• Bulk Resistance, as we have discussed previously,
  is the natural resistance of the n and p type
  materials that make up the diode. The effects of
  RB is seen in Quadrant I of the graph in figure
  2.19 of your text (on page 40) in the Forward
  bias mode.
• VF is not constant and changes with the value of
  IF. ?IF (change in IF) is the result of the
  current passing through the RB of the diode.
• The total voltage being dropped across the diode
  is the 0.7 V barrier potential and the voltage
  being produced by IF x RB.
• VF 0.7 IF RB (for Si)
• NOTE As IF increases so does the
• voltage across the diode by ?IF RB

23
PN Junction Diodes
24
PN Junction Diodes
See example 2.12 page 41
25
PN Junction Diodes

• Reverse Current (IR)
• Ideally we have discussed that conduction ceases
  during the reverse biasing of a diode. However,
  again we don not live in an ideal world. As such
  there is a minimal amount of current that does
  flow during reverse bias mode. This current is
  referred to as REVERSE CURRENT (IR). IR is made
  up of two separate currents. REVERSE SATURATION
  CURRENT (IS) and SURFACE LEAKAGE CURRENT
  (ISL)..thus, IR IS ISL
• Reverse Saturation Current (IS)
• .is the result of thermal activity in the diode
  material and is directly proportional to
  temperature. It is unaffected by the reverse
  voltage (VR). IS is typically much larger than
  ISL.
• Surface Leakage Current (ISL)
• .is generated along the surface of the pn-diode
  and controlled directly by VR (reverse voltage).
  It is independent of temperature. Since IS gtgt ISL
  there is very little effect on IR with a change
  in ISL.
• The effect of the reverse current can be used to
  find the effective reverse bias resistance of a
  diode. See figure 2.21 page 42.

26
PN Junction Diodes
27
PN Junction Diodes

• Diffusion Current
• IF below the Knee voltage (VK) does not instantly
  drop to zero. Once VF is below the barrier
  potential the depletion layer begins to form. Not
  until a reverse bias mode, or at least VF 0
  will the depletion layer reach its maximum width,
  thus its maximum resistance. The small amount of
  current between VR and VK is know as the
  Diffusion Current.
• Temperature Effects (IF VF, and IR IS)
• Diode characteristics are affected by
  temperature. Increased thermal activity causes a
  decrease in device resistance. (See figure 2.23)
• NOTE In forward bias mode first we look at IF
  VF
• 1. As temperature increases, IF increases at a
  specified value of VF. _at_ 0.7V line
• I15mA _at_ 25ºC, and I225mA_at_100 ºC
• 2. VF decreases at a specified value of IF
• _at_20mA line
• V10.75V _at_ 25 ºC and V20.68V _at_ 100 ºC
• Thus with increased thermal activity IF goes up
  and VP goes down

28
PN Junction Diodes
29
PN Junction Diodes

• Temperature Effects (IR IS)
• First IR IS ISL
• but IS gtgt ISL
• therefore IS IR
• The effect on Is in reverse bias mode is very
  similar to the effects on IF in forward bias
  mode. Is increases as temperature increases. IT
  increases at a rate of 2? for every 10? rise in
  temperature. (It doubles with every 10 ? rise).
• IR IR (2 ?)
• where
• IR new value at new temperature
• IR value of IR _at_ 25?C
• ? ( high temperature -25) / 10
• See figure 2.24 on page 44 and
• See example 2.13 on page 45

30
PN Junction Diodes
31
PN Junction Diodes
32
PN Junction Diodes

• Answer