Power Semiconductor Diode - Power Semiconductor Devices

# Power Semiconductor Diode - Power Semiconductor Devices - Electronics and Communication Engineering (ECE)

``` Page 1

Module
1

Power Semiconductor
Devices
Version 2 EE IIT, Kharagpur 1
Page 2

Module
1

Power Semiconductor
Devices
Version 2 EE IIT, Kharagpur 1

Lesson
2

Constructional Features,
Operating Principle,
Characteristics and
Specification of Power
Semiconductor Diode
Version 2 EE IIT, Kharagpur 2
Page 3

Module
1

Power Semiconductor
Devices
Version 2 EE IIT, Kharagpur 1

Lesson
2

Constructional Features,
Operating Principle,
Characteristics and
Specification of Power
Semiconductor Diode
Version 2 EE IIT, Kharagpur 2
Instructional Objective

On Completion the student will be able to

1. Draw the spatial distribution of charge density, electric field and electric potential in a
step junction p-n diode.
2. Calculate the voltage drop across a forward biased diode for a given forward current and
vice-verse.
3. Identify the constructional features that distinguish a power diode from a signal level
diode.

4. Differentiate between different reverse voltage ratings found in a Power Diode speciation
sheet.

5. Identify the difference between the forward characteristic of a power diode and a signal
level diode and explain it.

6. Evaluate the forward current specifications of a diode for a given application.
7. Draw the “Turn On” and “Turn Off” characteristics of a power diode.
8. Define “Forward recovery voltage”, “Reverse recovery current” “Reverse Recovery
charge” as applicable to a power diode.
Version 2 EE IIT, Kharagpur 3
Page 4

Module
1

Power Semiconductor
Devices
Version 2 EE IIT, Kharagpur 1

Lesson
2

Constructional Features,
Operating Principle,
Characteristics and
Specification of Power
Semiconductor Diode
Version 2 EE IIT, Kharagpur 2
Instructional Objective

On Completion the student will be able to

1. Draw the spatial distribution of charge density, electric field and electric potential in a
step junction p-n diode.
2. Calculate the voltage drop across a forward biased diode for a given forward current and
vice-verse.
3. Identify the constructional features that distinguish a power diode from a signal level
diode.

4. Differentiate between different reverse voltage ratings found in a Power Diode speciation
sheet.

5. Identify the difference between the forward characteristic of a power diode and a signal
level diode and explain it.

6. Evaluate the forward current specifications of a diode for a given application.
7. Draw the “Turn On” and “Turn Off” characteristics of a power diode.
8. Define “Forward recovery voltage”, “Reverse recovery current” “Reverse Recovery
charge” as applicable to a power diode.
Version 2 EE IIT, Kharagpur 3
Power Semiconductor Diodes

2.1 Introduction

Power semiconductor diode is the “power level” counter part of the “low power signal diodes”
with which most of us have some degree of familiarity. These power devices, however, are
required to carry up to several KA of current under forward bias condition and block up to
several KV under reverse biased condition. These extreme requirements call for important
structural changes in a power diode which significantly affect their operating characteristics.
These structural modifications are generic in the sense that the same basic modifications are
applied to all other low power semiconductor devices (all of which have one or more p-n
junctions) to scale up their power capabilities. It is, therefore, important to understand the nature
and implication of these modifications in relation to the simplest of the power devices, i.e., a
power semiconductor diode.

2.2  Review of Basic p-n Diode Characteristics

A p-n junction diode is formed by placing p and n type semiconductor materials in intimate
contact on an atomic scale. This may be achieved by diffusing acceptor impurities in to an n type
silicon crystal or by the opposite sequence.
In an open circuit p-n junction diode, majority carriers from either side will defuse across the
junction to the opposite side where they are in minority. These diffusing carriers will leave
behind a region of ionized atoms at the immediate vicinity of the metallurgical junction. This
region of immobile ionized atoms is called the space charge region. This process continues till
the resultant electric field (created by the space charge density) and the potential barrier at the
junction builds up to sufficient level to prevent any further migration of carriers. At this point the
p-n junction is said to be in thermal equilibrium condition. Variation of the space charge density,
the electric field and the potential along the device is shown in Fig 2.1 (a).

Version 2 EE IIT, Kharagpur 4
Page 5

Module
1

Power Semiconductor
Devices
Version 2 EE IIT, Kharagpur 1

Lesson
2

Constructional Features,
Operating Principle,
Characteristics and
Specification of Power
Semiconductor Diode
Version 2 EE IIT, Kharagpur 2
Instructional Objective

On Completion the student will be able to

1. Draw the spatial distribution of charge density, electric field and electric potential in a
step junction p-n diode.
2. Calculate the voltage drop across a forward biased diode for a given forward current and
vice-verse.
3. Identify the constructional features that distinguish a power diode from a signal level
diode.

4. Differentiate between different reverse voltage ratings found in a Power Diode speciation
sheet.

5. Identify the difference between the forward characteristic of a power diode and a signal
level diode and explain it.

6. Evaluate the forward current specifications of a diode for a given application.
7. Draw the “Turn On” and “Turn Off” characteristics of a power diode.
8. Define “Forward recovery voltage”, “Reverse recovery current” “Reverse Recovery
charge” as applicable to a power diode.
Version 2 EE IIT, Kharagpur 3
Power Semiconductor Diodes

2.1 Introduction

Power semiconductor diode is the “power level” counter part of the “low power signal diodes”
with which most of us have some degree of familiarity. These power devices, however, are
required to carry up to several KA of current under forward bias condition and block up to
several KV under reverse biased condition. These extreme requirements call for important
structural changes in a power diode which significantly affect their operating characteristics.
These structural modifications are generic in the sense that the same basic modifications are
applied to all other low power semiconductor devices (all of which have one or more p-n
junctions) to scale up their power capabilities. It is, therefore, important to understand the nature
and implication of these modifications in relation to the simplest of the power devices, i.e., a
power semiconductor diode.

2.2  Review of Basic p-n Diode Characteristics

A p-n junction diode is formed by placing p and n type semiconductor materials in intimate
contact on an atomic scale. This may be achieved by diffusing acceptor impurities in to an n type
silicon crystal or by the opposite sequence.
In an open circuit p-n junction diode, majority carriers from either side will defuse across the
junction to the opposite side where they are in minority. These diffusing carriers will leave
behind a region of ionized atoms at the immediate vicinity of the metallurgical junction. This
region of immobile ionized atoms is called the space charge region. This process continues till
the resultant electric field (created by the space charge density) and the potential barrier at the
junction builds up to sufficient level to prevent any further migration of carriers. At this point the
p-n junction is said to be in thermal equilibrium condition. Variation of the space charge density,
the electric field and the potential along the device is shown in Fig 2.1 (a).

Version 2 EE IIT, Kharagpur 4

(a)  (b) (c)

Fig 2.1: Space change density the electric field and the electric potential in side a p-n
junction under (a) thermal equilibrium condition, (b) reverse biased condition,
(c) forward biased condition.

When an external voltage is applied with p side move negative then the n side the junction is
said to be under reverse bias condition. This reverse bias adds to the height of the potential
barrier. The electric field strength at the junction and the width of the space change region (also
called “the depletion region” because of the absence of free carriers) also increases. On the other
hand, free minority carrier densities (n
p
in the p side and p
n
in the n side) will be zero at the edge
of the depletion region on either side (Fig 2.1 (b)). This gradient in minority carrier density
causes a small flux of minority carriers to defuse towards the deletion layer where they are swept
immediately by the large electric field into the electrical neutral region of the opposite side. This
will constitute a small leakage current across the junction from the n side to the p side. There
will also be a contribution to the leakage current by the electron hole pairs generated in the space
change layer by the thermal ionization process. These two components of current together is
called the “reverse saturation current I
s
” of the diode. Value of I
s
is independent of the reverse
voltage magnitude (up to a certain level) but extremely sensitive to temperature variation.
When the applied reverse voltage exceeds some threshold value (for a given diode) the reverse
current increases rapidly. The diode is said to have undergone “reverse break down”.
Reverse break down is caused by "impact ionization" as explained below. Electrons accelerated
by the large depletion layer electric field due to the applied reverse voltage may attain sufficient
knick energy to liberate another electron from the covalent bonds when it strikes a silicon atom.
The liberated electron in turn may repeat the process. This cascading effect (avalanche) may
produce a large number of free electrons very quickly resulting in a large reverse current. The
power dissipated in the device increases manifold and may cause its destruction. Therefore,
operation of a diode in the reverse breakdown region must be avoided.
Version 2 EE IIT, Kharagpur 5
```

### Top Courses for Electronics and Communication Engineering (ECE)

Track your progress, build streaks, highlight & save important lessons and more!
 (Scan QR code)

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

,

;