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