1. Instrumentation & Power Electronic Systems
POWER SEMICONDUCTOR DIODES
By: Engr. Irfan Ahmed Halepoto 1

2. Power Semiconductor Devices: Categorization
Power Semiconductor Devices can be categorised into three groups:
Un-Controlled:
Diode
Fully-Controlled:
Power transistors: e.g. BJT, MOSFET, IGBT.
Semi-Controlled:
Thyristor (SCR).
GTO

3. POWER SEMICONDUCTOR DEVICES : Classification
POWER SEMICONDUCTOR DIODES (Un-Controlled)
General purpose power diodes
Fast recovery power diodes
Schottky power diodes
POWER TRANSISTOR (Fully-Controlled)
Power BJT
Power MOSFETs
Insulated Gate Bipolar Transistors (IGBTs)
POWER THYRISTOR (Semi-Controlled)
Silicon Controlled Rectifier (SCR)
Gate turn-off Thyristor (GTO)
Diacs, Triacs, Unijunction transistors, etc.
Complete range of family members (more than 15 members)

4. Constructional Geometry of P-N Diode
P-N junction diode is formed by placing p & n type semiconductor materials in intimate contact.
This is achieved by diffusing acceptor impurities (p type) in to an donor impurities (n type) silicon crystal or vice versa .
Normally majority carriers from either side will diffuse across the junction to the opposite side where they are in minority.
These diffusing carriers will create a region of ionized atoms at the immediate vicinity of the metallurgical junction.
This region of immobile ionized atoms is called the Depletion region (space charge region).
This process continues till the resultant electric field (created by space charge density) and 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 (no more migration).

5. Space charge density: electric field & electric potential in p-n junction under Forward biased condition
When diode is forward biased (i.e., p side more positive than n side), potential barrier is lowered and large number of minority carriers are injected to both sides of the junction.
The injected minority carriers eventually recombines with the majority carries as they defuse further into electrically neutral drift region.
The excess free carrier density in p & n side follows exponential decay characteristics length , known as the "minority carrier diffusion length“.

6. Space charge density: electric field & the electric potential in p-n junction under reverse biased condition
At the edge of depletion region, minority carrier densities causes a flux (movement of molecules), which defuse them towards the deletion layer where they are swept by electric field in opposite side .
This will constitute a small leakage current across the junction from the n side to the p side, while electron hole pairs also generates additional leakage current, known as “reverse saturation current (Is) of the diode.
When the applied reverse voltage exceeds some threshold value, reverse current increases rapidly.
Diode is said to have undergone “reverse break down”, caused by "impact ionization“.

7. Power Semiconductor Devices Specialty
Power semiconductor devices represent the heart of modern power electronics, with two major desirable characteristics guiding their manufacturing and design development:
Switching speed
(Turn-On times)
(Turn-Off times)
Power-handling capabilities
Voltage blocking capabilities
Current carrying capabilities

8. Power Semiconductor Devices (Power Switches)
Power switches operates in two states:
Fully on. i.e. switch closed.
Conducting state
Fully off , i.e. switch opened.
Blocking state

9. Switching Characteristics: Ideal Switch
When device is in the nonconduction state (off-state),No limit on the amount of device voltage (forward or reverse blocking voltage)
Infinite off-state resistance, i.e., zero leakage.
When device is in the conduction state (On -state), no limit on the amount of current (forward or reverse current) that the device can carry.
Zero-on state voltage drop
No limit on the operating speed of the device when it changes state
zero rise and fall times
Ideal Switch: current & voltage waveforms

10. Switching Characteristics: Practical Switch
Limited power-handling capabilities (within some specific range)
limited conduction current when switch is in the on state
limited blocking voltage when the switch is in the off-state.
Limited switching speed, due to the finite turn-on and turn-off times.
Finite on-state and off-state resistances i.e., existence of forward voltage drop in the on state, and reverse current flow (leakage) in the off-state
Due to these characteristics, practical switch experiences power losses in the on and off states and during switching transitions.

11. Switching Characteristics: Practical Switch
Practical switch: current and voltage waveforms

12. Reverse Recovery Time IF
IRM VR t0 t2
trr= ( t2 - t0 )
VRM
When a diode is switched quickly from forward to reverse bias, it still continues to conduct due to the minority carriers which remains in the p-n junction.
The minority carriers require finite time, i.e, trr (reverse recovery time) to recombine with opposite charge and neutralise.
This time is called reverse recovery time of the diode.
Effects of reverse recovery are increase in switching losses, increase in voltage rating, over-voltage (spikes) in inductive loads.

13. Reverse Recovery Characteristics: Soft Recovery
ta is the time to remove the charge stored in junction’s depletion region
tb is the time to remove the charge stored in the bulk semiconductor material
Reverse recovery time = trr = ta+tb
Peak Reverse Current = IRR = ta(di/dt)

14. Reverse Recovery Characteristics: Abrupt Recovery
Reverse recovery time = trr = ta+tb
Peak Reverse Current = IRR = ta(di/dt)

15. POWER SEMICONDUCTOR DIODES…………
Power semiconductor diodes have a pn-junction similar to signal diodes.
Power semiconductor diode P-N junction is normally formed by allowing Diffusion, Epitaxial Growth process, which allows modern power diodes to produce desire characteristics.
Power diodes have larger power, voltage & current-handling capabilities than those of ordinary diodes.
Frequency response or switching speed is low compared with that of signal diodes.
Power diodes can operate at high junction temperatures.

16. Diffusion & Epitaxial Growth process
Power semiconductor diode P-N junction is formed by allowing Diffusion, Epitaxial Growth process
Diffusion process is a transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion.
The result of diffusion is a gradual mixing of material.
Adjust of molecule concentration for required results.
Epitaxial Growth process refers to the process of growing a thin layer of single crystal silicon over a single crystal silicon substrate.
growth of an Epitaxial layer over the substrate offers improvements in the performance of devices, prevention of latch-up in CMOS circuits and improved doping control .

17. Diffusion & Epitaxial Growth process Epitaxial Growth process
Diffusion process
Epitaxial Growth process

18. Power Semiconductor Diode Characteristics
Power Diode:
(a) Simplified structure (b) Circuit symbol (c) Current limits
(d) Voltage limits (e) Safe Operating Area

19. Power Semiconductor Diode Classifications
Power diodes are classified according to
Constructional features,
Type of material used in doping process,
Power ratings,
Switching characteristics,
Reverse recovery time,
Application wise
General purpose power diodes
Fast recovery power diodes
Schottky power diodes

20. General Purpose Power diode
A general purpose power diode is a typical two-terminal pn-junction device.
General purpose power diode are manufactured by Diffusion process.
Used in low power applications where recovery time (slow response) is not an issue.
diode rectifiers
converters for a low input frequency up to 1 KHz.
Line commutated converters
Alloyed type rectifier diodes are used in welding power supplies.

21. General Purpose Power diode: Characteristics
On state voltage: very low (below 1V)
Large trr (about 25us) (very slow response)
Very high current ratings (more than 3kA - up to 5kA)
Very high voltage ratings (50V to 5kV)
Used in line-frequency (50/60Hz) applications such as rectifiers

22. General Purpose Power diode Biasing

23. Fast Recovery Power Diode
Low recovery time (faster switching) , normally less than 5uS.
Current ratings from 1A to 1000A.
Voltage ratings from 50V to 3kV.
For high rating (more than 400V), these diodes are manufactured by DIFFUSION process
Recovery time is controlled by Platinum or Gold diffusion
For less than 400V ratings, EPITAXIAL diodes provides very fast recovery time.
Epitaxial diodes have a very narrow base width resulting in recovery time of about 50nS.

24. Fast Recovery Power Diode
The major field of applications is in electrical power conversion i.e., in free-wheeling ac-dc and dc-ac converter circuits.
Use of fast recovery diodes are preferable for free-wheeling in SCR circuits because of low recovery loss, lower junction temperature and reduced di/dt 
These diodes are primarily used in communication circuits above 1 GHz (high frequency circuits)

25. Schottky Power diodes
Schottky diode is a semiconductor diode with a low forward voltage and a very fast switching action .
Reverse recovery time is almost zero (in ideal conditions).
Schottky diode use a Metal–Semiconductor junction as Schottky barrier, instead of a semiconductor junction in a conventional diodes.
Schottky diode is a majority carrier semiconductor device.
When a Schottky diode is forward biased, free electrons on the N-side gain enough energy and travel to the metal side and cause a forward current,
since the metal does not have any boles, there is no charge storage, which results in ZERO Reverse recovery time.

26. Schottky diodes

27. Schottky diodes: Characteristics
Power Schottky diodes are available up to forward current ratings of 300A.
The main limitation of Schottky diode is their low reverse voltage (Limited blocking voltage) in order of 30 to 100V.
Power rating are in the range of 100V/300A.
Very low forward voltage drop (barrier potential is of 0.15 to 0.45V) typically 0.3V
Used in low voltage, high current such as switched mode power supplies.
The operating frequency may be as high kHz as the device is suitable for high frequency application.

28. Power Semiconductor Diode Summary

29. Characteristics of Power Semiconductor Diodes
Power Diodes with largest power rating are required to conduct several kA in the forward direction with very little power loss while blocking several kV in the reverse direction.
Large blocking voltage requires wide depletion layer to restrict electric field strength below the “impact ionization” level.
For a wide depletion layer, space charge density in the depletion layer should also be low.
But such a construction, will result in a device with high resistively in the forward direction.
Consequently, power loss (switching characteristics) at the required rated current will be unacceptably high.
On the other hand if forward resistance (and hence power loss) is reduced by increasing the doping level, reverse break down voltage will reduce.
This apparent contradiction in the requirements of a power diode is resolved by introducing a lightly doped “drift layer” of required thickness between two heavily doped p and n layers.

30. Construction of Power Semiconductor Diodes
For more than 400V , DIFFUSION process.
For less than 400V ratings, EPITAXIAL growth process
Cross-sectional view of power diode

31. Constructional Structure of Power Semiconductor Diodes
Ordinary diodes have zigzag type boundary, due to this impurities diffuse fast laterally as they do vertically into the substrate, so radius of curvature becomes variable to the width of depletion layer.
Electric field becomes non-uniform and is highest where the curvature is small.
This leads to a lower breakdown voltage.
In the case of power semiconductor diodes special steps are taken in fabrication to control the radius of curvature of depletion layer boundary , one such step is the use of GUARD RING.
Guard rings are mostly of p-type and their depletion layers merge with the depletion layer of reverse biased p-n junction.
This (Guard rings) prevents the radius of curvature becoming too small, so breakdown strength is increased.
The coating of silicon dioxide layer also helps in controlling the electric field at the surface.
Use of guard ring to improve the breakdown strength of power diode

32. Power Diode Bias Conditions
For the neutrality the number of ionized atoms in the p+ region should be same as that in the n- region.
However, since NdD<< NaA, space charge region will extends into the n- drift region.
Physical width of the drift region can be either larger or smaller than the depletion layer width.
Consequently two type of diodes exist,
non punch through type & punch through type.
If the thickness of lightly doped (n-) layer is more than the thickness of depletion layer width at breakdown, then it is non-punch through diode
In “non-punch through” diodes, depletion layer boundary doesn’t reach the end of the drift layer (non-uniform electric field- unexpected results) .
So, electric field strength is maximum at the p+ n- junction and decrease to zero at the end of the depletion region.
However if the thickness of n- layer is less than the depletion layer width at breakdown, diode is called punch through diode.
In “punch through” diodes the depletion layer spans the entire drift region and is in contact with the n+ cathode (uniform electric field- expected results ).
(a) Non-punch through type
(b) punch through type.

33. Observed Turn ON behavior of a power Diode
Power Diodes take finite time to make transition from reverse bias to forward bias condition (switch ON) & vice versa (switch OFF).
A typical turn on transient is shown in Fig.
It is observed that the forward diode voltage during turn ON may transiently reach a significantly higher value Vfr compared to the steady state voltage drop at the steady current IF.
In some power converter circuits (e.g voltage source inverter) where a free wheeling diode is used across an asymmetrical blocking power switch (i.e GTO) this transient over voltage may be high enough to destroy the main power switch.
Forward recovery voltage (Vfr) is given as a function of the forward di/dt has typical values lie within the range of 10-30V.
Forward recovery time (tfr) is within 10 us.
Forward current & voltage waveforms of a power diode during Turn On operation.

34. Observed Turn OFF behavior of a Power Diode
Diode current does not stop at zero, instead it grows in the negative direction to Irr (“peak reverse recovery current”).
Voltage drop across the diode does not change appreciably from its steady state value till the diode current reaches reverse recovery level.
Towards the end of the reverse recovery period if the reverse current falls too sharply, (low value of snappiness factor, S), stray circuit inductance may cause dangerous over voltage (Vrr) across the device.
Snappiness factor S depends mainly on the construction of the diode (e.g. drift region width, doping lever, carrier life time etc.)
Turn OFF characteristics of a power diode