Structure More complicated structure & operating

characteristics

Complexity : to make the device to suit to high power

applications

C / S view of PN junction diode

structure Heavily doped n type substrate - cathode Over n type is lightly doped n- layer Heavily doped p region anode

C/ S area - depends on I through the device 4 inch max n- region Drift region not in signal diodes n- region absorbs depletion layer

wide at large reverse voltages

VI Characteristics

VI Characteristics FB : I grows linearly, not exponentially Ohmic drops mask exponential VI characteristics RB : Small leakage current flows till breakdown is reached Reverse I can be limited only by external circuit Large V & I damage of the device

Breakdown V ConsiderationsNon Punch through diodes 2. Punch through diodes1.

Non Punch through diodes Length of Drift region > Depletion Layer -> During

Breakdown Dep.Layer has not reached the lighly doped n+ Layer

Punch through diodes Dep. Layer has extended across drift region & is in

contact with n+ Layer Punch through. After punch through, increase in reverse voltage will not widen the depletion layer This is because, doping density of n+ layer is very high So, electric field profile flattens.

Punch Through Diode

Punch through Diode

Inference Large breakdown voltages require lightly doped

junctions atleast on one side

Drift layer of diode must be long in HV devices to

accommodate long depletion layers

Boundary Layer Control

ON State losses

ON State losses At large current levels, dissipation of drift region

should be accounted Losses in drift region limit the power capability of the device Resistance of drift region in ON state is much less than the apparent ohmic value on the basis of geometric size Care should be taken in calculating resistance & power dissipation

Conductivity modulation During on state, R of drift region

because large amount of excessive charge carriers are injected into drift layer So, resistance of drift layer .(conductivity modulation) Low injection level : n- layer neutralizes the holes High injection level : e- are attracted from n+ region Double injection

Conductivity modulation If diffusion length > Wd , distribution of excess charges

are flat n- < na (1014 /cm3 ),the condition is called high level injection Thus, conductivity of the region is enhanced Total V = Vj + Vd Factors affecting Vd are Excess carrier density Life time of excess carriers Mobility of excess carriers

Impact of On State losses Low on state loss

low power dissipation Low on state loss due to large stored charge affects switching time For large Vbd Vd should be large Vd Wd 2

Switching Characteristics

Features of switching waveforms Voltage overshoot during turn on

not present in

signal diodes Sharpness of fall of reverse current during turn off

Switching Characteristics Transition time & Shape depends on Intrinsic properties of diode Circuit in which diode is connected

In general, switching properties will be given in data sheet of the device

Rate of change of current in Diode Parameters controlling di/dt depends on the

application of the diode If diode is used in circuits having inductances, rate of

change of I is controlled by value of inductance If diode is used as FWD then , turn off of a solid state

device controls di/dt

Turn On Growth of Excess charge carriers

Turn on transient Turn on time = t1 + t2 During turn on, Space charge stored in depletion region is removed by the growth of forward current When depletion layer is completely discharged, junction becomes forward biased At t1 , excess carriers are injected into the drift layer

Excess carriers are injected into the junction on both

p+ n- & n- n+ junctions

Turn on transient Initially, there is no conductivity modulation so

voltage drop is high. As the forward current grows, due to injection of large no. of carriers, voltage drops down. Inductance adds additional V drop if di/dt is high Vd reduces as drift layer is shorted due to injection of large amount of carriers When Vd falls from peak to steady state value, excess charge distribution in drift layer gets completed

Turn on transient Growth of excess charge carriers depends on Intrinsic properties of diode External circuit to which it is connected Faster turn on by reducing life time s increases on

state loss Switching time : t1 hundreds of ns

t2 - s

Turn Off Decay of excess charge carriers

Turn off transient Turn off time : t3 + t4 + t5 Inverse of turn on process Excess charge stored in junction should be removed

before junction becomes reverse biased Charge carriers are removed byrecombination sweep out by negative diode current

Once depletion layer acquires sufficient space charge

from RB voltage, it expands into depletion layer

Turn off transient As long as charge carriers are present in the junctions

of drift layer, diode remains in FB condition After carrier sweep out proceeds for interval t4 both

junctions become RB At t5, diode current stops growing & becomes zero

Schottky diode

Schottky diode