Lecture # 4

Power Transistors
Prof. Dr. Shahid Iqbal
Department of Electrical Engineering
University of Gujrat
Email: si@uog.edu.pk
 Power Transistors
• Transistors with higher voltage and current rating are called
Power Transistors.
• 3-layer NPN or PNP semiconductor device with 2 PN
junctions
• 4 types commonly used: power BJT, power MOSFET,
Power MD and IGBT
• All transistors have amplifier characteristics however, they
are used as switches in Power Electronics
• BJT = minority-carrier-based/ bipolar device
» current-controlled device
• MOSFET = majority-carrier-based/ unipolar device
» Voltage-controlled device
• IGBT = minority-carrier-based voltage-controlled device
 Power BJT

• NPN is more widely used than PNP

• Switching speed faster than thyristors.
(turn on time < 1μs, turn off < 2μs)
• High frequency (up to 50kHz
applications)
• Complex base drive circuit
 I-V Characteristics of Power BJT
Hard Quasi-saturation
saturation Second breakdown
Ic

1
I B3 > I B2 > I B1
Rd

Primary breakdown
I B3

Conventional avalanche
I B2 b/d, large Ic

I B1 IB<0
IB=0

0 VSUS VCEO VCBO VCE

 I-V Characteristics of Power BJT
Secondary Breakdown
If high voltage and current occur simultaneously during turn off,
a hot spot is formed and the device fails by thermal runaway
Quasi Saturation
Excessive carriers in lightly doped collector drift region
Hard Saturation
Excess-carriers density reach the other side (n+) of the drift region
Hard Quasi-saturation
saturation Second breakdown
Ic

1
I B3 > I B2 > I B1
Rd

Primary breakdown
I B3

I B2

I B1
IB<0
IB=0
0 VSUS VCEO VCE
VCBO
 DC current gain and on-state voltage
Two important parameters which circuit designers need to consider:
(1) DC current gain, hFE or  (= IC/IB) and
(2) On-state or saturation voltage, VCE(sat).
Log(hFE) V @hFE= 10, TJ = 25C
TJ = 150C ~2V
TJ = 25C
~1.6V
VBE(sat)

~0.6V
VCE(sat)
~0.1V
I
DC current gain characteristics C(max) Log(IC) Typical on-state voltage I Log(IC)
C(max)
(in 2-cycle full log graph) (in 2-cycle semi-log graph)

• Typical current gains for power BJTs are in between 5 – 50 at the rated current in which it is
lower for higher voltage capability BJT.
• The on-state voltage of CE is usually in the 1 - 2V range. A ~0.6V is required to forward-bias
BE junction. The increase of VCE(sat) and VBE(sat) is mainly due to the lightly doped collector
drift region.
 Switching Characteristic of BJT
Switching times depend on device characteristic and external circuit
vb VCC
V1
RL
iC
t1 t2 t vb Rb
-V2 vCE
ib I i b vBE
1

t
-I2
td ts
vCE VCC tf v tr v
90% ton
10% 50% VCE(sat)
td ts t
iC tr i IC(on) tf i
90% IC(leakage)
10% 50% tW
on off t
During turn-on charges are supplied to BE junction and then the drift region
During turn-off all stored and space charges are removed
Turn-on and –off time can be reduced by peaking of positive and negative
base current respectively
 Safe Operating Areas (SOAs) of BJT
TJM= 200C • SOA is bounded by four
Log(iC) 2nd breakdown
limits:
ICM 1. current,
Avalanche 2. thermal,
breakdown 3. second breakdown
and
FBSOA 4. voltage.
@TC = 25C
 ICM - limited by bonding
BVCEO Log(vCE) wires or metalizations on
wafer.
 TJM - power dissipation limit set by the thermal resistance of the
transistor and the max allowable average junction temperature.
 Second breakdown - max permissible combinations of voltage and
current without getting into the region of the iC-vCE plane where second
breakdown may occur.
 VCEO - avalanche breakdown voltage limit.
 Safe Operating Areas (SOAs) of BJT
Log(iC) Pulse duration Shorter switching time
ICM
FBSOA 10s
Switch mode
100s
@TC = 25C operation Smaller
TJM= 200C 1ms energy
2nd BD: D10%, dc operation
dc
TJM  200C
Consideration

VCEO Log(vCE)
Log(iC) Larger SOA
ICM

RBSOA • SOA is bounded by four

vBE(off) < 0 limits:
@TJ = 25C 1. current,
2. thermal,
vBE(off) = 0 BVCBO
3. second breakdown
and
BVCEO Log(vCE) 4. voltage.
 PNP Power BJT

 npn BJT is preferred over pnp BJT because in npn BJT the
dominant charge carriers are electrons which have higher
mobilities (~3 times higher). Thus npn transistors have;
1. thus faster switching speed
2. & smaller on-state power loss
3. higher voltage & current ratings.

iC
IB C-
vEC
B
E+

Circuit symbol of a pnp BJT

Monolithic Darlington Transistors (MDs)
They are used to have high current gain with high breakdown
voltage. However, they have higher on-state voltage drops and
slower switching speeds. hFE of a few hundreds are available.
VCE(sat) = 2 to 5V, depending on current and voltage rating.

iC iC
IB C IB C

B + B -
vCE vEC
- +
E E
Circuit symbol Circuit symbol
of an npn MD of a pnp MD
 Power MOSFET
D
N-Channel MOSFET
G
 Shows great promise for
applications involving high S
frequency (up to 1MHz) N-channel MOSFET

D
 Low power (up to few kW)
 No secondary breakdown G
problem S
 Fast switching speed (few ns to a P-channel MOSFET

few hundred ns) D

 High gate input impedance n+
n-
 Modern P-MOSFETs have very p p
low Ron n+ n+
S S
G SiO2
Structure of N-channel MOSFET
 I-V Characteristic of Power MOSFETs

VDS = VGS-VGS(TH)
iD Ideal
VDS<VGS-VGS(TH) VDS > VGS-VGS(TH)
on
iD Ohmic Active region
region VGS5 off
IDS5

Increasing VGS
VGS4 iD vDS
IDS4
actual
VGS3
IDS3
VGS2
IDS2
VGS1 VGS > VGS(TH)
IDS1 VGS(TH) vGS
BVDSS vDS linearized
VGS < VGS(TH) Cut-off region
Transfer curve
Current-voltage characteristics of an n-channel power MOSFET
ΔID/ΔVGS= transconductance (for constant VDS)
No secondary b/d – positive temperature coefficient of resistive
Static drain-source on-state resistance
The on-state resistance of a MOSFET is quite
constant over the entire drain current.
IDM is single-pulse (tp  10s) peak drain
current.
RDS(on)/m
40

35
IDM iD/A
Example of static on-state resistance

VDSS = 150V, ID = 35A, IDM = 105

 Parasitic Capacitances
• Typical values of input (Ciss), output (Coss) and reverse
transfer (Crss) capacitances -- determine circuit
component values
• equivalent circuit capacitances:
• Ciss = CGS + CGD, CDS shorted D
• Coss = CDS + CGD, CGS shorted Cgd

• Crss = CGD G
Cds
• Even though Cgs >> Cgd, the Cgs
later capacitance undergoes a S
much larger voltage excursion, Parasitic Capacitances of
so its effect on switching time MOSFET
cannot be neglected.
 Effect of Voltage Excursion Parasitic
Capacitances

VDSS = 150V, ID = 35A, IDM = 105A

 Switching Characteristic of MOSFET

vg V1 VDD

vGS t1 t2 RL
t
VGS(TH) V1 D
90% vg G
10% S
td(on) td(off) t RG
tr ton tf
iD ID(on)
90% ID(leakage)
50% tW
10%
on off t
 Effect of RG on Switching Time
Variation
VDD

RL
D
G

vg S
RG

VDSS = 150V, ID = 35A, IDM = 105A

Safe operating areas (SOAs) of MOSFET
• Bound by 3 limits: log (iD)
1. Maximum drain current,
IDM IDM
2. The internal junction
temperature (Tj) that is 10µ
governed by power
100µ
dissipation in the device.
3. Breakdown voltage TJ, max 1000µ
BVDSS .
DC
MOSFET does not have any
second breakdown, so none log (VDSS)
appear in SOA
BVDSS
•TJmax = 150°C
•RBSOA = FBSOA Example of FBSOA. VDSS = 150V, ID = 35A and
IDM = 105A.
 Insulated Gate Bipolar Transistor (IGBT)
• BJT + MOSFET
- Gate behavior similar to MOSFET n-channel C
- Low losses like BJT, Low ON state voltage MOSFET
pnp
- Switching frequency up to 100kHz BJT
• Two types: Asymmetric/punch-through (PT) G
(different reverse and forward blocking E
capability) and symmetric/non-punch-through n-channel IGBT
(NPT). PT-IGBTs have lower on-state losses
and shorter turn-off times.
• Forward direction (I-V characteristic) similar to C
logic-level BJT
Cgc
• Transfer curve & Switching waveforms identical
to P.MOSFET G Cce
• No second b/d problem Cge E
• Ron = Ron(MOSFET)/10 Parasitic Capacitances of IGBT
 Circuit symbols of IGBTs

C Drain(D)
Gate (G)
G or n-Channel

E
Source(S)

n-channel IGBT

D
C

G or
G
E S

p-channel IGBT
 Equivalent circuits of IGBTS
p-channel
n-channel C
C MOSFET
MOSFET npn
pnp
BJT
BJT
G
G
E
E
n-channel IGBT p-channel IGBT
C C
RMod
p+ h h pnp
n+ G
n- npn
MOSFET

p+ RB
p n+ n+ p e E
E Equivalent circuit with parasitic thyristor,
G SiO2 body region spreading resistance, RB and
drift region resistance RMod
structure (one cell), n-channel PT-IGBT
 I-V characteristics of IGBTS
iC Increasing VGE
VGE5 iC actual

For PT-IGBTs VGE4 Active region

A few tens VGE3
of volts VGE(th) vGE
due J1 linearized
VGE2
breakdown
Transfer curve
VRM VGE1 VGE > VGE(th)

VGE < VGE(th) Cut-off region BVCES vCE

~1V
Due to J1 forward bias

VGE(th) ~ 3- 8V, VGE(max) = 20 to 25V, VCE(sat) ~ 2- 4V

 I-V characteristics appears qualitatively similar to that of a logic-level BJT
except that the controlling parameter is an input voltage.
 Transfer curve iC-vGE is identical to that of the power MOSFET except
VGE(th) and the slope values.
 Saturation Voltage characteristics
Major current flow
VCE(sat) = VJ1 + Vdrift + IDRchannel
RMod VJ1
,where VJ1 ~ 0.7-1.0V, C

Vdrift < that of power MOSFET, and

VCE(sat)
IDRchannel ~ that of power MOSFET G Vdrift

• Normally, the on-state or saturation voltage drop is used E

IDRchannel
instead of on-state resistance.

Saturation voltage characteristics Saturation voltage vs. TC

 Parasitic capacitances

Same like MOSFET, there are 3 parasitic capacitances and

the measured capacitances are
 Input capacitance, Cies = Cgc + Cge (Cce is shorted)
 Output capacitance, Coes = Cgc + Cce (Cge is shorted)
 Reverse transfer capacitance, Cres = Cgc
Cies is of a few nF and Cres < Coes < Cies.
Even though Cge >> Cgc, the later capacitance undergoes a
much larger voltage excursion, so its effect on switching
C
time cannot be neglected.
Cgc
The capacitance variations with VGE and
VCE are about the same as those G Cce
of MOSFET. Cge E
Parasitic Capacitances of IGBT
 Switching characteristics of IGBTs
 The shapes of the switching waveforms are
about the same as those of power MOSFET.
 However, both internal MOSFET and BJT of an
IGBT involved in turn-on and turn-off the IGBT.
 Hence, the turn-on (td(on), tr) and turn-off (td(off),
tf) times vary with IC (refer to the graph for BJT)
and RG (refer to the graph for MOSFET).
 Note that td(off) is used as for power MOSFET
instead of the storage time of power BJT.
 The switching times of IGBT are smaller than
those of BJT but larger than those of MOSFET.
 Safe operating areas (SOAs) of IGBTs
• FBSOA identical to power MOSFET.
• RBSOA (for turn-off) is different from
the FBSOA. The reapplied dvCE/dt is Log(iC) Same like MOSFET
ICM(pulsed)
limited to avoid the latch-up of IGBT ICM(continuous) 10s
(or latch-on of the parasitic thyristor). 100s
But the values are quite large which 1ms
FBSOA DC
can be easily controlled by the gate. If
latch-up occurs, it must be terminated
quickly, otherwise the IGBT will be VCE Log(vCE)
destroyed.
• The allowable maximum temperature,
TJ(max) is 150ºC. Log(iC) 1kV/s
• The maximum collector current can be 2kV/s
Reapplied
dVCE/dt
4 to 10 times the nominal rated RBSOA 3kV/s
current for 5 - 10s depending on the Log(vCE)
value of VCE.
 Commercial individual IGBTs and IGBT modules

•Commercial available individual IGBTs have nominal current ratings

as large as 200 - 400A and voltage ratings as large as 1700V.
Voltage rating up to 2 - 3kV are projected. (The voltage ratings of
IGBTs are higher than those of BJTs due to the small current gain of
the internal pnp BJT.)
•For a 1kV device, the on-state voltage is 2 - 3V at rated current.
•The turn-on and turn-off times are less than 1s.
•IGBTs are available in module in which 4 to 6 individual IGBTs are
connected in parallel. Hence, the current ratings are in the ranged of
1000 to 1500A.
 IGBT characteristics comparison with BJT,
MOSFET of similar sizes and ratings
Features BJT MOSFET IGBT
Drive method current voltage voltage
Drive circuit complex simple simple
Input impedance low high high
Drive power high low low
Switching speed slow (s) fast (ns) medium
Operating frequency low (< 100 kHz) fast (<1MHz) medium
SOA narrow wide wide
Saturation voltage low high low
 SCR: highest power capability and slowest switching speed
 GTO: high power and slow speed
 TRIAC: main ac applications
 power BJT: medium power and medium speed
 power MD: medium power and medium speed (<BJT)
 power MOSFET: lowest power and fastest speed
 IGBT: medium power and medium speed
 MCT: medium power and medium speed
 Desired characteristics of controllable switches

•Hence, the following characteristics in a controllable switches are

desirable:
1. Small leakage current in the off state.
2. Small on-state voltage drop.
3. Short turn-on and turn-off times.
4. Large forward- and reverse-voltage-blocking capability. (Reverse-
voltage-blocking capability is not required in certain cases).
5. High on-state current rating.
6. Positive temperature coefficient of on-state resistance.
7. Small control power required to switch the device.
8. Capability to withstand rated voltage and rated current
simultaneously while switching. (This eliminates the need for
snubber circuits)
9. Large dv/dt and di/dt ratings.
 Design considerations on power
semiconductor switches
The design consideration will cover;
(A) Power Losses in Power Semiconductor Switches
(B) Gate and Base Drive Circuits
(C) Protection Techniques and Snubber Circuits and
(D) Temperature control by heat sinks

Instantaneous power, p(t) = v(t)i(t)

•For voltage and current source: p(t) > 0 indicates power is being
supplied by the source.
•For passive component: p(t) > 0 indicates power is being absorbed
by the component.
t2
E or W   p(t )dt
Energy or work, t1

1 t0 T WT
T t0
Average power, P p (t ) dt 
T