1.2 Conventional Power Devices
Power semiconductor devices have been grouped into two categories: the old or conventional
devices that appeared before 1980, i.e., thyristor, GTO, triac, BJT and power MOSFET; and the
second category of modern devices which appeared in 1980's, - IGBT, Superjunction MOSFETs.
All power switches share a common rule. They all have a reverse biased space charge region to
hold voltage in the off-state and develop a highly conductive path in the on-state. It would be
worth looking through the evolution of power switches at a glance.
GTO
Figure 1.2 Simplified structure of a GTO and its equivalent circuit.
A gate turn-off thyristor (GTO) is a special type of thyristor, a high-power semiconductor
device. GTOs, as opposed to normal thyristors, are fully controllable switches which can be
turned on and off by their third lead, the GATE lead. Normal thyristors (Silicon-controlled
rectifier) are not fully controllable switches (a "fully controllable switch" can be turned on and
off at will). Thyristors can only be turned ON and cannot be turned OFF. Thyristors are switched
ON by a gate signal, but even after the gate signal is de-asserted (removed), the thyristor remains
in the ON-state until any turn-off condition occurs (which can be the application of a reverse
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
voltage to the terminals, or when the current flowing through (forward current) falls below a
certain threshold value known as the "holding current"). Thus, a thyristor behaves like a normal
semiconductor diode after it is turned on or "fired".
The GTO can be turned-on by a gate signal, and can also be turned-off by a gate signal of
negative polarity.
Turn on is accomplished by a "positive current" pulse between the gate and cathode
terminals. As the gate-cathode behaves like PN junction, there will be some relatively small
voltage between the terminals. Turn off is accomplished by a "negative voltage" pulse between
the gate and cathode terminals. Some of the forward current (about one-third to one-fifth) is
"stolen" and used to induce a cathode-gate voltage which in turn induces the forward current to
fall and the GTO will switch off (transitioning to the 'blocking' state.)
GTO thyristors suffer from long switch off times, whereby after the forward current falls,
there is a long tail time where residual current continues to flow until all remaining charge from
the device is taken away. This restricts the maximum switching frequency to approx 1 kHz.
The state-of-the-art devices are available up to 4500 V, 2500 A ratings.
BJT
Figure 1.3 Schematic of a n-p-n Bipolar Junction Transistor and its circuit
symbol.
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
One of the first types of power semiconductors, the BJT is a three layered semiconductor
consisting of a sandwich of p-n-p or n-p-n materials. In addition, it has three terminals: the
emitter, the collector, and the base (Figure 1.3). The base is lightly doped, whereas the emitter is
heavily doped and wider. The emitter-base region is forward biased so that majority carriers will
flow across the junction. On the other hand, the collector-base region is reverse biased, which
results in a small minority carrier flow.
Operational Advantages and Disadvantages
When used in a common emitter mode, as it is most often, the BJT acts as a current-controlled
switch. The base current is in the input and the collector current is the output. Because it is
current-controlled, it has a fairly low saturation voltage, which is desirable. In addition, BJTs are
able to handle high voltages and currents with few problems.
Of course, there are many drawbacks. The BJT has low gain at high frequencies, so it is not
useful for amplification under those conditions. Additionally, it does not have a very high surge
rating—the peak current is only about twice the maximum continuous current rating. Unlike
MOSFETs, BJTs also have a relatively slow switching speed because it takes time to charge the
emitter and collector depletion capacitances, which consequently slows the turn-on time. There
are also two breakdown areas associated with the BJT that reduce its safe operating area. The
first is the avalanche breakdown, which causes a rapid rise in current, and a second breakdown
can be brought on by inductive loads, which can overheat and destroy the transistor.
Power BJTs also have a thick and low-doped collector region. Such collector regions result in a
large blocking voltage. Extremely low doping densities, down to 1013 cm-3, are use to obtain
blocking voltages as large as 3000 V. As a result, one finds that the structure needs to be
redesigned to a) effectively manage the power dissipation and b) avoid the Kirk effect. The
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
power dissipation is managed by minimizing the power dissipation and spreading the resulting
heat dissipation onto a large area. The Kirk effect is normally avoided by increasing the collector
doping density. However, for devices with a very high blocking voltage, this may not be an
option. Power BJTs therefore are operated at rather low current density of 100 A/cm2 since the
lower current density reduces the power dissipation per unit area and eliminates the Kirk effect.
Large currents – up to 1000 A – are obtained by making a large area device. Silicon BJTs
dominated the power device market, in part because of the low cost of large area silicon devices
and the high thermal conductivity of silicon compared to GaAs. Silicon carbide (SiC) is being
called as the perfect material for high-power BJTs. The higher thermal conductivity (3x) and
breakdown field (10x) compared to silicon give it a clear performance advantage. The high
saturation velocity (3x compared to silicon) also shifts the onset of the Kirk effect to higher
current densities. The proliferation of its use will heavily depend on the material cost and quality
of the SiC wafers.
The BJT switching speed is considerably faster than thyristor-type devices because excess
minority carriers in the base are almost entirely removed by negative base current (for NPN
transistor). Modern high power transistors normally comprise of multiple matched devices in
parallel within a package. Power transistor applications in industry range from a few kWs to
several hundred kWs size in voltage-fed choppers and inverters with switching frequency up to
10-15 KHz. The state-of-the-art devices are available with ratings up to 600 V, 500 A.
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
Power MOSFET
Figure 1.4 Schematic of a Power MOSFET- DMOS.
A power MOSFET is a unipolar, majority carrier, voltage controlled device. Figure 1.4 shows
the most dominating power MOSFET called DMOS. During the last decade, the power
ratings and characteristics of power MOSFET have improved dramatically with sharp fall of
prices, and it is now a key competitor to other power devices. The N- channel enhancement
mode device is common because of the higher mobility of electron. Originally, devices with
surface groove technology, called VMOS, were used but today planer DMOS structure is very
common. Being a voltage controlled device, the gate circuit impedance is extremely high.
However, during fast turn-on and turn-off, the gate needs a current pulse to charge and
discharge, respectively, the effective gate-source capacitance. Being a majority carrier device,
there is no inherent delay and storage switching time as that of BJT. The MOSFET devices are
therefore extremely fast compared to other devices. The high switching speed causes low
switching loss. Power MOSFETs have been used in converters with hundreds of KHz switching
frequency. However, the device has a reverse body diode which is slow due to large storage
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
charge. Although the body diode has full by- pass current capability, high speed applications
often require bypassing this diode with external fast recovery diodes. The on-resistance of a
device is a key parameter that determines the conduction drop. The on-resistance increases with
voltage rating making the device very lossy at high current.
The resistance has positive
temperature coefficient and therefore permits easy paralleling of large number of devices. The
second breakdown effect of MOSFET is negligible due to this positive temperature coefficient
effect. If localized heating occurs for any reason, increase of resistance forces the current
distribution to be uniform. The peak current of a device can therefore be increased on duty cycle
basis. Power MOSFETS are generally used in high frequency switching applications within the
rating of a few watts to a few kilowatts. The device is very popular in switching mode power
supplies. The state-of-the-art devices are available with 500 V, 40 A ratings.
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
IGBT
Figure 1.5 Schematic of an IGBT.
The IGBT is the most popular power semiconductor currently used today. It combines the MOS
gate structure with the bipolar current conduction to create a device that is the best of both the
MOSFET and the BJT. For terminals, it is a hybrid between the BJT and the MOSFET. It has
three terminals: the collector, the gate, and the emitter. Schematically as shown in Figure 1.5, the
IGBT is basically a p-n-p BJT where the base current is provided by a voltage controlled nchannel MOSFET.
Operation Modes
Forward-Blocking and Conduction Modes
When a positive voltage is applied across the collector-to-emitter terminal with
gate shorted to emitter shown in Figure 1.5, the device enters into forward blocking mode with
junctions J1 and J3 are forward-biased and junction J2 is reverse-biased. A depletion layer extends
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
on both-sides of junction J2 partly into p- base and n--drift region. An IGBT in the forwardblocking state can be transferred to the forward conducting state by removing the gate-emitter
shorting and applying a positive voltage of sufficient level to invert the Si below gate in the pbase region. This forms a conducting channel which connects the n+-emitter to the n--drift region.
Through this channel, electrons are transported from the n+ emitter to the n-drift. This flow of
electrons into the n--drift lowers the potential of the n--drift region whereby the p+ -collector/ n-drift becomes forward-biased. Under this forward-biased condition, a high density of minority
carrier holes is injected into the n--drift from the p+ -collector. When the injected carrier
concentration is very much larger the background concentration, a condition defined as a ‘plasma
of holes’ builds up in the n--drift region. This plasma of holes attracts electrons from the emitter
contact to maintain local charge neutrality. In this manner, approximately equal excess
concentrations of holes and electrons are gathered in the n--drift region. This excess electron and
hole concentrations drastically enhance the conductivity of n--drift region. This mechanism in
rise in conductivity is referred to as the conductivity modulation of the n--drift region.
Reverse-Blocking Mode
When a negative voltage is applied across the collector-to-emitter terminal shown in Figure 1.5,
the junction J1 becomes reverse-biased and its depletion layer extends into the n--drift region.
The break down voltage during the reverse-blocking is determined by an open-base BJT formed
by the p+- collector/ n--drift /p-base regions. The device is prone to punch-through if the N--drift
region is very lightly-doped. The desired reverse voltage capability can be obtained by
optimizing the resistivity and thickness of the n--drift region.
The width of the n--drift region that determines the reverse voltage capability and the forward
voltage drop which increases with increasing width can be determined
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
Where,
Lp = Minority carrier diffusion length
Vm = Maximum blocking voltage
εo = Permittivity of free space
The device was commercially introduced in 1983, and since then the ratings and characteristics
have improved significantly. The IGBT offers significant advantages over BJT and power
MOSFET in medium power (a few kWs to a few hundred kWs) medium frequency (up to 50
KHz) power converter applications. The device has the high input impedance of a MOSFET
but BJT-like
conduction
characteristics. Recently, IGBTs are finding wide popularity in
medium power applications, such as dc and ac motor drives, UPS systems, power supplies and
drivers for solenoids, relays and contactors. Present IGBT inverter induction motor drives using
15- 20 kHz switching frequency are finding favor where audio noise is objectionable. The stateof-the-art devices are available up to 1200 V, 400 A.
Following table compares the three major contributors to power electronics market.
Comparison Criterion
Drive Method
Drive Circuit complexity
Switching speed
Switching frequencies
Forward Voltage drop
Current carrying capability
Breakdown Voltage
BJT
Current
High
Slow (µs)
Few Khz
Low
High
High
MOSFET
Voltage
Low
Fast (ns)
Upto 1 MHz
Medium
Medium
Medium (< 1500V)
IGBT
Voltage
Low
Medium
< 50 Khz
Low
High
Very High
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
Figure 1.6 The higher frequency regime still remains unconquered where GaN –
based Power devices could make a big difference.
1.3 Why Gallium Nitride?
Although until now the most commercially viable power devices are based on silicon
technologies, where cost reductions continuously occur, they have now approached a
performance plateau. Concurrently, next generation and emerging applications are demanding
further substantial leaps in power conversion performance. Hence, to meet the new requirements
of forthcoming applications, new materials and transistor structures are needed to fill this gap.
Although, silicon carbide (SiC) FETs have emerged on the scene in the past 10 years to address
these issues, they suffer from significant cost premiums due to limited quality material supply, as
well as the intrinsic cost structure of the material. Additionally, SiC based technology has limited
scalability.
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
Figure 1.7 Calculated material limit curves for unipolar devices shows AlGaN/GaN
material system ahead in the competition.
Gallium Nitride promises performance that is at least 10 times better than existing silicon devices
and looks to be the most promising all over the range from 20V- 1200V!
Benefits of the AlGaN/ GaN material system over SiC
A combination of high electron mobility and higher bandgap provides GaN with a significant
reduction on device specific on-resistance Ron for a given reverse hold-off voltage capability than
both SiC and silicon devices, as shown in the calculated material limit curves unipolar devices in
Figure 1.7 It is clear that an order of magnitude improvement in specific on resistance can be
achieved for GaN based devices over silicon counterparts, even at these early stages of GaN
power device development. Since GaN based power devices achieve a combination low gate
capacitance and low on-resistance, it permits much higher frequency switching converters than
competing silicon transistors. With SiC, that has very similar bandgap and breakdown field, GaN
gains in the mobility of the 2DEG which is formed when a thin AlGaN layer is grown on top.
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury
The other benefit of GaN comes from the fact that it could be grown on Si-substrates thereby
making the performance/cost ratio much better than SiC.
Chapter1: Dissertation: “AlGaN/GaN CAVET for High Power Application” – Srabanti Chowdhury