Question

Power Semiconductor device - history

Answer 1

Power electronics and converters utilizing them made a head start when the first device the

Silicon Controlled Rectifier was proposed by Bell Labs and commercially produced by General

Electric in the earlier fifties. The Mercury Arc Rectifiers were well in use by that time and the

robust and compact SCR first started replacing it in the rectifiers and cycloconverters. The

necessity arose of extending the application of the SCR beyond the line-commutated mode of

action, which called for external measures to circumvent its turn-off incapability via its control

terminals. Various turn-off schemes were proposed and their classification was suggested but it

became increasingly obvious that a device with turn-off capability was desirable, which would

permit it a wider application. The turn-off networks and aids were impractical at higher powers.

The Bipolar transistor, which had by the sixties been developed to handle a few tens of

amperes and block a few hundred volts, arrived as the first competitor to the SCR. It is superior

to the SCR in its turn-off capability, which could be exercised via its control terminals. This

permitted the replacement of the SCR in all forced-commutated inverters and choppers.

However, the gain (power) of the SCR is a few decades superior to that of the Bipolar transistor 

and the high base currents required to switch the Bipolar spawned the Darlington. Three or more

stage Darlingtons are available as a single chip complete with accessories for its convenient

drive. Higher operating frequencies were obtainable with a discrete Bipolars compared to the

'fast' inverter-grade SCRs permitting reduction of filter components. But the Darlington's

operating frequency had to be reduced to permit a sequential turn-off of the drivers and the main

transistor. Further, the incapability of the Bipolar to block reverse voltages restricted its use.

The Power MOSFET burst into the scene commercially near the end seventies. This device

also represents the first successful marriage between modern integrated circuit and discrete

power semiconductor manufacturing technologies. Its voltage drive capability – giving it again a

higher gain, the ease of its paralleling and most importantly the much higher operating

frequencies reaching upto a few MHz saw it replacing the Bipolar also at the sub-10 KW range

mainly for SMPS type of applications. Extension of VLSI manufacturing facilities for the

MOSFET reduced its price vis-à-vis the Bipolar also. However, being a majority carrier device

its on-state voltage is dictated by the RDS(ON) of the device, which in turn is proportional to about

V 2.3

DSS rating of the MOSFET. Consequently, high-voltage MOSFETS are not commercially

viable.

Improvements were being tried out on the SCR regarding its turn-off capability mostly by

reducing the turn-on gain. Different versions of the Gate-turn-off device, the Gate turn-off

Thyristor (GTO), were proposed by various manufacturers - each advocating their own symbol

for the device. The requirement for an extremely high turn-off control current via the gate and

the comparatively higher cost of the device restricted its application only to inverters rated above

a few hundred KVA.

The lookout for a more efficient, cheap, fast and robust turn-off-able device proceeded in

different directions with MOS drives for both the basic thysistor and the Bipolar. The Insulated

Gate Bipolar Transistor (IGBT) – basically a MOSFET driven Bipolar from its terminal

characteristics has been a successful proposition with devices being made available at about 4

KV and 4 KA. Its switching frequency of about 25 KHz and ease of connection and drive saw

it totally removing the Bipolar from practically all applications. Industrially, only the MOSFET

has been able to continue in the sub – 10 KVA range primarily because of its high switching

frequency. The IGBT has also pushed up the GTO to applications above 2-5 MVA.

 Subsequent developments in converter topologies – especially the three-level inverter

permitted use of the IGBT in converters of 5 MVA range. However at ratings above that the

GTO (6KV/6KA device of Mitsubishi) based converters had some space. Only SCR based

converters are possible at the highest range where line-commutated or load-commutated

converters were the only solution. The surge current, the peak repetition voltage and I2

t ratings

are applicable only to the thyristors making them more robust, specially thermally, than the

transistors of all varieties. 

Presently there are few hybrid devices and Intelligent Power Modules (IPM) are marketed by

some manufacturers. The IPMs have already gathered wide acceptance. The 4500 V, 1200 A 

IEGT (injection-enhanced gate transistor) of Toshiba or the 6000 V, 3500 A IGCT (Integrated

Gate Commutated Thyristors) of ABB which are promising at the higher power ranges.

However these new devices must prove themselves before they are accepted by the industry at

large.

 Silicon carbide is a wide band gap semiconductor with an energy band gap wider than about

2 eV that possesses extremely high thermal, chemical, and mechanical stability. Silicon carbide

is the only wide band gap semiconductor among gallium nitride (GaN, EG = 3.4 eV), aluminum

nitride (AlN, EG = 6.2 eV), and silicon carbide that possesses a high-quality native oxide suitable

for use as an MOS insulator in electronic devices The breakdown field in SiC is about 8 times

higher than in silicon. This is important for high-voltage power switching transistors. For

example, a device of a given size in SiC will have a blocking voltage 8 times higher than the

same device in silicon. More importantly, the on-resistance of the SiC device will be about two

decades lower than the silicon device. Consequently, the efficiency of the power converter is

higher. In addition, SiC-based semiconductor switches can operate at high temperatures

(~600C) without much change in their electrical properties. Thus the converter has a higher

reliability. Reduced losses and allowable higher operating temperatures result in smaller heatsink

size. Moreover, the high frequency operating capability of SiC converters lowers the filtering

requirement and the filter size. As a result, they are compact, light, reliable, and efficient and

have a high power density. These qualities satisfy the requirements of power converters for most

applications and they are expected to be the devices of the future.

 Ratings have been progressively increasing for all devices while the newer devices offer

substantially better performance. With the SCR and the pin-diodes, so called because of the

sandwiched intrinsic ‘i’-layer between the ‘p’ and ‘n’ layers, having mostly line-commutated

converter applications, emphasis was mostly on their static characteristics - forward and reverse

voltage blocking, current carrying and over-current ratings, on-state forward voltage etc and also

on issues like paralleling and series operation of the devices. As the operating speeds of the

devices increased, the dynamic (switching) characteristics of the devices assumed greater

importance as most of the dissipation was during these transients. Attention turned to the

development of efficient drive networks and protection techniques which were found to enhance

the performance of the devices and their peak power handling capacities. Issues related to

paralleling were resolved by the system designer within the device itself like in MOSFETS,

while the converter topology was required to take care of their series operation as in multi-level

converters.

 The range of power devices thus developed over the last few decades can be represented as a

tree, Fig. 1.5, on the basis of their controllability and other dominant features. 

Version 2 EE IIT, Kharagpur 

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