Power devices Power BJT - Large devices to reduce current density - - PowerPoint PPT Presentation

Power devices

◼ Power BJT

• “Large” devices to reduce

current density

• Structure is similar to the

integrated BJT, but there is no need to have the collector contact on the upper face -> collector contact at the bottom

• interleaved B/E structure to

reduce emitter crowding

◼ Power FET

• to reduce the electric

field: double implantation (with n- on the drain side)

• Vertical structures are
• possible. E.g.: VMOS

In a MOSFET

• when T increases (e.g. due to

power dissipation) I decreases (because m decreases: it’s a drift based device)

• > it’s easy to put many of them in

parallel. E.g.: HexFET

Four-layer devices

◼ Although transistors make excellent switches,

they have limitations when it comes to switching high currents at high voltages

◼ In such situations we often use devices that are

specifically designed for such applications

◼ These are four-layer devices

 they amplify in the sense that a small current

controls a large current

 but can only be on or off, as relays  moreover, they can only be switched on (not

• ff!!)

9.5

◼ operation

 construction resembles

two interconnected bipolar transistors

 turning on T2 holds on

T1

 T1 keeps T2 on…  device then conducts

until the current goes to zero.

Four-layer devices

◼ This was the basic 4-layer device ◼ Bilateral version of the 4-layer device: Diac ◼ If current is injected in the base of (e.g.) the npn, the

switch-on (but not the switch-off!) can be controlled: SCR

• r thyristor

◼ Bilateral version of the SCR: Triac

Four-layer devices

◼ The SCR

 a four-layer

device with a pnpn structure

 three terminals:

anode, cathode and gate

 gate is the

control input.

Four-layer devices (contd.)

◼ Use of a thyristor in

AC power control

 once triggered the device

conducts for the remainder

• f the half cycle

 varying firing time

determines output power

 allows control from 0–50%

• f full power

Four-layer devices (contd.)

◼ Full-wave power

control using thyristors

 full-wave control

requires two devices

 allows control from

0–100% of full power

 requires two gate

drive circuits

 opto-isolation often

used to insulate circuits from AC supply

Four-layer devices (contd.)

◼ The triac

 resembles a bidirectional

thyristor

 allows full-wave control

using a single device

 the triac is often used with a

bidirectional trigger diode (a diac) to produce the necessary drive pulses

 the latter breaks down at

particular voltage and fires the triac

Four-layer devices (contd.)

◼ A simple lamp-dimmer using a triac

Four-layer devices (contd.)

Other power devices

Besides

◼ power BJTs ◼ power MOSFETs ◼ thyristors

• ther power devices do exist:

◼ GTOs, gate turn-off thyristors ◼ IGBTs, insulated gate bipolar transistors ◼ MCTs, MOS controlled thyristors

◼ Other power devices

• GTOs, gate turn-off thyristors
• can also be switched off (however, a rather

large current is needed)

• IGBTs, insulated gate bipolar transistors
• high input impedence (similar to MOSFETs)
• low VON (similar to BJTs)
• MCTs, mos controlled thyristors
• it’s more or less a thyristor controlled by a

MOSFET

◼ A (rough) comparison of performances

device power speed BJT medium medium MOSFET medium high thyristor very high very low GTO high low IGBT medium medium MCT medium medium

[Spectrum, May 2014]

◼ Wide-bandgap semiconductors can operate stably at high

temperatures and frequencies

◼ For medium currents and thermal loads where extremely

fast and efficient switching is required, Gallium nitride (GaN) is optimal

◼ For very high currents and thermal loading where large

amounts of energy need to be processed in a small area (e.g. in a vehicular motor drive) silicon carbide (SiC) is the best choice.

Application map of power devices (2014)

Cross section of the elementary cell of (a) traditional planar MOSFETs and (b) state-of-the-art (2014) trench MOSFETs