Today s subject MOSFET and IGBT 2018-05-22
MOSFET metal oxide semiconductor field effect transistor Drain Gate n-channel Source p-channel
The MOSFET - Source Gate G D n + p p n + S body body n - drift region + Drain
Diffusion structure It is in the p-doped region, the body, where the channel is formed There is always one reversed biased pnjunction, so the conduction is not based on minority carrier injection A small signal MOSFET can either be operating in normally on or normally off. A power MOSFET operates in normally off
MOSFET output characteristic V DS =V GS -V GS(th) V GS i D ohmic active cut-off, V GS <V GS(th) V DS(max) V DS
MOSFET transfer characteristic i D V GS(th) V GS
Active region The gate source voltage is higher than the threshold voltage V GS >V GS(th) The drain current is independant of the applied drain source voltage The drain current is determined by the applied gate-source voltage and is proportional to the squared difference between the applied gate source voltage and its threshold
The MOSFET equivalent circuits in the cut-off and active region D C GD R G i D (V GS ) C DS C GS S
The ohmic region The drain current is proportional to and not independant of the drain source voltage (resistive) The resistance is determined by the geometrical size and the conductivity of the drift region The region with the lowest drain-source voltage for a certain current
The MOSFET equivalent circuits in the ohmic region D C GD R G R DS(on) C DS C GS S
Cut off region The gate source voltage is lower than the threshold voltage V GS <V GS(th) (3-5 V) The MOSFET must be able to block the applied DRAIN-SOURCE voltage, which must be lower than the breakdown DRAIN-SOURCE voltage, V DS <V DS(max) to avoid avalanche
The conduction mechanism, V GS <V GS(th ) The SiO 2 between the gate and the body forms the MOS capacitance Source + + + + + + Gate n + - - - - ++ ++ p n - Drain When a voltage, <V GS(th) is applied to the gate a depletion area is formed in the p-body, becaise the majority carriers (holes) are repelled, Acceptor atoms are exposed
The conduction mechanism, V GS >V GS(th) Source +++++++++++++ Gate n + p n - V GS >V GS(th) - - - - - - - - n-channel Drain The conductivity increases strongly and a channel between drain and source of free electrons, minority carriers, is formed The inversion layer slightly increases
The MOSFET parasitic elements Source Gate metallisation The npn structure must not be turned on as the bjt cannot be turned off, as there is no connection between the gate and the p (body) region. body n - n + insulation p p n + body drift region The parasitic diode structure can be used as a freewheeling diode Drain
The MOSFET npn GE short circuit The source metalisation is covering both the n + and the p region to avoid turning on the npn structure Source metallisation Gate short circuit n + insulation p n - body With too high resistance in the body, hole conduction can cause a voltage drop between base and emitter of the stray transistor, latch up, because there is no access to this base
The buck converter, step down chopper with a MOSFET i S D G S v L i L v R V Dc i D V load v D
Switching A power MOSFET switches faster than the power BJT, since the excess carriers does not have to be established and removed at turn-on and turn-off In most cases the MOSFET can be modelled as being capacitive between all three terminals The gate-drain capacitance is formed of the gate-oxide and the depleted part of the drift region. The latter increases with higher gatedrain voltage and thus the capacitance reduces
V cc V GS (I load ) V GS(th) 0 v GS MOSFET Time diagram at turn-on V EE V CC -V EE R G(on) 0 i G D I load i D C GD 0 v D V CC G R V GS,C GS V EE R DS(on ) C DS V dc S V DS(on) 0 S t d(on ) t ri t fv1 t fv2 turn-on
MOSFET Time diagram at turn-off V cc V GS (I load ) V GS(th) 0 v GS V EE D V CC -V EE R G(on) 0 i G i D C GD I load V CC V EE R V GS,C GS G R DS(on ) C DS 0 V dc v D S turn-on S V DS(on) 0 t d(off t rv1 t rv2 t fi ) turn-off
IGBT
The NPT-IGBT Emitter D Gate G n + p p n + E body body n - p + drift region injection layer Collector
IGBT output characteristic i C V CE
IGBT transfer characteristic i C V GE
IGBT gate-collector capacitance versus gate-emitter voltage C GC V GE
IGBT Time diagram at turn-on and turn-off V CC V GE (I load ) V GE(th) 0 v GE V EE V CC -V EE R G(on) 0 i G V EE -V CC R G(off) I load i C 0 v C V dc E V CE(on) 0 t d(on ) t ri t fv1 t fv2 turn-on t d(off t rv1 t rv2 t fi ) turn-off
The IGBT stray thytistor Source Gate n + p body n - drift region With too high resistance in the body, hole conduction can cause a voltage drop between base and emitter p + injection layer
Darlington-thyristor Latch up C G The thyristor structure must not be trigged E