A+Novel+Lateral+IGBT+with+a+Controlled+Anode+for+On-off-stat

Vol.32,No.7Journal of Semiconductors July2011 A novel lateral IGBT with a controlled anode for on-off-state loss trade-off improvement

Chen Wensuo(陈文锁) ,Zhang Bo(张波),Fang Jian(方健),and Li Zhaoji(李肇基)

State Key Laboratory of Electronic Thin Film and Integrated Devices,University of Electronic Science and Technology

of China,Chengdu610054,China

Abstract:A new lateral insulated-gate bipolar transistor with a controlled anode(CA-LIGBT)on silicon-on-

insulator(SOI)substrate is reported.Benefiting from both the enhanced conductivity modulation effect and the

high resistance controlled electron extracting path,CA-LIGBT has a faster turn-off speed and lower forward drop,

and the trade-off between off-state and on-state losses is better than that of state-of-the-art3-D NCA-LIGBT,which

we presented earlier.As the simulation results show,the ratios of figure of merit(FOM)for CA-LIGBT compared

to that of3-D NCA-LIGBT and conventional LIGBT are1.45:1and59.53:1,respectively.And,the new devices

can be created by using additional silicon direct bonding(SDB).So,from the power efficiency point of view,the

proposed CA-LIGBT is a promising device for use in power ICs.

Key words:controlled anode;turn-off time;forward drop;power IC

DOI:10.1088/1674-4926/32/7/074005EEACC:2560

1.Introduction

The lateral insulated-gate bipolar transistor(LIGBT)is a

promising power device for power ICs.Conductivity modu-

lation permits LIGBT to have low forward drop,but it also

causes slow turn off due to the removal of stored electron-

hole plasma in the drift region,which is strongly dependent

on the recombination process of the electron–hole pairs during

the turn-off period.Recently,various approaches?1 have been

reported for optimization of the LIGBT structures to achieve

fast turn-off speed or/and make good trade-off between on-

state and off-state losses.These include passive PMOS driving

LIGBT?2 ,shorted anode LIGBT(SA-LIGBT)?3 ,segmented

anode LIGBT(SA-LIGBT)?4 ,gradual hole injection dual gate

LIGBT(GHI-LIGBT)?5 ,segmented anode NPN controlled

LIGBT?6 ,dual gate inversion layer emitter transistor(DG-

ILET)?7 and n-region controlled anode LIGBTs?8;9 .

In this paper,a novel LIGBT with a controlled anode(CA-

LIGBT)on silicon-on-insulator(SOI)substrate,of which the

trade-off between off-state and on-state losses is better than

that of state-of-the-art3-D NCA-LIGBT?8 ,is proposed and

discussed for the first time.Two-dimensional numerical simu-

lations?10 are carried out to help with the analysis of the char-

acteristics for the proposed CA-LIGBT.

2.Device concept and operation

The simplified schematic of the proposed CA-LIGBT is

illustrated in Fig.1(a).A trench oxide is added at the anode

part for the new structure.A high resistance controlled n-region

whose doping concentration is as low as that of the n-drift re-

gion is formed under anode P C diffusion.Figure1(b)is for CA-LIGBT with only anode trench oxide to show the net ef-

fect of n-drift enhanced conductivity modulation because of anode trench oxide.Figures1(c)and1(d)are for the3-D NCA-LIGBT and conventional LIGBT.It is noted that the new de-vices can be created by using additional silicon direct bonding (SDB).

For a LIGBT,the common base current gain of the inher-ent PNP transistor(anode P C/N-drift/cathode P-base)is?PNP D?T M E,where?T is the base transport factor,which is de-pendent on lifetime and minority carrier diffusivity,M is the avalanche multiplication factor,which in high voltage devices can be considered as unity, E is the emitter injection effi-ciency,which is dependent on the base and emitter Gummel numbers.For given cathode and drift region,changing the de-sign of the anode scheme only influences the emitter Gummel number,which depends on the dimensions and doping concen-tration of the anode region(s).

As for our proposed CA-LIGBT,trench oxide can enhance the conductivity modulation effect in the n-drift region during the on-state.At the forward bias,the electrons diffusing toward the anode part in the proposed CA-LIGBT cannot be collected rapidly and accumulate in the n-buffer and n-drift regions,lead-ing to an increase in electron concentration in these regions. The electrons,providing the base drive for the PNP transistor inherent in the LIGBT,in turn enhance the hole injection from the anode P C to maintain electrical neutrality in conductivity modulation regions.Thus,the effect of conductivity modula-tion in the n-drift and n-buffer regions is enhanced.

The high resistance controlled n-region affects both the forward state and the off state:during the turn-on state,ac-cording to the analysis in Ref.[8],hole injection can be initi-ated at a much lower applied anode voltage,which effectively suppresses snapback in the conducting state without sacrificing the high current handling capability.During the turn-off state, it acts as the electron extracting path,and the proposed CA-LIGBT has a faster switching speed and lower turn-off loss.

*Project supported by the National Natural Science Foundation of China(Nos.61076082,60876053).

Corresponding author.Email:wensuochen@4993bf5c804d2b160b4ec0bb

Received17January2011,revised manuscript received3March2011c 2011Chinese Institute of Electronics

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Semicond.2011,32(7)Chen Wensuo et al.

Fig.1.Simplified schematic of (a)the proposed CA-LIGBT,(b)CA-LIGBT with only anode oxide,(c)3-D NCA-LIGBT and (d)conventional LIGBT.n-drift thickness T d D 5 m,length L d D 22 m and doping concentration N d D 1.5 1015cm 3;

anode oxide thickness T ox D 4 m.

Fig.2.I –V characteristics of (a)conventional LIGBT,CA-LIGBT,CA-LIGBT with only anode oxide and 3D NCA-LIGBT,and (b)elec-tron–hole concentration in n-drift of conventional LIGBT and CA-LIGBT with only anode oxide at an anode current density of 100A/cm 2.Lifetimes of carriers are 1 s,temperature is 300K;for CA-LIGBT,high resistance controlled n-region length,L D 3 m and thickness,d D 0.4 m

The novel design of the proposed CA-LIGBT inherits the advantages of both the enhanced conductivity modulation ef-fect by the trench oxide and the high resistance controlled n-region as the electron extracting path during the turn-off state.

3.Results and discussion

Figure 2(a)shows the I –V characteristics of conventional LIGBT,CA-LIGBT,CA-LIGBT with only anode oxide and 3D NCA-LIGBT.All of these devices have the same struc-ture and parameters except the anode part.Hole injection for CA-LIGBT is initiated at low applied anode voltage (the turn in point is about 1.15V),and there is no snapback but hold-

ing high current handling capability.Figure 2(b),illustrating electron–hole concentration in n-drift of conventional LIGBT and CA-LIGBT with only anode oxide,shows the net effect of n-drift enhanced conductivity modulation because of anode trench oxide.It is worth noting that the forward drop of CA-LIGBT with only anode oxide is the lowest due to the net en-hanced conductivity modulation effect by trench oxide at the anode part.That the forward drop of CA-LIGBT is slightly higher than that of the conventional one but lower than that of 3D NCA-LIGBT is because of both effects of trench oxide and the high resistance controlled n-region for the anode part.

Equations ((1)and (2))show the figure of merit (FOM).For drive application,neglecting the turn-on,driving,and non-

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Fig.3.Turn-off time/forward voltage drop trade-off curves obtained through control of the anode hole injection efficiency of conventional LIGBT,3D NCA-LIGBT and CA-LIGBT.

Fig.4.Turn-off time/forward voltage drop trade-off curves obtained through control of the key parameters of the proposed CA-LIGBT:the anode P C concentration;high resistance controlled n-region depth and length.

conducting state losses the conducting loss and turn-off loss are the most important parts in the total loss of LIGBT ?11 .

FOM D J C

v on e off ;

(1)e off D

1

6

V s J C t off ;(2)

where J C is the current density,100A/cm 2;v on is the forward drop at current density J C ;e off is the turn-off switching energy per pulse of operation;V s is the applied voltage,100V for sim-ulation.The ratios of FOM for CA-LIGBT compared to that of 3-D NCA-LIGBT and conventional LIGBT are 1.45:1and 59.53:1,respectively.

Also,Figure 3shows turn-off time/forward voltage drop trade-off curves obtained through control of the anode hole injection efficiency of conventional LIGBT,3D NCA-

LIGBT and the proposed CA-LIGBT.Figure 4shows turn-off

Fig.5.Breakdown characteristics of conventional LIGBT and CA-LIGBT;lifetimes of carriers t D 1 s,temperature T D 300K.

time/forward voltage drop trade-off curves obtained through control of the key parameters of the proposed CA-LIGBT.As can be observed clearly from these two figures,the trade-off curve of the proposed CA-LIGBT lies below that of 3D NCA-LIGBT and conventional LIGBT,and all of the proposed struc-tures with different parameters lie on the same trade-off curve.So,from a power efficiency point of view,the proposed CA-LIGBT is a promised device used in power ICs.

Figure 5shows breakdown characteristics of conventional LIGBT and CA-LIGBT.The proposed CA-LIGBT has the same high breakdown voltage as that of conventional one.

4.Conclusion

The operation mechanism and simulation analysis of novel fast speed LIGBT are explained in detail.The new device has higher FOM and better trade-off curves between off-state and on-state losses than that of state-of-the-art 3-D NCA-LIGBT.The proposed CA-LIGBT also has the advantages of effective snapback suppression in forward I –V characteristics and high breakdown voltage.

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