IEEE TRANSACTIONS INDUSTRY APPLICATIONS, VOL. JANUARY/FEBRUARY 2002

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IEEE TRANSACTIONS INDUSTRY APPLICATIONS, VOL. JANUARY/FEBRUARY 2002

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IEEE TRANSACTIONS INDUSTRY APPLICATIONS, VOL. JANUARY/FEBRUARY 2002
Punch-Through IGBT Having n-Buffer Layer
Hideo Iwamoto, Hideki Haruguchi, Yoshifumi Tomomatsu, John Donlon, Senior Member, IEEE, Eric Motto, Member, IEEE
Abstract-Insulated gate bipolar transistors (IGBTs) based non-punch-through (NPT) design approach exhibit excellent safe operating area (SOA) short-circuit endurance, positive temperature coefficient on-state voltage over operating current range, silicon cost. These merits have supported development commercialization IGBTs above 1200-V class. However, need quite thin silicon obtain competitive on-state losses 1200-V below classes hindered approach this area. punch-through (PT) IGBT been developed which exhibits merits approach, rugged short-circuit endurance, while also having better tradeoff relation between on-state voltage turn-off loss than either existing third-generation IGBTs. Index Terms-Insulated gate bipolar transistor, power semiconductor.
past several years, special processing permitted buffer layer approach achieve high-voltage IGBTs (2500 3300 which combine best characteristics both approaches. This tact been taken 1200-V design area produce IGBT that only exhibits best merits IGBT, rugged short-circuit endurance positive temperature coefficient on-state voltage over operating current range, also shows better tradeoff relation between on-state voltage turn-off loss compared with current third-generation IGBTs. DEVICE STRUCTURE Comparison Between IGBTs comparison characteristics IGBT IGBT shown Table [3]. shown this table, IGBT following disadvantages compared with IGBT: higher crossover point on-state curves (the crossover point device current which forward characteristic device transitions from negative temperature coefficient positive temperature coefficient) slightly lower destruction threshold, i.e., lower short-circuit endurance RBSOA. Described below structure production methods IGBT that were developed order achieve better characteristics than IGBT. Breakover Voltage equivalent IGBT's breakover voltage under open base condition p-n-p transistor's calculated from following formula using collector-base breakdown voltage p-n-p transistor's current gain when voltage applied across IGBT, collector-to-emitter (when drift layer depleted). shown above formula, IGBT's lower than collector-base breakover voltage [equivalent diode's breakbreakdown voltage raised increasing down voltage]. However, much possible. off-state, when high voltage applied across collector-to-emitter, significantly higher p-n-p transistor's current gain given than that on-state current gain. following equation:
INTRODUCTION manufacture power semiconductors always involved tradeoffs between conduction losses, switching speed, safe operating area (SOA). insulated gate bipolar transistors (IGBTs) these tradeoffs further complicated such factors silicon processing limitations costs. approaches IGBT design have evolved, punch-through (PT) non-punch-through (NPT). Each approach characteristics that beneficial detrimental user. approach on-state voltage, leakage current, good SOA, small negative temperature coefficient on-state voltage. approach uses lower cost silicon, very rugged short circuit endurance, high leakage current, large positive temperature coefficient on-state voltage. approach requires relatively thin (when compared approach) silicon achieve on-state characteristics approaching those approach. This been practical voltage designs above 2000 resulted high processing cost lower voltage designs. Over
Paper IPCSD 01-076, presented 1999 Industry Applications Society Annual Meeting, Phoenix, October 3-7, approved publication IEEE TRANSACTIONS INDUSTRY APPLICATIONS Power Electronics Devices Components Committee IEEE Industry Applications Society. Manuscript submitted review October 1999 released publication October 2001. Iwamoto Haruguchi with Mitsubishi Electric Corporation, Fukuoka 819-01, Japan (e-mail: iwamotoh@mail.oka.melco.co.jp; haruguch@mail.oka.melco.co.jp). Tomomatsu with Fukuryo Semiconductor Engineering Corporation, Fukuoka 819-01, Japan (e-mail: tomomaty@mail.oka.melco.co.jp). Donlon Motto with Powerex, Inc., Youngwood, 15697-1800 (e-mail: jdonlon@pwrx.com; emotto@pwrx.com). Publisher Item Identifier 0093-9994(02)00783-1.
0093-9994/02$17.00 2002 IEEE
IWAMOTO al.: IGBT
TABLE IGBT CHARACTERISTICS
where base transport factor, efficiency. calculated
Injection
drift layer width, diffusion length. buffer layer. Also, general, IGBT does have injection efficiency controlled limiting concentration collector layer IGBT does involve lifetime control. order increase breakover voltage needs minimized increasing thickNPT IGBT, other hand, IGBT's buffer layer ness short thin. Sufficient voltage obtained even made thin because low. other words, IGBT's breakover voltage improved leakage curlayer because rent reduced spite thinner buffer layer. IGBT
On-State Voltage IGBT tion [1]: obtained from following equa-
Fig. Cross-sectional views 1200-V IGBTs. IGBT with buffer layer. IGBT. IGBT.
voltage drop caused channel current where forward voltage drop MOSFET region, collector layer, diode region that consists buffer layer, drift layer. aforementioned, drift layer needs thicker than that IGBT's IGBT order secure breakdown voltage because buffer layer. This increases IGBT does have Since greatly transistor's current gain IGBT's increases and, depends becomes higher than that therefore, IGBT's IGBT. Thus, IGBT exhibits advantage that on-state voltage lower than that IGBT. Switching Characteristics tail current generated switching process when exbase cess carriers, which stored p-n-p transistor's area, disappear their recombination. reported that tail current calculated from following equation [2]:
current higher tail time longer. This that results because IGBT neither lifetime high buffer layer. other hand, IGBT, control buffer with lifetime control, short lifetime tail current layer. Therefore, IGBT's tail time short. Additionally, local lifetime redrift layer reduce tail time while still duction controlling saturation voltage. Cross Point On-State Characteristics Described below reasons IGBT's cross characpoint (the point where teristic curve versus on-state cross) lower than that IGBT. Because built-in potential decreases junction temperature rises, saturation voltage low-current region drops temperature rises. inclination slope curve's straight-line area function mobility lifetime. Because IGBT does lifetime control, lifetime value very long barely affects saturation voltage. characteristics are, therefore, influenced mobility's temperature dependency alone. mobility drops temperature rises, inclination curve becomes smaller temperature rises (i.e., on-state voltage significantly increases), crossover point becomes lower result.
carrier lifetime base area. This equation inwhere needs reduced order decrease dicates that tail current tail time. However, IGBT's tail
IEEE TRANSACTIONS INDUSTRY APPLICATIONS, VOL. JANUARY/FEBRUARY 2002
Fig. Off-state characteristics IGBTs whose relative impurity concentration layer shown Fig.
Fig.
Impurity concentration profile IGBT.
Fig. Electrical field distribution IGBT (simulation). 1200
Fig. Simulation result 1200-V IGBTs. Case without n-buffer layer; case with n-buffer layer case with n-buffer layer
Ruggedness Destruction tolerance greatly influenced both layer thickness order secure required layer needs breakover voltage IGBT, thicker. Consequently, destruction tolerance becomes high. Therefore, destruction tolerance IGBT could increased optimizing thickness layer. However, this approach also increases saturation voltage switching loss. reduce these drawbacks forming local lifetime-controlled layer adjacent layer optimizing doping density layer. That possible ensure high destruction tolerance without much effect saturation voltage switching losses buffer layer which lifetime controlled adopting layer. short increasing doping density slightly Realize Superior IGBT IGBT's characteristics improved following.
case standard IGBT, whole IGBT lifetime controlled electron beam irradiation lifetime greatly affects saturation voltage. lifetime dependent temperature becomes longer temperature rises. Therefore, increase lifetime offsets drop mobility difference function temperature between curve's inclination becomes small. result, crossover point rises. seen from above descriptions, crossover point IGBT lower because lifetime control because structure. means crossover point IGBT lowered reducing lifetime control. With conventional IGBTs, whole chip treated lifetime control electron beam irradiation. Therefore, crossover point lowered using local lifetime control [4].
IWAMOTO al.: IGBT
Fig. Off-state characteristic IGBT. With proton irradiation. Without proton irradiation. (Horizontal: V/div; vertical: A/div.)
Fig. switching loss tradeoff. Measurement conditions: switching loss:
Fig. Forward characteristics (horizontal: mV/div, vertical: A/div). IGBT Conventional IGBT
drift layer should made thin possible order reduce saturation voltage. reduce turn-off time, effective reduce lifedrift layer that depleted when time off-state voltage applied. -buffer layer needs constructed order secure high breakover voltage IGBT that thin drift layer. lifetime control should minimized order lower crossover point on-state characteristic curves. drift layer should optimized thickness improve destruction tolerance. Structure IGBT Based aforementioned analysis, -buffer layer constructed newly developed methods, IGBT drift layer thickness been developed. with optimized
Fig. compares schematic cross sections newly developed IGBT [Fig. 1(a)] with conventional IGBT [Fig. 1(b)] IGBT [Fig. 1(c)]. Fig. 1(a) IGBT with newly developed -buffer layer. collector region formed with approximately 100- boron diffused layer over -type single layer designed crystal wafer. thickness that depletion layer extends collector region when layer rated voltage applied state. That some thicker than that IGBT shown Fig. 1(b). This -buffer layer formed first applying proton irradiation near border between collector area layer then annealed. buffer layer having substrate formed changing lower resistivity than proton irradiated area donor area. Also, IGBT's controlled changing anneal time control local lifetime [5]. Fig. shows resulting measurement newly developed IGBT's impurity concentration
IEEE TRANSACTIONS INDUSTRY APPLICATIONS, VOL. JANUARY/FEBRUARY 2002
Fig. Turn-off switching waveforms. IGBT. Conventional IGBT. A/div; V/div; ns/div).
profile after annealing. This curve confirms that resistivity drops when proton irradiated area turns donor area -buffer layer formed. Fig. 1(b) shows structure conventional IGBT that uses epitaxial wafer, Fig. 1(c) shows structure conventional IGBT that uses -type single crystal wafer 250- thickness. conventional IGBT uses electron beam irradiation whole chip lifetime controlled. IGBT does involve lifetime control maintained within appropriate value limiting collector concentration. IGBT chips that were used conventional fabricated models exA/cm amined this comparison 1200 III. DEVICE SIMULATION Breakover voltage simulation results shown three 1200-V designs Fig. Case IGBT without -buffer layer, Case IGBT with -buffer layer layer, Case having same lifetime IGBT with -buffer layer having shorter lifetime than drift layer. concentration -buffer layer comes from value obtained from Fig. -layer thickness kept constant (150 concentration varied. simulation confirms that, Case breakover decreases when impurity concentration voltage drops below 1.2. That because depletion layer extends collector drift layer concentration. other hand, Case breakover voltage increases impurity concentration decreases. This because -buffer layer prevents depletion layer from extending collector. Case breakover voltage much higher p-n-p transistor, than Case This because which composed IGBT's base area, area, collector area, decreases. Fig. shows off-state characteristics three layer concentration (relative cases, value). From these figures, clear that breakover voltage drift layer concentration Case peaks particular value Case highest breakdown voltage. needs reduced that According (1), high breakover voltage attained. IGBT with
Fig. Switching waveform IGBT under condition short circuit. Test conditions: V/div; A/div; s/div).
-buffer layer that formed aforementioned methods short lifetime resistance. Therefore, higher breakover drift layer made voltage attained even though thinner than that IGBT. Shown Fig. electrical field distribution inside chip when 1200 applied across collector-to-emitter when impurity concentration Fig. Case Fig. shows that electric field suddenly drops near -buffer layer Case That depletion layer blocked from reaching through collector layer -buffer layer. stable breakover voltage obtained even slight variadrift layer thickness drift layer impurity tions concentration observed during production. Thus, this merits forming -buffer layer. Simulation also carried assess temperature dependence on-state characteristic. confirmed that, with local lifetime control, crossover point below curves rated current. EXPERIMENTAL RESULTS fabricated model IGBT, whose drift layer thickness impurity concentration case Fig. 1.0, been made evaluated. evaluation results follows.
IWAMOTO al.: IGBT
Fig. Switching waveform RBSOA test IGBT. Measurement conditions: V/div; A/div; ns/div).
1200
Breakover Voltage IGBT's characteristic off-state shown Fig. Fig. 6(b) shows characteristics before proton irradiation (equivalent Case simulation results) Fig. 6(a) shows them after proton irradiated area became donor area (equivalent Case simulation.) Fig. 6(a) shows that IGBT sufficient breakover voltage 1200-V class result formation -buffer layer reduction proton irradiated area's lifetime. Also, simulation results shown Fig. very much agree with measurement results shown Fig. those figures point trend breakover voltage shift before after formation -buffer layer match. Tradeoff Between On-State Voltage Switching Loss turn-off switching loss Tradeoff between shown Fig. Generally, turn-off loss defined loss generated turn-off from when collector voltage reaches rise when collector current falls 10%. However, turn-off loss measured according this definition, turn-off losses IGBT IGBT turn understated inaccurately compared since current tail drags long into area where current low. order make fair comparison against conventional IGBT, turn-off loss here defined loss from when voltage reaches rise when current falls Fig. measurement result based this definition. values chips used IGBT changed altering anneal time. Also, IGBT changed altering boron concentration drift layer IGBT collector layer. made thinner than that IGBT and, therefore, IGBT better characteristics. Also, IGBT better characteristics than conventional IGBT drift layer. This because despite having thicker IGBT uses local lifetime control with proton irradiation. On-State Characteristics Shown Fig. output characteristics IGBT conventional IGBT crossover point IGBT that uses local
lifetime control very low. Therefore, this IGBT merit that current easily balanced when connected parallel. Switching Characteristics Fig. shows current voltage switching waveforms IGBT conventional IGBT load turn-off. These waveforms show that turn-off loss IGBT does increase much compared conventional IGBT because small increase tail current even though temperature high (125 This because IGBT uses local lifetime control proton irradiation little affected lifetime temperature shift compared conventional IGBT whose whole wafer's lifetime controlled electron beam irradiation. However, IGBT's tail time longer than that conventional IGBT because drift layer thicker than that IGBT's conventional IGBT. Yet, IGBT's turn-off loss lower since tail current lower length shorter. Ruggedness Shown Fig. switching waveform PT-IGBT under short-circuit condition. shown here, IGBT damaged, even short-circuit duration Thus, more than sufficient short-circuit endurance actual application use. Fig. shows switching waveform RBSOA test IGBT. confirmed that IGBT capable turning (current A/cm which rated current under such density harsh conditions. Thus, IGBT larger turn-off capability (RBSOA) than that conventional IGBT. CONCLUSION Experimental results demonstrated that heat treatment (annealing) reduced resistivity proton irradiated -type region IGBT. Application this phenomenon allowed development 1200-V IGBT having -buffer layer. Experiments verified that IGBT merits IGBT, with crossover point curves below rated current with high short-circuit endurance. addition, lower losses than with existing third-generation IGBTs were achieved.
IEEE TRANSACTIONS INDUSTRY APPLICATIONS, VOL. JANUARY/FEBRUARY 2002
REFERENCES
Baliga, Modern Power Devices. Melbourne, Krieger, 1992, 358. al., "Modeling turn-off characteristics bipolar transistor," IEEE Electron Device Lett., vol. EDL-6, 211-214, June 1985. Yamashita, Yamada, Uchida, Yamaguchi, Ishizawa, relation between dynamic saturation characteristics tail current nonpunchthrough IGBT," Conf. Rec. IEEE-IAS Annu. Meeting, vol. 1996, 1425-1432. Mochizuki, Ishii, Takeda, Hagino, Yamada, "Examination punch through IGBT high voltage high current applications," Proc. Int. Symp. Power Semiconductor Devices, 1997, 237-240. Wondrak Silber, "Buried recombination layers with enhanced -type conductivity silicon power devices," Phisica vol. 129, 322-326, 1985.
Yoshifumi Tomomatsu born 1967. received B.S. degree applied physics from Fukuoka University, Fukuoka, Japan, 1989. 1989, joined Fukuryo Semiconductor Engineering Corporation, Fukuoka, Japan, subsidiary group Mitsubishi Electric Corporation. Design Development Engineer power semiconductors (IGBT diode). Tomomatsu member Institute Electrical Engineers Japan.
Hideo Iwamoto born Japan 1943. received B.E. degree electrical engineering from Osaka University, Osaka, Japan, Doctor Engineering degree from Yamaguchi University, Yamaguchi, Japan, 1967 2001, respectively. with Mitsubishi Electric Corporation from 1967 1991. with Powerex, Inc., Youngwood, from 1992 1998, prior reurning Mitsubishi Electric Corporation, Fukuoka, Japan, 1999. been engaged design development power semiconductor devices. Iwamoto member Institute Electrical Engineers Japan.
John Donlon (S'64-M'66-SM'93) received B.S. degree with high honors from University Lowell, Lowell, M.S. degree from Syracuse University, Syracuse, 1966 1970, respectively, both electrical engineering. While Syracuse University, studied under National Aeronautics Space Administration Traineeship. Senior Application Engineer Powerex, Inc., Youngwood, been involved rating, evaluation, application power semiconductors past years. represents Powerex Electronic Industries Alliance JEDEC Standards Committee Rectifiers Thyristors chairs Committee Transistors. been active publication application notes technical papers describing characteristics proper application power semiconductors. Donlon senior member IEEE Industry Applications Society member IEEE Power Electronics Society, Epsilon Sigma, Kappa
Hideki Haruguchi born Kagoshima Prefecture, Japan, 1968. received B.S. degree electrical engineering from Kumamoto University, Kumamoto, Japan, 1991. Since 1991, been with Engineering Development Design Section, Power Semiconductor Division, Mitsubishi Electric Corporation, Fukuoka, Japan, where worked design power semiconductor devices, particular, design IGBT chips.
Eric Motto (M'90) received B.S. degree electrical engineering from Pennsylvania State University, University Park, B.A. degree mathematics from Saint Vincent College, Latrobe, 1987. Principal Application Engineer with Powerex, Inc., Youngwood, been involved evaluation application IGBT intelligent power modules since 1990. written presented more than conference technical papers authored numerous application notes trade magazine articles.

IEEE TRANSACTIONS INDUSTRY APPLICATIONS, VOL. JANUARY/FEBRUARY 2002

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