Miscellaneous active electrical nonlinear devices – circuits – and – Gating – Utilizing three or more electrode solid-state device
Reexamination Certificate
1999-05-17
2001-11-27
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Gating
Utilizing three or more electrode solid-state device
C327S435000, C327S436000, C327S403000
Reexamination Certificate
active
06323717
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device used to control high power and a drive method and drive apparatus.
Recently, IGBTs (Insulated Gate Bipolar Transistors) and IEGTs (Injection Enhanced Gate Transistors) have received a great deal of attention as power control semiconductor devices. These IGBT and IEGT are bipolar devices with MOS structures, and have the high-speed switching characteristics of the power MOSFET and the high-breakdown voltage, short-turn-on time characteristics of the bipolar transistor. For this reason, the IGBT and IEGT can be applied to power converters such as an inverter. The IGBT will be exemplified.
FIG. 1
is a circuit diagram showing the arrangement of a general inverter. High-side IGBT
1
and low-side IGBT
2
, which respectively have reflux diodes D
1
and D
2
and gate resistances RG
1
and RG
2
, are series-connected to a power supply voltage Vcc.
As for low-side IGBT
2
, a gate voltage of ±15V is applied to IGBT
2
from a gate drive circuit (not shown) via the gate resistance RG
2
to flow (ON) or cut off (OFF) a collector current Ic flowing through IGBT
2
in correspondence with the gate signal. For example, if a positive gate signal is applied to a gate G of IGBT
2
, the collector current Ic flows to turn on IGBT
2
; if a negative gate signal is applied, the collector current Ic is cut off to turn off IGBT
2
.
When the gate signal changes from negative to positive, IGBT
2
is turned on. IGBT
2
shifts from the OFF state to the ON state to flow the collector current Ic. When the gate signal changes from positive to negative, IGBT
2
is turned off. IGBT
2
shifts from the ON state to the OFF state to cut off the collector current Ic.
FIG. 2
is a waveform chart showing an example of the turn-off waveform of the IGBT.
FIG. 3
is a sectional view showing the structure of the IGBT for explaining turn-off operation. As shown in
FIG. 3
, the IGBT is constituted by forming a heavily doped p-type emitter layer
2
on one surface of a lightly doped n-type base layer
1
and forming a collector electrode
3
on the p-type emitter layer
2
.
A p-type base layer
4
is selectively formed on the other surface of the n-type base layer
1
, and a heavily doped n-type source layer
5
is formed on the surface of the p-type base layer
4
. A gate electrode
7
is formed on the p-type base layer
4
between the n-type source and base layers
5
and
1
via a gate oxide film
6
. An emitter electrode
8
is formed on the n-type source layer
5
and p-type base layer
4
.
In this IGBT, if the gate signal supplied from the gate drive circuit changes from +15V to −15V, a gate voltage V
G
of IGBT
2
connected to the gate drive circuit via the RB first falls to a given value (time t
1
) and keeps this value for a while (time t
2
). In this specification, this constant V
G
period (time t
1
to t
2
) is called a mirror time in the MOSFET mode. During the mirror time in the MOSFET mode, a collector voltage V
CE
rises to about 15V.
After that, in the IGBT, a depletion layer having a high electric field starts extending from below the gate oxide film
6
and p-type base layer
4
into the n-type base layer
1
, and the collector voltage V
CE
abruptly rises (from time t
2
). At the same time, the gate voltage V
G
starts gradually falling but is still higher than a threshold voltage Vth of the IBGT.
If the collector voltage is clamped by the diode, the collector current Ic is commutated to the diode (D
1
in
FIG. 1
) and cut off. At the same time, the gate current also abruptly falls (time t
3
), and the gate voltage V
G
falls to the threshold voltage Vth or less (from time t
3
). In this specification, the period (time t
2
to t
3
) required to reach the threshold voltage Vth of the IGBT after the gate voltage V
G
starts falling is called a mirror time in the IGBT mode.
This switching method is used in all current IGBTs. The switching method is advantageous in small drive force of the gate drive circuit and switching control-lability by the gate resistance RG. Particularly, this method is most easily, widely used in low-resistance, small-capacity IGBTs. Conventionally, RG is generally set large in order to stably operate devices such as the IGBT, which is adopted in all device applications at present.
However, the present inventors have studied to find that this switching method poses serious problems in stability upon switching.
FIG. 3
shows carriers inside the device in the mirror time in the IGBT mode, in addition to the structure of the IGBT. Since the gate voltage V
G
is higher than the threshold voltage Vth, electrons are still injected (e− in FIG.
3
), while holes flow from the collector side (h+ in FIG.
3
). Thus, both holes and electrons exist in the high electric field (depletion layer). The presence of both holes and electrons degrades stability. Note that the broken line in
FIG. 3
represents that the n-type base layer
1
above the broken line is in a high electric field and accumulated carriers remain in the n-type base layer
1
below the broken line.
For example, a space charge density &rgr; in the high electric field is given by equation (1) using a donor density N
D
in the n-type base layer
1
, a hole density p in the high electric field, and an electron density n in the high electric field:
&rgr;=q
(
N
D
+p−n
) (1)
A voltage applied to the IGBT is obtained by dividing the integrated value of the space charge density &rgr; in the high electric field by a silicon permittivity &egr;
Si
.
A current density J is given by equation (2) using an electronic current density Jn in the high electric field, a hole current density Jp in the high electric field, and a carrier saturation rate vs (about 10
7
cm/s):
J=Jn+Jp=q·vs
(
p+n
) (2)
In this case, it should be noted that holes and electrons have opposite charge polarities and cancel each other, i.e., are represented by the difference (p−n) as for the space charge density &rgr;, like equation (1), whereas holes and electrons have the same elementary electric charges and can be represented by the sum (p+n) of the hole and electron densities as for the current density J in the high electric field, like equation (2).
This shows that even if the electric field distribution inside the device keeps a constant value under conditions such as the collector voltage V
CE
, the current density is not determined in one-to-one correspondence but has a high degree of freedom, i.e., the current density cannot be made constant.
Further, if the collector voltage V
CE
and collector current Ic are positively fed back to the gate, the current density varies to make the stability of the current density J more unstable, and the current concentrates to destruct the device.
Next, various problems arising when the current capacity and breakdown voltage of one IGBT increase along with a larger capacity of the IGBT will be explained.
In recent years, since the current capacity of the IGBT increases, a plurality of IGBT chips are parallel-connected in one IGBT package (one device). For example, 4 to 6 chips are parallel-connected in the package for a 1,700-V, 400-A IGBT, about 6 chips are parallel-connected for a 2,000-V, 400-A IGBT, and 20 to 24 chips are parallel-connected for a 3.3-kV, 1,200-A IGBT. Each chip generally has a size of about 7 to 15 mm square. Connecting a larger number of chips increases the package size.
FIG. 4
is a circuit diagram showing the arrangement of IGBT
1
and IGBT
2
for two chips or devices that are parallel-connected. Gates G
1
and G
2
of the IGBT
1
and IGBT
2
are combined into one via corresponding gate resistances RG
1
and RG
2
, and this gate is properly connected to a gate drive circuit via a resistance (not shown).
FIG. 5
shows the turn-off waveform of this circuit. The difference between the gate voltages V
G1
and V
G2
of the two IGBT
1
and IGBT
2
increases in the mirror time in the IGBT mode to make collector currents Ic
1
Domon Tomokazu
Eicher Simon
Ogura Tsuneo
Ohashi Hiromichi
Omura Ichiro
Kabushiki Kaisha Toshiba
Lam Tuan T.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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