Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Field effect transistor
Reexamination Certificate
2002-08-01
2004-07-06
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Field effect transistor
C438S202000
Reexamination Certificate
active
06759696
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a bipolar transistor comprising
a collector region with a first doping type,
a base region with a second doping type,
and an emitter region with the first doping type,
a junction being situated between the emitter region and the base region, and, viewed from said junction, a depletion region extending in the emitter region,
and, said emitter region comprising a layer of a first semiconductor material and a layer of a second semiconductor material.
The invention also relates to a method of manufacturing a bipolar transistor comprising a collector region with a first doping type and a base region with a second doping type, on which an emitter region with the first doping type is formed, said emitter region including a layer of a first semiconductor material and a layer of a second semiconductor material.
BACKGROUND AND SUMMARY OF THE INVENTION
U.S. Pat. No. 5,535,912 discloses a bipolar transistor that can suitably operate at high frequencies. Said bipolar transistor has a cutoff frequency of typically 100 GHz, as a result of which the transistor can suitably be used as a component in optical communications networks for transporting 40 Gb/s.
The bipolar transistor is made from silicon and includes a base region with a Ge
x
Si
1−x
strained layer. As the bandgap of Ge
x
Si
1−x
is smaller than that of Si, with the conduction band coinciding with that of silicon, and the valence band energetically moved by &Dgr;Ev with respect to the valence band of Si, the charge storage in the base region and the emitter region is reduced relative to silicon bipolar transistors at comparable current levels. In order to maximize the speed of the transistor, the percentage of Ge in the base region is as high as possible.
In the known bipolar transistor, the charge storage in the emitter is also reduced, which can be attributed to the fact that the bandgap, viewed from the junction, decreases linearly in the direction of the emitter contact. During operation of the bipolar transistor, minority charge carriers are injected into the emitter region from the base region and accelerated by the internal electric field in the emitter, as a result of which the average residence time decreases.
The Ge
x
Si
1−x
strained layer in the base region causes a change of the bandgap &Dgr;Ev, as a result of which the collector current increases exponentially by &Dgr;Ev. As a result, the current gain, which is defined as the quotient of the collector current and the base current, increases substantially. A drawback of a base region with Ge
x
Si
1−x
resides in that the current gain is too high, as a result of which collector-emitter breakdown occurs rapidly. The device is not robust because the bipolar transistor amplifies the current internally. For practical applications, a current gain of only approximately 100 is desired.
In the known heterojunction bipolar transistor, the collector current is reduced by increasing the base doping. In addition, the emitter contact is made of a metal instead of the customarily used polysilicon. The recombination of minority charge carriers at a metal contact exceeds that at a polysilicon contact by approximately one order or magnitude, as a result of which the base current is increased by approximately one order of magnitude.
A drawback of the known bipolar transistor resides in that setting the value of the base current is difficult. As the metal contact borders on the emitter region, and reacts at the interface with the second semiconductor material of the emitter region, the width of the emitter region, viewed from the junction, is highly subject to variations.
As the width of the emitter region of a bipolar transistor intended for high-speed applications is very small, the decrease of the emitter width due to the interface reaction, causing a part of the emitter region to be consumed, is comparatively large. The base current depends very substantially on the width of the emitter region and the interface between the emitter region and the metal. A metal contact leads to a substantial variation in base current between bipolar transistors and hence to a substantial variation in current gain.
It is an object of the invention to provide a bipolar transistor of the type described in the opening paragraph, which enables the current amplification to be very accurately adjustable via the base current.
As regards the bipolar transistor in accordance with the invention, this object is achieved in that the intrinsic carrier concentration of the second semiconductor material exceeds the intrinsic carrier concentration of the first semiconductor material, the layer of the second semiconductor material is situated outside the depletion region, and the second semiconductor material is doped such that Auger recombination occurs.
When the bipolar transistor is in operation, minority charge carriers injected from the base region into the emitter region diffuse from the depletion region in the direction of an emitter contact that borders on the emitter region. In the layer of the second semiconductor material, the intrinsic concentration n, of minority charge carriers is greater than the intrinsic concentration in the first semiconductor material due to a smaller bandgap of the second semiconductor material. In a semiconductor, n
i
2
=np, where n is the concentration of electrons and p is the concentration of holes, so that an increased concentration of minority charge carriers is present in the layer of the second semiconductor material. The physical effect causing an increase in base current is referred to as Auger recombination.
Auger recombination occurs if excess charge carriers recombine in semiconductor material having a high doping concentration. The probability of direct recombination between holes and electrons must not be negligible relative to the recombination speed due to traps (Schottky Read Hall recombination). In the case of Auger recombination, there are three charge carriers that interact with each other, i.e. either two electrons and one hole, or two holes and one electron. Two charge carriers recombine and the third charge carrier takes over the impulse from the incident charge carriers and the energy released by said recombination.
For an n-type emitter, the Auger recombination depends quadratically on the electron concentration and linearly on the hole concentration. Auger recombination contributes dominantly to the base current if the hole concentration is increased by a number of orders of magnitude by the use of the second semiconductor material having a smaller bandgap and hence a higher intrinsic concentration. The increase of the minority charge carriers depends exponentially on the decrease in bandgap. Thus, by accurately setting the bandgap as a function of the composition of the second semiconductor material, the base current can be very accurately set, so that also the current amplification can be very accurately set.
The first semiconductor material in the emitter region may be, for example, InAlAs, and the second semiconductor material may be, for example, InGaAs. An N-type doping for these materials is, for example, silicon, and a p-type doping is, for example, beryllium. Alternatively, silicon may be used for the bipolar transistor comprising Si as the first semiconductor material and a Ge
x
Si
1−x
composition as the second semiconductor material. For the N-type doping use can be made, for example, of As or P, and for the p-type doping use can be made, for example, of B.
Owing to the comparatively high intrinsic concentration, Ge can particularly suitably be used as the second semiconductor material.
Advantageously, the second semiconductor material has a composition that is at least substantially constant over at least a part of the layer. As a result, the bandgap is at least substantially constant over said part as well as the intrinsic carrier concentration. In comparison with a situation where the composition of the second semiconductor material varies, a better setting
Aksen Eyup
Huizing Hendrik Gezienus Albert
Klootwijk Johan Hendrik
Slotboom Jan Willem
Terpstra Doede
Koninklijke Philips Electronics , N.V.
Nelms David
Vu David
Waxler Aaron
LandOfFree
Bipolar transistor, semiconductor device and method of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Bipolar transistor, semiconductor device and method of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Bipolar transistor, semiconductor device and method of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3257360