Schottky-clamped bipolar transistor with reduced self heating

The self heating of a high-performance bipolar transistor that is formed on a fully-isolated single-crystal silicon region of a silicon-on-insulator (SOI) structure is substantially reduced by forming a Schottky structure in the same fully-isolated single-crystal silicon region as the bipolar transistor is formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to bipolar transistors and, more particularly, to a Schottky-clamped bipolar transistor with reduced self heating.

2. Description of the Related Art

A bipolar transistor is a well-known structure that has an emitter, a base connected to the emitter, and a collector connected to the base. The emitter has a first conductivity type, the base has a second conductivity type, and the collector has the first conductivity type. For example, an npn bipolar transistor has an n-type emitter, a p-type base, and an n-type collector.

An npn bipolar transistor turns on when the voltage on the p-type base exceeds the voltage on the n-type emitter by approximately 0.7V, thereby forward biasing the junction. When the base-to-emitter junction becomes forward biased, the operation of the bipolar transistor depends on the voltage present on the collector.

When the voltage on the n-type collector is greater than the voltage on the p-type base, the base-to-collector junction is reverse biased and the transistor enters an active mode of operation. In this mode, a large number of electrons flow into the p-type base from the n-type emitter across the forward biased junction. A large portion of these electrons are captured by the electric field that lies across the reverse-biased base-to-collector junction, and then collected by the collector.

Alternately, when the voltage on the n-type collector is less than the voltage on the p-type base by approximately 0.7V, the base-to-collector junction becomes forward biased and the transistor enters a saturation mode of operation. In the saturation mode of operation, the transistor provides high current conduction from the collector to the emitter.

The saturation mode of operation is not preferred for high-frequency applications because once the bias voltages change, it takes a relatively long period of time for the transistor to recover from operating in the saturation mode. Thus, due to the long recovery time, bipolar transistors which operate in the saturation mode typically have a limited operating frequency.

To prevent a bipolar transistor from entering into the saturation mode, a Schottky diode is commonly placed between the base and collector, such that the anode (input) of the Schottky diode is connected to the base and the cathode (output) of the Schottky diode is connected to the collector.

In operation, a Schottky diode typically has a forward voltage drop of approximately 0.3V-0.4V. Thus, when the base-to-collector junction of a bipolar transistor is clamped by a Schottky diode, the base can never rise more than 0.3V-0.4V above the collector. As a result, the base-to-collector junction of a Schottky-clamped bipolar transistor can never become forward biased and, as a result, can never enter into the saturation mode.

FIG. 1shows a cross-sectional view that illustrates an example of a prior-art Schottky-clamped bipolar transistor100. As shown inFIG. 1, Schottky-clamped bipolar transistor100includes a Schottky diode110and an npn bipolar transistor112. As further shown inFIG. 1, Schottky-clamped bipolar transistor100utilizes a silicon-on-insulator (SOI) wafer which has been conventionally processed to have a bulk region114, an n-single-crystal silicon layer116, and a buried isolation layer118that lies between and electrically isolates single-crystal silicon layer116from bulk region114.

In addition, the SOI wafer has also been conventionally processed to have a deep trench isolation (DTI) structure120and a number of shallow trench isolation (STI) structures122. DTI structure120extends through single-crystal silicon layer116to touch isolation layer118and form a large number of fully-isolated single-crystal silicon regions124, including a fully-isolated n-single-crystal silicon region124-1that supports Schottky diode110and a fully-isolated n-single-crystal silicon region124-2that supports bipolar transistor112. The STI structures122, in turn, include an STI ring122-1that is formed in fully-isolated single-crystal silicon region124-1, and an STI region122-2that is formed in fully-isolated single-crystal silicon region124-2.

As also shown inFIG. 1, Schottky diode110includes an n+ ring130and a p+ guard ring132that are formed in fully-isolated single-crystal silicon region124-1on opposite sides of STI ring122-1. Schottky diode110also includes a metal ring134that touches the top surface of n+ ring130, and a metal region136that touches the top surface of fully-isolated single-crystal silicon region124-1and p+ guard ring132. Metal ring134and metal region136are commonly formed with a silicide, such as platinum silicide.

As additionally shown inFIG. 1, bipolar transistor112includes a collector structure140. Collector structure140, in turn, includes an n+ buried layer142that is formed in fully-isolated single-crystal silicon region124-2to touch the top surface of buried isolation layer118, an n-well144that is formed in fully-isolated single-crystal silicon region124-2to extend down and touch n+ buried layer142, and an n+ collector sinker region146that is formed in fully-isolated single-crystal silicon region124-2to extend down and touch n+ buried layer142.

Bipolar transistor112also includes a p-type silicon germanium (SiGe) base150that touches the top surface of fully-isolated single-crystal silicon region124-2, and an n-type silicon emitter152that touches the top surface of SiGe base150. A SiGe base, which forms a heterojunction bipolar transistor (HBT), is commonly used in high-frequency applications.

As further shown inFIG. 1, Schottky-clamped bipolar transistor100includes a non-conductive layer154that touches the top surfaces of DTI structure120, the STI structures122, metal ring134, metal region136SiGe base150, and emitter152. Transistor100further includes a number of contacts156that extend through non-conductive layer154to make electrical connections with metal ring134, metal region136, a silicided top surface of n+ collector sinker region146, SiGe base150, and silicon emitter152. In addition, as schematically illustrated, metal region136of Schottky diode110is electrically connected to SiGe base150, while metal ring134of Schottky diode110is electrically connected to the silicided top surface of n+ collector sinker region146.

In operation, metal region136functions as the anode of Schottky diode110and silicon region124-1functions as the cathode of Schottky diode110. In addition, n+ ring130functions as the cathode contact, while p+ guard ring132reduces the leakage current. As a result, when the voltage applied to SiGe base150, and thereby metal region136, rises above the voltage applied to n+ collector sinker region146by approximately 0.3V-0.4V, a current flows from SiGe base150to metal region136to n+ ring130to n+ collector sinker region146. On the other hand, when the voltage applied to metal region136falls below the voltage applied to n+ collector sinker region146, substantially no current flows from n+ ring130to metal region136.

One of the drawbacks of Schottky-clamped bipolar transistor100is that DTI structure120significantly limits the lateral dissipation of heat, which limits the heat that can be generated by bipolar transistor112which, in turn, limits the operation of bipolar transistor112. Heat is produced when current flows through a bipolar transistor. This type of heating, which is known as self heating, increases as the density of the current increases.

As bipolar transistors are scaled downward, the density of the current flowing through the transistors increases, which produces increasing levels of heat. Thus, as the transistors are scaled downward and the levels of heat rise through self heating, the inability to significantly dissipate heat laterally through DTI structure120affects the conductivity of the transistor and limits the safe operating area of the transistor.

One approach to reducing self heating is to increase the length L shown inFIG. 1, which is the distance from the edge of the shallow portion of the DTI structure120that lies below SiGe base150to the edge of the deep portion of the same DTI structure120.FIG. 2shows a prior-art graph200that illustrates a collector-emitter voltage V versus a collector current A for different values of the length L. As shown inFIG. 2, the upper portion of the collector current curve can be substantially improved (flattened out) by increasing the length L from 0.25 μm to 5.00 μm.

Although the collector current curve can be substantially improved by increasing the length L from 0.25 μm to 5.00 μm, increasing the length L increases the area or footprint of a fully-isolated single-crystal silicon region, such as fully-isolated single-crystal silicon region124-2. However, increasing the footprint of fully-isolated single-crystal silicon region124-2is not a realistic solution when the goal is to reduce the footprints of the devices and the die, which includes reducing the footprint of the fully-isolated single-crystal silicon regions. Thus, there is a need for an approach to reducing the self heating experienced by a SiGe bipolar transistor in a fully-isolated single-crystal silicon region.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3shows a cross-sectional view that illustrates an example of a Schottky-clamped bipolar transistor300in accordance with the present invention. As described in greater detail below, Schottky-clamped bipolar transistor300reduces self heating by forming a Schottky structure in the same fully-isolated single-crystal silicon region as the bipolar transistor.

As shown inFIG. 3, Schottky-clamped bipolar transistor300includes a Schottky structure310and an npn bipolar transistor312. As further shown inFIG. 3, Schottky-clamped bipolar transistor300utilizes a silicon-on-insulator (SOI) wafer which has been conventionally processed to have a bulk region314, an n-single-crystal silicon layer316, and a buried isolation layer318that lies between and electrically isolates single-crystal silicon layer316from bulk region314.

In addition, the SOI wafer has also been conventionally processed to have a deep trench isolation (DTI) structure320and a number of shallow trench isolation (STI) structures322. DTI structure320extends completely through n-single-crystal silicon layer316to touch isolation layer318and form a large number of n-single-crystal silicon regions324that touch isolation layer318. (One single-crystal silicon region324and a small portion of a second are shown for clarity.)

Each single-crystal silicon region324is horizontally spaced apart and horizontally electrically isolated from each other single-crystal silicon region by DTI structure320. Each single-crystal silicon region324is vertically spaced apart and vertically electrically isolated from bulk region314by isolation layer318. Thus, each n-single-crystal silicon region324is fully laterally electrically isolated from each other n-single-crystal silicon region324.

In the present invention, the n-single-crystal silicon regions324include an n-single-crystal silicon region324-1that supports both Schottky structure310and bipolar transistor312. The STI structures322, in turn, include an STI region322-1and an STI region322-2that are both formed in single-crystal silicon region324-1. As shown, the bottom surfaces of the STI regions322-1and322-2are spaced apart from a top surface of isolation layer318.

As also shown inFIG. 3, Schottky structure310includes a p+ guard ring332that is formed in n-single-crystal silicon region324-1, and a metal region336that touches the top surface of n-single-crystal silicon region324-1and p+ guard ring332. Metal region336can be formed with a silicide, such as platinum silicide. The portion of n-single-crystal silicon region324-1that is touched by metal region336has a dopant concentration that is less than the dopant concentration of buried layer342.

As additionally shown inFIG. 3, bipolar transistor312includes a collector structure340. Collector structure340, in turn, includes an n+ buried layer342that is formed in single-crystal silicon region324-1to touch the top surface of buried isolation layer318, an n-well344that is formed in single-crystal silicon region324-1to extend down from the top surface of single-crystal silicon region324-1and touch n+ buried layer342, and an n+ collector sinker region346that is formed in single-crystal silicon region324-1to extend down and touch n+ buried layer342and n-well344. In the present example, n+ buried layer342lies directly vertically below metal region336, and lies below and spaced apart from the bottom surfaces of the STI regions322-1and322-2.

As further shown inFIG. 3, bipolar transistor312additionally includes a p-type silicon germanium (SiGe) base350that touches a portion of the top surface of n-single-crystal silicon region324-1and the top surfaces of the STI regions322-1and322-2, and an n+ silicon emitter352that touches the top surface of SiGe base350.

Schottky-clamped bipolar transistor300also includes a non-conductive layer354that touches the top surfaces of DTI structure320, the STI structures322, metal region336, SiGe base350, and emitter352. Transistor300further includes a number of contacts356that extend through non-conductive layer354to make electrical connections with metal region336, a silicided top surface of n+ collector sinker region346, SiGe base350, and silicon emitter352. In addition, as schematically illustrated, metal region336of Schottky diode310is electrically connected to SiGe base350.

In operation, metal region336functions as the anode of a Schottky diode, while the portion of n-single-crystal silicon region324-1that touches and lies directly vertically below metal region336function as the cathode of the Schottky diode. In addition, n+ buried layer342and n+ collector sinker region346of collector structure340function as the cathode collector of the Schottky diode. Further, p+ guard ring332reduces the leakage current.

As a result, when the voltage applied to SiGe base350, and thereby metal region336, rises above the voltage applied to n+ collector sinker region346by approximately 0.3V-0.4V, a current flows from SiGe base350to metal region336to n-single-crystal silicon region324-1to n+ buried layer342and to n+ collector sinker region346. On the other hand, when the voltage applied to metal region336falls below the voltage applied to n+ collector sinker region346, substantially no current flows from n+ collector sinker region346to metal region336.

One of the advantages of Schottky-clamped bipolar transistor300is that since Schottky structure310and bipolar transistor312are formed in the same fully-isolated single-crystal silicon region324-1, the effective length L is significantly increased. Significantly increasing the effective length L, in turn, significantly reduces the self heating experienced by a SiGe bipolar transistor in a fully-isolated single-crystal silicon region.

FIG. 4shows a cross-sectional view that illustrates an example of a Schottky-clamped bipolar transistor400in accordance with an alternate embodiment of the present invention. Schottky-clamped bipolar transistor400is similar to Schottky-clamped bipolar transistor300and, as a result, utilizes the same reference numerals to designate the elements that are common to both devices.

As shown inFIG. 4, Schottky-clamped bipolar transistor400differs from Schottky-clamped bipolar transistor300in that Schottky-clamped bipolar transistor400has two Schottky structures, Schottky structure310and Schottky structure410; and an STI region322-11that separates the structures310and410. STI region322-11has a bottom surface that is spaced apart from the top surface of isolation layer318and, in the present example, from the top surface of n+ buried layer342. Schottky structures310and410are identical and, as a result, utilize the same reference numerals. As a result, in the present example, metal region336of Schottky structure410also lies directly vertically over a portion of n+ buried layer342.

Schottky structures310and410share a common cathode collector, i.e., n+ buried layer342and n+ collector sinker region346of collector structure340. However, while metal region336of Schottky structure310, which is connected to SiGe base150, functions as the anode of a Schottky diode, metal region336of Schottky structure410functions as the anode of another Schottky diode which can be connected to another device, thereby providing support for additional types of circuits and further improving the effective length L (which further reduces the self heating). For example, the anode of Schottky structure410, as well as the anodes of any additional Schottky structures410, can be biased at different reverse bias conditions to support varactor and other applications.

Schottky-clamped bipolar transistors300and400can be formed by modifying conventional fabrication processes. For example, single-crystal silicon region324-1is formed in the same manner that the single-crystal silicon regions124are formed, except that single-crystal silicon region324-1is larger.

Similarly, the STI regions322-1and322-2are formed at the same time that STI ring122-1and STI region122-2are formed, except that STI region322-1is formed as a strip in the same single-crystal silicon region324-1that STI region322-2is formed. Further, transistor312is formed in the same manner that transistor112is formed.

In addition, Schottky structures310and410are formed in the same manner that Schottky diode110was formed, except that the steps required to form n+ cathode contact ring130, metal ring134, the contacts156connected to metal ring134, and the metal trace connecting the contacts156to n+ collector sinker region146have been eliminated.

It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.