Vertical bipolar transistors are widely employed in semiconductor devices, because they offer high operating speeds and high current gain. A vertical bipolar transistor can be fabricated in a small substrate area because the elements of the bipolar transistor can be formed as layers of semiconductor material, thereby taking advantage of the vertical dimensions of a semiconductor substrate. The ability to fabricate many bipolar devices in a small surface area makes the vertical bipolar transistor an ideal circuit element in combination with metal-oxide-semiconductor (MOS) devices. Vertical bipolar transistors are widely used in switching devices and as current sources in complex bipolar-complementary-MOS (BiCMOS) integrated circuits.
Vertical bipolar transistors can be fabricated to have extremely high current gain by fabricating a very thin base region in the device. High output current is achieved by a small variation in voltage of the base region. By varying the base voltage, the vertical bipolar transistor is set to an on-state, an intermediate state, or an off-state. Thus by small changes in base voltage, the vertical bipolar transistor produces either a large amount of electrical current output, an intermediate amount of current, or alternatively, under ideal conditions, no output current at all. The on/off characteristics of the vertical bipolar device make this device suitable for application in digital devices, and for coupling to other digital circuits using MOS devices.
While the vertical bipolar transistor provides a high output current in digital applications, complex semiconductor devices also require the fabrication of resistive elements for the regulation of electrical current in the device. Typically, resistive elements are provided by forming regions of semiconductor material having a controlled resistivity ranging from a high near intrinsic resistivity to very low resistivity. Additionally, resistive devices can also be created through fabrication of MOS load transistors. The load transistors resist the flow of current when proper voltage levels are applied to the terminals of the load transistor to place the transistor in an off-state. In the case of either a load transistor, or a layer of resistive semiconductor material, a single level of resistance is provided. Neither the MOS load transistor nor the semiconductor resistor is capable of assuming a uniformly variable resistance value. Therefore, when coupling conventional resistive elements to a high current source, such as a vertical bipolar transistor, only a single resistive level can be attained.
Although circuit elements, such as a vertical bipolar transistor and an MOS transistor conventionally, operate as switches in either an on-state or an off-state, the need for variable current levels often arises in the design of complex semiconductor devices. Numerous types of semiconductor devices benefit from variable resistance current sources, such as multi-state logic devices, operational amplifiers, analog-digital converters, and digital signal processing devices, and the like. Accordingly, further development is necessary to enable the fabrication of vertical bipolar devices having voltage controlled variable current output.