PNP Transistor Base-Diode Circuit for Normally-On Logic Switching
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Solution Overview
Problem
The use of PNP bipolar junction transistors as 'normally on' transistors in digital logic circuits is impractical due to the large temperature coefficient of resistors and the physical size requirements for current limiting, making it unsuitable for integrated circuits.
Innovation Solution
A circuit design utilizing a first diode for current limiting, which is smaller and more stable across temperatures, and a second diode for non-metallic connection to the transistor base, allowing for polysilicon conductive traces and reducing parasitic effects, enabling the creation of logic gates with reduced transistor count.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large resistor is used for current limiting in the base circuit, then the transistor can be switched off with maximum current from the current source, but the resistor becomes physically large and unsuitable for integrated circuits
Solution Approach 1:
The patent changes the component type from a resistor to a diode, fundamentally altering the electrical characteristics. The diode provides current limiting through its forward voltage drop characteristic rather than resistance, enabling compact integration while maintaining the ability to switch off the transistor with available current
2Reliability
If a large resistor is used for current limiting, then sufficient current limiting is achieved, but the circuit becomes unsuitable for integrated circuits due to physical size constraints
Solution Approach 1:
The invention changes from resistive current limiting to diode-based current limiting, where the diode's forward voltage drop characteristic provides the current limiting function. This parameter change enables compact physical dimensions suitable for integrated circuit fabrication processes
Solution Approach 2:
The patent replaces the resistive element (which would require large physical dimensions) with a diode element that achieves the same current limiting function through semiconductor physics, specifically the forward bias voltage-drop characteristic, enabling integration
3Reliability
If a resistor with large temperature coefficient is used, then current limiting is provided, but operational stability over wide temperature range deteriorates
Solution Approach 1:
The patent changes from using a resistor with temperature-dependent resistance to a diode with temperature-compensated forward voltage drop characteristics. By selecting a zener diode with specific breakdown voltage, the circuit achieves operational stability across wide temperature ranges while maintaining current limiting functionality
4Object-generated harmful factors
If polysilicon conductive traces are used to connect to the transistor base, then parasitic effects between neighbouring transistors are reduced, but a non-metallic connection method is required
Solution Approach 1:
The patent replaces metal conductive traces with polysilicon conductive traces for connecting to the transistor base. This substitution reduces parasitic effects between neighbouring transistors while still providing adequate electrical connection through the polysilicon material's conductive properties
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution provides operational stability over a wide temperature range, reduces parasitic effects, and increases transistor density in integrated circuits, allowing for the implementation of logic gates with fewer transistors, such as NOR gates with a single transistor, and improves switching speed and power efficiency.
Implementation Method 1
The first diode provides the current limiting function of a large resistor of the prior art circuit; however, the first diode, unlike a resistor, is a relatively small electronic component
Implementation Method 2
When reverse biased, current flow through the diode may be a result of a leakage current due to quantum tunnelling
Implementation Method 3
The first diode may be a zener diode. Compared with many other types of diode, zener diodes, when biased below their zener voltage (breakdown voltage), operate reliably compared with other diodes
Data Source
Figure 1a
Figure 1b
Figure 2a
AI summary
A circuit comprising: a transistor, a base of the transistor being switchably connectable to a signal source; a first diode connected between the base of the transistor, and a reference voltage; wherein the circuit arranged such that when the signal source is not connected to the base of the transistor, a voltage applied at an emitter of the transistor causes a current flow through the base of the transistor and through the first diode such that the transistor is in an ON state; the impedance of the signal source is lower than the impedance of the transistor through the emitter and base; the first diode is selected to provide a current limiting function such that when the signal source is connected to the base of the transistor, current flow through the base reduces such that the transistor switches to an OFF state; and in which the circuit comprises a second diode, the second diode comprised from a semiconductor region that provides the base of the transistor and a further semiconductor region in direct contact with the semiconductor region that provides the base of the transistor; and in which the signal source is connectable to the base of the transistor through the second diode.