Constant Current Circuit Using Depletion and Enhancement Transistors
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Solution Overview
Problem
Existing constant current circuits in semiconductor devices require a long channel length to achieve minute currents, leading to a large layout area and increased manufacturing complexity due to variations in threshold voltage caused by manufacturing variations.
Innovation Solution
A constant current circuit configuration using a depletion type first transistor and an enhancement type second transistor, where the gate and drain of the second transistor are connected to a reference node, and the source of the second transistor is connected to the source of the first transistor, allowing for a voltage corresponding to the threshold voltage of the second transistor to be applied to the first transistor, reducing its channel length and enabling common production of channel dope layers for threshold control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the channel length of the depletion type transistor is lengthened to obtain a minute current, then the constant current output is improved, but the layout area of the constant current circuit becomes large
Solution Approach 1:
An enhancement type transistor is introduced as an intermediary component to generate a threshold voltage that is applied to the depletion type transistor. This mediator transistor enables the depletion type transistor to operate with a shorter channel length while still achieving the desired minute current output, thereby resolving the contradiction between current precision and layout area.
Solution Approach 2:
The invention changes the operating parameters of the depletion type transistor by applying a gate-source voltage equal to the threshold voltage of the enhancement type transistor. This parameter change allows the depletion type transistor to achieve minute current output with a shorter channel length, thus reducing the layout area while maintaining manufacturing precision.
2Reliability
If the channel length is increased to reduce manufacturing variations, then the reliability is improved, but the layout area increases
Solution Approach 1:
The enhancement type transistor provides a feedback mechanism where its threshold voltage is applied to the depletion type transistor's gate. This feedback loop compensates for manufacturing variations by dynamically adjusting the operating point, thereby improving output current stability without requiring an increased channel length.
Solution Approach 2:
The enhancement type transistor acts as a mediator that compensates for manufacturing variations in the depletion type transistor. By introducing this intermediary component, the system achieves improved reliability through threshold voltage control without the penalty of increased layout area.
3Manufacturing precision
If different mask processes are used for threshold control of each transistor, then the manufacturing precision is improved, but the device complexity increases
Solution Approach 1:
The invention merges the threshold control processes of both transistors into a single common mask process. By designing the transistors with opposite conductivity types, the channel dope layers for both devices can be formed simultaneously using the same mask, thereby reducing device complexity while maintaining manufacturing precision.
Solution Approach 2:
The common mask process serves multiple functions: it controls the threshold voltage of the depletion type transistor and simultaneously controls the threshold voltage of the enhancement type transistor. This universal approach simplifies the production process while maintaining precise threshold control for both devices.
Data Source
AI summary
In an N-channel depletion type first transistor, a gate is connected to a reference node and a drain is connected to a current output node. In a P-channel enhancement type second transistor, a gate and a drain are connected to the reference node and a source is connected to a source of the N-channel depletion type first transistor.