Enhanced-Reliability Darlington Pair Gain Blocks
A new family of Darlington pair gain blocks that solve a number of potential reliability issues with pre-existing gain blocks.
This new family of gain blocks will reduce field failures, and lead to lower maintenance and warranty costs for systems manufacturers. The enhanced-reliability features are included in the CGB 7000 family without compromising RF performance. On December 22, 1953, US patent No. 2 663 806 was issued to Sidney Darlington. The patent included a number of transistor and resistor combinations, including the well-known configuration now known as the Darlington pair, shown in figure 1.
Fifty years after the original patent was issued, this circuit configuration is still widely used today. Millions of Darlington pair-based gain blocks are used every year, in numerous applications associated with infrastructures such as CATV and wireless, industries such as test and measurement, homeland security and defense markets. For these and virtually all electronic-based industries, A variety of Darlington pair-based gain blocks are available, providing a range of bandwidths, powers, and gains.
Gain Block Advantages and Disadvantages
Gain blocks offer a number of advantages including:
• Useable performance from DC to >10 GHz.
• Good broadband matching into 50Ω
• Tight performance distributions
However, currently available Darlington-pair gain blocks generally have some disadvantages as well:
• Poor ESD resistance, typically <250 V, using the human body model (HBM)
• Poor thermal resistance
• Poor testability.
In the Darlington configuration schematic shown in Figure 1, resistors are connected in parallel with the base-collector and base-emitter junctions of the transistors. The resistors, while essential to proper operation, prevent thorough testing of the transistors. When tested, grossly defective parts are detected, however, less obvious defects may not be detected until they cause field failures.
These drawbacks have led to low levels of reliability and durability for some of the Darlington pair-based gain blocks available today. Despite this, these parts have been designed into a variety of applications requiring high reliability, including military products, cellular base stations, and test and measurement equipment.
The Technology
To address these issues, Celeritek has developed the CGB 7000 series of gain blocks.
The technology behind this development incorporates InGaP HBT technology and offers low thermal resistances. ESD protection circuits on every die in the family provide immunity to ESD pulses of up to 1000 V, using the human body model (HBM), and pulses of up to 2000 V, using the charged device model (CDM).
The die includes extra pads to enhance testability unique to this design allows greatly enhanced testing of each die, allowing the detection of parts with subtle defects that might lead to field failures. Some of the tests enabled by the inclusion of extra pads include:
• Current leakage tests.
• Breakdown voltage tests.
• DC current gain (beta) tests.
• Measurements of base-collector and base-emitter diode forward voltage drops at low current levels.
• Measurements to check for proper functionality of the ESD protection circuitry.In figure 3, a portion of a wafer map is shown. A transition region exists in which many die that pass the standard tests fail the more sensitive tests enabled by the additional pads.
Conclusion
The low thermal resistance, ESD protection, and enhanced testability features on the CGB 7000 family will reduce field failures, and lead to lower maintenance and warranty costs for systems manufacturers using these parts. These enhanced-reliability features are included without compromising RF performance.
Figure 1. Schematic of a Darlington Pair
Celeritek
3236 Scott Boulevard
Santa Clara, CA, 95054
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