Schottky Diode Guard Ring for Parasitic Transistor Suppression
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Solution Overview
Problem
Conventional Schottky diodes suffer from low breakdown voltages and poor leakage characteristics due to large edge electric fields, which are exacerbated by parasitic bipolar junction transistors activated by guard rings in ohmic contact, degrading both forward and reverse bias performance.
Innovation Solution
A silicon substrate with an insulating element and doped well regions, where a conductive layer forms both a primary n-type Schottky diode and a p-type guard ring Schottky diode in parallel, with the guard ring being reverse biased during forward bias to inhibit parasitic transistor action and forward biased during reverse bias to reduce edge electric fields, thereby enhancing breakdown voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a guard ring in ohmic contact is used to reduce edge electric fields, then reverse breakdown characteristics are improved, but parasitic bipolar junction transistors are activated which amplify small emitter-base currents to produce large substrate currents, degrading forward characteristics
Solution Approach 1:
An n-type intermediate layer is introduced between the metal contact and the p-type guard ring. This intermediate layer forms a rectifying contact with the guard ring, preventing direct ohmic contact and thereby blocking the activation of parasitic bipolar junction transistors while still allowing the guard ring to perform its field relief function
Solution Approach 2:
The contact structure is made non-uniform: the central region maintains direct metal-to-semiconductor contact for primary diode operation, while the peripheral region uses an intermediate layer to create rectifying contact with the guard ring. This local differentiation allows simultaneous optimization of both forward and reverse characteristics
2Reliability
If the edge of the Schottky contact layer is placed on a field relief guard ring, then reverse breakdown voltage is improved, but leakage characteristics deteriorate due to parasitic transistor action
Solution Approach 1:
The n-type intermediate layer acts as a mediator that forms a rectifying barrier with the p-type guard ring. This barrier prevents the formation of parasitic bipolar transistors that would otherwise cause excessive leakage current, while preserving the voltage breakdown enhancement provided by the guard ring structure
Solution Approach 2:
The contact structure employs a composite configuration with multiple semiconductor regions (n-type intermediate layer and p-type guard ring) with different electrical properties. This composite structure creates both rectifying and ohmic contacts in specific regions, achieving superior electrical characteristics compared to single-material contacts
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
The solution effectively suppresses parasitic bipolar transistor action, maintaining good Schottky diode performance by increasing breakdown voltage and reducing substrate currents, while maintaining expected forward current characteristics.
Implementation Method 1
Conduction in the Schottky diode is controlled by thermionic emission of majority carriers over the barrier
Implementation Method 2
large electric fields at the edges of the contact layer or rectifying barrier can lead to relatively low breakdown voltages
Data Source
AI summary
In one general aspect, an apparatus includes a metal or metal silicide contact layer disposed on an n-well region of a semiconductor substrate to form a primary Schottky diode. The apparatus includes a p-well guard ring region of the semiconductor substrate abutting the primary Schottky diode. The metal silicide contact layer has a perimeter portion extending over the p-well guard ring region of the semiconductor substrate and the p-well guard ring region has a doping level establishing a work function difference relative to the perimeter portion of the metal silicide contact layer to form a guard ring Schottky diode. The guard ring Schottky diode is in series with a p-n junction interface of the p-well region and the n-well region and the guard ring Schottky diode has a polarity opposite to that of the primary Schottky diode.