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Minimum Size Cross Point Bipolar ROS Cell

IP.com Disclosure Number: IPCOM000108672D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 4 page(s) / 119K

Publishing Venue

IBM

Related People

Arienzo, M: AUTHOR [+2]

Abstract

Disclosed is a cross-point configured bipolar Read-Only-Store (ROS) cell which offers the minimum cell size achievable with a poly-base (or epi-base) transistor.

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This is the abbreviated version, containing approximately 52% of the total text.

Minimum Size Cross Point Bipolar ROS Cell

       Disclosed is a cross-point configured bipolar
Read-Only-Store (ROS) cell which offers the minimum cell size
achievable with a poly-base (or epi-base) transistor.

      Pure bipolar ROS (or Read-Only-Memory (ROM)) is crucial for the
control store applications in main-frame computer systems.  Small
cell size for the ROS is essential for high-density,
high-performance, as well as low-power applications.

      Fig. 1 shows the schematics of the single-transistor ROS cells
together with simplified bit-line drivers and sense amplifier
circuits.  The state of the cell is determined by whether the emitter
of the cell transistor is connected to the bit-line.  During the Read
operation, the selected word-line is pulled 'high' and the bit-line
current source Ics is steered to the selected bit-line via the column
(bit-line)-select circuit.  Depending on the state of the selected
cell, the current will either flow through the selected cell
transistor (when its emitter is connected to the bit-line) or through
the reference transistor Tref (when the emitter of the selected cell
transistor is 'open') which has its base connected to a reference
voltage Vref placed one diode drop above the middle of the 'high' and
'low' states of the word-line voltage.

      Noticing that all the ROS cell can share a common collector bed
(buried-layer) and assuming a poly- (or epi-) base transistor, one
can arrive at the layout configuration shown in Fig. 2. Note that in
Fig.  2, the X-dimension of the cell is essentially limited by the
width of the emitter-poly plus their spacing, i.e.,
Xcell  = (E-poly Width) + (E-poly to E-poly Spacing) = (Emitter width
+ 2 x E-poly coverage of Emitter) + (E-poly to E-poly Spacing)

      This is the minimum dimension one can achieve with a non-butted
poly-emitter transistor.  In the Y-direction, however, the dimension
is much larger due to the necessity to accommodate the base contact.
Hence, the Y-dimension of the cell i...