Metal-insulator-semiconductor bipolar transistor as a 4F2 vertical RRAM selection device E. Yalon and D. Ritter Department of Electrical Engineering Technion – Israel Institute of Technology Haifa 3200, Israel
[email protected] Abstract— The metal-insulator-semiconductor bipolar transistor detects the injection of minority carriers through a resistive switching element into a semiconductor electrode. We propose that this device may serve as a selection device for bipolar resistive switching elements in large memory crossbar arrays. The selection is accomplished by inspection of the highly nonlinear minority carrier current. The experimental device provides non-linearity of about ~80 mV/decade, but 60 mV/decade is feasible. The device may potentially be confined into the minimal 4F2 footprint. The selection properties of the device are discussed, and compared to those of alternative selection devices suitable for bipolar switching elements.
quantify the non-linearity is the semi-logarithmic V-I slope (i.e. 60 mV/decade for an ideal diode).
Keywords: resistive switching; crossbar array; selection device; bipolar transistor; resistive random access memory (RRAM)
Using MOSFETs and BJTs as selectors is straight-forward. They are highly non-linear (ideally ~60mV/decade), but their drawback is a footprint larger than 4F2 and incompatibility with 3D configurations and BEOL process. The footprint of the “vertical 3D BJT” that was recently reported [7] does not exceed 4F2 yet requires a high quality p-n junction, and is therefore not suitable for 3D configuration and BEOL process. The CRS is composed of two anti-serial bipolar resistive switching elements, one for the data storage and the other for the selection operation. The select RS element is normally in its high resistance state (HRS), except during the read operation of the selected cell. The CRS is therefore highly non-linear (175 mV/decade) that was demonstrated thus far.
I.
INTRODUCTION
Resistive switching (RS) elements located at the junction of a crossbars array achieve the desired 4F2 footprint, where F is the minimum feature size of a technology node. However, a passive crossbar array suffers from an inherent read-disturb problem due to sneak path current [1-3]. A selection device is therefore required to suppress the sneak path current through unselected cells in large arrays [1]. An ideal selection device should maintain the minimal footprint of 4F2, exhibit high nonlinearity, carry high current density, and preferably also be compatible with 3D stack and back-end-of-line (BEOL) technology [1, 2, 4, 5]. Semiconductor p-n junctions satisfy all the above mentioned requirements for unipolar RS elements, including 3D stack compatibility using poly-silicon [6]. Yet, their reverse current is too low for reset operation in scaled bipolar RS devices. Several other types of selection devices were demonstrated for bipolar RRAM arrays [5, 7-10], but none of them fully satisfy the selector specifications. Here, we propose a new type of selection device based upon the detection of injected minority carriers into a semiconductor electrode of a RS element. We have previously referred to this device as a metal-insulator-semiconductor bipolar transistor [11]. This selection device may potentially satisfy all selector specifications of a bipolar RS element. The magnitude of the non-linearity of the selection device determines its on/off ratio, which dictates the maximal array size. The figure of merit used throughout this publication to
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II.
SELECTION DEVICES FOR BIPOLAR RESTIVE SWITCHES
The minority carrier injection selection device, proposed here, is a possible solution for the selection of bipolar RS elements. We therefore briefly review previously proposed selection devices for bipolar RS elements, including planar and vertical MOSFETs [9, 12], bipolar transistors [7, 10, 13], complementary RS device (CRS) [5, 14], and rectifying MIM stacks (referred to as “1S1R” crossbar arrays) [8, 15-18].
III.
THE PROPOSED MINORITY CARRIER INJECTION SELECTION DEVICE
The proposed selection device is based upon the metalinsulator-semiconductor bipolar transistor structure, which is a three terminal device integrating a RS element and a p-n junction. This device was previously used to study the conduction mechanism [11] and filament temperature [19] in RS materials. Here, we suggest that it can be used as a 4F2 vertically integrated selection device. The structure is similar to the vertical BJT selection device [7], but the device physics is fundamentally different. As in the BJT case, the proposed device is suitable for bipolar RS elements. To create the device, the dielectric RS layer is deposited directly on the p-type base layer, rather than on the emitter layer as in the vertical BJT
The switching characteristics of a device composed of a 5 nm thick HfO2 as the RS layer and InGaAs as semiconductor are presented in Fig. 2. The InGaAs device was selected as an example (such transistors were designed in our lab in previous projects). Silicon and poly-silicon are considered for future work. The base current (Ib) shown in Fig. 2 is the majority carrier current, flowing between the bit line (BL) and the word line (WL). The collector current (Ic) is the minority carrier current between the BL and the “read” line. The principle of operation of the proposed selection device can be understood by inspection of Fig. 2. The collector current is generated by minority carriers that are injected by thermionic assisted tunneling through the RS material into the semiconductor [11]. Minority carriers are injected only in the low resistance state (LRS), and therefore any detected collector current is an indication that the RS material is at LRS. Due to the threshold–like exponential dependence of the minority carrier current on the read voltage (Ic in Fig. 2), minority carriers injection is suppressed through elements in the sneak paths. Thus, minority carriers are injected only in the selected device and only if it is at LRS. The high non-linearity of the read current (~80mV/decade) is indicated in Fig. 2. The deviation from the expected 60 mV/decade is due to elevated temperature in the conductive filament [19], and can in principle be avoided by appropriate device design.
10 10
|I| [A]
case. The resulting structure is a single integrated device which includes both the switching element and the selection device. Electrons are injected through the dielectric RS layer into the semiconductor conduction and valence bands. The shorted or backward biased p-n junction detects the minority carrier current, measured at the collector terminal. The majority carrier current is measured at the base terminal. A schematic cross section of the device and an illustration of its proposed implementation as a selection device are shown in Fig. 1. The experimental device details are given in [11].
-4
-6
LRS
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LRS I
C
~80 mv/dec slope
10 10 10
-8
-10
HRS
HRS
-12
-2 -1.5 -1 -0.5
0
0.5
Vb [V]
1
1.5
2
Figure 2. Measured resistive switching characteristics of a single metalinsulator-semiconductor bipolar transistor with 5nm HfO2 layer. Base (holedashed red line) and collector (electron-solid blue line) currents versus baseemitter voltage.Base-collector junction is shorted. Arrows mark bias sweep direction. The non-linearity of the collector (read) current at LRS is indicated.
As mentioned above, the structure of the proposed selection device is very similar to the vertical BJT [7]. The only difference is that the RS material is deposited directly on base layer, rather than on the emitter contact. The proposed device should therefore also achieve the minimal 4F2 footprint, along with the above described excellent non-linearity. The advantage of the device compared to the vertical BJT is that no current gain is required and the demands on the performance of the base-collector p-n junction can be relaxed. This device may therefore be suitable for 3D configurations using poly-silicon diodes [6]. The various selection devices discussed in this paper and their properties are summarized in Table I. We finally note that a drawback of the proposed approach is that although the sneak path does not disturb the read operation, its contribution to excessive power consumption is not eliminated. Additionally, there is no cell selection during write operation. IV.
Figure 1. Schematic cross section of the metal-insulator-semiconductor bipolar transistor and its implementation as an integrated resistive switching element and a selection device. During a read operation, bit line (base) is positively biased with respect to word line. Base-collector junction is reverse biased. Read line detetcts minority carriers if RRAM layer is in LRS.
LRS IB
CONCLUSIONS
A selection device based upon the detection of minority carriers injected through a RS element was proposed for bipolar RRAM crossbar arrays. The device can achieve the 4F2 cell area and is highly non-linear. The demonstrated experimental device was based upon compound III/V semiconductor technology. The feasibility of silicon (front end) and poly-Si based devices is yet to be demonstrated. ACKNOWLEDGMENT We thank Y. Roizin and E. Pikhay of TowerJazz for helpful discussions. E. Yalon gratefully acknowledges the support of The Clore Israel Foundation Scholars Programme and the Russel Berrie Nanotechnology Institute, Technion. Device fabrication was performed at the Micro-Nano Fabrication Unit (MNFU), Technion.
Selection Device MOSFET
TABLE I. SUMMARY OF REPORTED SELECTION DEVICES AND THEIR PROPERTIES Cell area Destructive Non-linearity BEOL\3D readout compatibility >8F2 No >60mV/decade No
[9]
2
No
~80mV/decade
No
[12]
2
5.5F
No
~60mV/decade
No
[3]
2
No
~60mV/decade
No
[7]
2
Yes
175mV/decade
Yes
[8,15-18]
4F (deduced from similarity of structure to the vertical BJT [7])
No
Demonstrated: ~80mV/decade Projected: ~60mV/decade
Yes, using poly-Si diodes [6] (not yet demonstrated)
This work
Vertical GAA MOSFET
5.3F
BJT Vertical BJT
4F
CRS
4F
1S1R Minority carrier detection device
Reference
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