Physical Devices Used to Construct Memories

1.  Semiconductor Flip-flop

      Semiconductor devices, magnetic surfaces and polycarbonate layers on glass patters are used to fabricate memory cells. The device used for making semiconductor memory cells is a semiconductor switch in Figure 1. The semiconductor device is modelled as a controlled switch. Referring to figure 1 .a. if a voltage 0 is applied to the point X, then the ”gate” g is open and the voltage at y equals V as no current flows from A to B. If a voltage V is applied to the point X, then the gate g closes. The path between Y and B is closed by the gate. As B is connected to ground (0, V), the voltage at Y will be 0 V. The input-output relationship of this device is Table 1 .a.  If we call the voltage V a binary 1 and 0 V a binary 0, then Tables 1. B. is obtained. Table 1.b is obtained. This table completely defines the logical behavior of the controlled switch.

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X	Y
0	V
V	0

X	Y
0	1
1	0

(b)

(a)     

Table 1. The operation of the Controlled Switch

In Figure 2. Two of these controlled switches are controlled switches are connected. The two switches are labelled T1 and T2. Observe that this configuration of switches can be in any one of four possible states. These states are shown in the first two columns of Table 2. Of these four states, g1 and g2 cannot open together. For if g1 is open, the voltage at Y1 will be v, which will make X2 also equal to V and thus close g2. Similarly, g1 and g2 cannot be closed together because if g1 is closed and open is possible. In this case, Y1 will be 0 as g1 is closed and X2 which is equal to Y1 will also be 0, which will open g2. This state is known as a stable state as circuit will continue to remain in this state as long as it is not distributed by external voltage. The state g1 open and g2 closed is also stable. In this case, g1 open will make Y1 equal to V, which in turn makes X2 go to V, which will close g2. This circuit which has two stable states can be used as a memory cells. One of the stable states may be used to represent the storage of a binary 1 and the other a binary 0. This circuit is known as a flip- flop. Table 2 summarizes the above discussion.

A flip –flop may be made to store a 1 or a 0 by using an external input voltage. Referring to Figure 2. If the input voltage is made V for a short period, the gate g1 will close. The voltage Y1 will become 0 and it will open gate g2. The voltage Y2 will equal V and the flip-flop will go to the 1 state. If the input voltage is made 0 for a short period, g1 will and g2 will close. Y2 will become 0 and flip –flop will go to the 0 state. The functioning of this flip-flop with input is summarized in Table 3.

Write and read controls may be introduced by two controlled switches which close the input and output paths as shown in figure 3.a. In Figure 3 b. The memory cell with the controlled switches (shown within dotted lines in Figures in Figure 3 a.) is shown as a memory cell of which was used in the logical description.

The memory cell we have illustrated is a flip-flop. The advantage of a flip-flop is that read out from it is non-destructive. It is also relatively fast. Another electronic memory cell which used widely is a capacitor shown in Figure below :

When a voltage V corresponding to a 1 is applied to the input line and the write switch is closed. C gets charged to voltage V. When a read- signal is applied to the, switch 2 closes and a voltage V (which corresponds to a 1) is sensed at the output. If 0 V is applied to the output. If 0 V is applied to the input, the capacitor C does not get charged and the output when read is 0. The major advantages of a capacitor memory cell are: (I) it is inexpensive and (ii) it is compact. Thus, large memories can be built at reasonable cost. The main memories of most computers use this technology for memories.  They are called DRAM (Dynamic Random Access Memory),  whereas memories built using flip-flops are called SRAM (Static Random Access Memory). The main disadvantages of capacitors as memory cell: (i) capacitors tend to discharge gradually. Thus, the contents of all the capacitors in memory require refreshing (i.e., the original values in them should be rewritten) periodically (normally after each read cycle). (ii) The read out destructive; that is, the contents is lost after it is read, requiring rewriting after reading. Thus, DRAMs are slower than SRAM. As they are cheaper, they are widely used as main memories of computers.

1.  Magnetic Surface Recording

The physical device which realizes the logical memory cell model of is a magnetic recording surface. A magnetic field is created in the gap in a magnetic head by passing a current though a coil wound on the head. The field may be created in one of two directions, depending on the direction of current in the coil surrounding the electromagnet. A plastic surface coated with a ferromagnetic material is placed below the head. The surface is magnetized in either of two directions, depending on the magnetic field across the recording head. This is illustrated in figure 1. If a portion of a surface magnetized in the left- to-right direction is assumed to represent a 1, and the right-to-left direction assumed to be 0, then the strips of magnetized surface may be assumed as storing 1s and 0s as shown in Figure 1.

In order to read the bits stored on a magnetic surface, the surface is moved at a high speed below the magnetic head. The magnetized surface acts as a tiny magnet and induces a voltage in the coil wound on the head (Faraday’s law of induction). The orientation of magnetization of a 0 is opposite to that of a 1. Thus, the polarity of the voltage induced when a 0 moves below it (Figure 2). Using amplifier which defects the polarity of the induced voltage, 1s and 0s are read.

In the method described above, both writing and reading are done by the same head. The reading method is called inductive read out. Using the same head for reading and writing has the disadvantage of requiring a larger number of windings on the head for reading the faint magnetic field produced by the tiny magnetic spot recorded on the on the recording surface. This increases the inductance of the head coil which leads to reduced read/write speed. Magnetic head designers thus looked for a different reading mechanism. This led to the development of magneto- resistive reading head. In this technology, an inductive head is used for writing and a separate magneto-resistive head for reading. A magneto-resistive material has the property that its resistance becomes low when a magnet with say S->N alignment is placed near it and high when a magnet of opposite alignment N<-S is placed near it made using this material. A constant current I is passed through the head (see Figure 3). This current value changes depending on the polarity of the polarity of the magnetic spot below the head. When a 1 (S->N alignment) moves below the head, the resistance of the head reduces and consequently the current through the head increases. When a 0 (S<- N alignment) moves below the head, the resistance of the increases. When a 0 (N<-S alignment) moves below the head, the resistance of the head increases and the head current decreases (see Figure 6. )The increase or decrease of the head current is detected by a sense amplifier and is interpreted as either a 1 or a 0. The magneto-resistive head (MR head) technology was first introduced by IBM in early 1990s and is now being adopted by other manufacturers of magnetic surface. Bit density with MR heads is around 150 Mb/ cm^2 compared to 30 Mb/cm^2 with earlier inductive head. Recently, IBM announced Giant Magneto- resistive heads which give packing density of Gb/cm62. The packing density is continuous increasing every Year.

Perpendicular Recording

We have illustrated magnetic recording of a ferromagnetic coated surface which has a plastic base (see Figure 9.). With this arrangement of write-head, the magnetic dipoles are aligned horizontally.

In the method described above, both writing and reading are done by the same head. The reading method is called inductive read out. Using the same head for reading and writing has the disadvantage of requiring a larger number of windings on the head for reading the faint magnetic field produced by the tiny magnetic spot recorded on the on the recording surface. This increases the inductance of the head coil which leads to reduced read/write speed. Magnetic head designers thus looked for a different reading mechanism. This led to the development of magneto- resistive reading head. In this technology, an inductive head is used for writing and a separate magneto-resistive head for reading. A magneto-resistive material has the property that its resistance becomes low when a magnet with say S->N alignment is placed near it and high when a magnet of opposite alignment N<-S is placed near it made using this material. A constant current I is passed through the head (see Figure 3). This current value changes depending on the polarity of the polarity of the magnetic spot below the head. When a 1 (S->N alignment) moves below the head, the resistance of the head reduces and consequently the current through the head increases. When a 0 (S<- N alignment) moves below the head, the resistance of the increases. When a 0 (N<-S alignment) moves below the head, the resistance of the head increases and the head current decreases (see Figure 6. )The increase or decrease of the head current is detected by a sense amplifier and is interpreted as either a 1 or a 0. The magneto-resistive head (MR head) technology was first introduced by IBM in early 1990s and is now being adopted by other manufacturers of magnetic surface. Bit density with MR heads is around 150 Mb/ cm^2 compared to 30 Mb/cm^2 with earlier inductive head. Recently, IBM announced Giant Magneto- resistive heads which give packing density of Gb/cm62. The packing density is continuous increasing every Year.

Perpendicular Recording

We have illustrated magnetic recording of a ferromagnetic coated surface which has a plastic base (see Figure 9.). With this arrangement of write-head, the magnetic dipoles are aligned horizontally.

The two most popular magnetic surface recording devices used currently are hard disk and magnetic tape. Earlier floppy disks made with flexible magnetic tape material enclosed in an envelope were used for storage. They are now obsolete and not available in the market.