Chapter 2 Theoretical Background and Device Fabrication
2.2 Dielectric characteristics …
From Figure 2.4(a), we can get the different breakdown voltage corresponding to different thickness of silicon oxide. When the operating voltage is 1V with thickness of bottom dielectric is 50 nm, the leakage current level is 10-3mA/cm2. When the operating voltage is 1V the leakage with thickness of top dielectric is 30 nm, the leakage current level is 10-2 to 10-3mA/cm2. The current of transistor can be low off depending on these measurement results.
Figure 2.4(b) shows comparison the characteristics of silicon oxide with/without Polystyrene Spheres. From Figure 2.4(b), the leakage current of silicon oxide significantly increase after used Polystyrene spheres as mask. The reason is Polystyrene Spheres occupied the location of silicon oxide (SIO). It is inevitable in the process of Polystyrene Spheres.
Au(400A) S i O (5 00A)Al (400A) S i O (3 00A) Organic Al (5 00A)
Organic
2 000A
Hole size:2000A
Au(400A) S i O (5 00A)Al (400A) S i O (3 00A) Organic Al (5 00A)
Organic
2 000A
Hole size:2000A
Chapter 2
Figure 2.1 The structure of space-charge-limited transistor.
Figure 2.2 The process flow of the vertical transistor proposed in this work.
(a) (b)
(c) (d)
(a) (b)
(c) (d)
Figure 2.3(a) The structure of polystyrene spheres.(b) The polystyrene spheres on the Au film surface.(c) The polystyrene spheres are removed by an adhesive tape (Scotch, 3M) without damage to the metal. (d) The gold surface without the boiling IPA treatment.
-1.0 -0.5 0.0 0.5 1.0 10-6
10-5 10-4 10-3 10-2 10-1 100 101
E(MV/cm)
J(mA/cm
2 )
with Polystyrene Spheres without Polystyrene Spheres
-1.0 -0.5 0.0 0.5 1.0
10-4 10-3 10-2 10-1
J(mA/cm
2 )
E(MV/cm)
(a)
(b)
Figure 2.4(a) The characteristics of silicon oxide with Polystyrene Spheres. (b) Comparison the characteristics of silicon oxide with/without Polystyrene Spheres.
Chapter 3
Result and Discussion
3.1 Transfer characteristic of SCLT
3.1.1 P3HT-based space-charge-limited transistor
P3HT-based SCLT was fabricated based on chapter 2. The characteristics of P3HT-based SCLT with opening diameters of 2000 Å on the top insulator (SIO) are shown in Figure 3.1 (a). In P3HT-based SCLT the carriers in the Al grid are blocked by the 30-nm silicon oxide and 14-nm P3HT between Al (base) and Al (collector).
The grid current is the reverse current of the Al/SIO/P3HT/Al diode which is almost zero. The current gain is an important value to be maximized. As shown in Figure 3.1 (a) and Figure 3.1 (b), the grid current density (JG) of P3HT-based devices is in the order of 10-5 mA/cm2 which is much smaller than JC. The current gain which is defined JC/JGis as large as 104. The collector current (JC) for fixed collector voltage is modulated by the grid voltage (VG) and the Au (emitter) is the common ground.The positive grid voltage VG, is used to introduce energy barrier for holes at the openings, and the off current can be reduced by increasing VG until a large leakage current between the grid and collector occurs. Figure 3.1(a) shows the on/off ratio of JC is 24310 at VC = – 1 V for transistors with opening diameters of 2000 Å on the top insulator and the highestJC output is 1.331mA/cm2in for device dimension is 1 mm2. The total output current can be scaled up by using a larger area in the same condition.
The device characteristics in double logarithmic scale with fixed VG are shown in Figure 3.2 for tracing the signature of SCLC. Three regions belonging to ohmic, trap filling and SCLC can be distinguished [45]. The slope of log I − logV is equal to 1 for ohmic conduction, while the slope is equal to 2 for SCLC. The dashed lines with slope equal to 1 and 2 are drawn in the Figure.3.2 for indication. Indeed, the current
follows the SCLC once the barrier at the opening is suppressed by a sufficiently negative VC. There is always a small ohmic current at the low voltage. The polymer diode has a turn-on voltage where the current varies from a small leakage ohmic current to a quadratic SCLC current. The turn-on voltage is determined by both the level of the leakage and the difference between the work functions of the cathode and anode. Figure3.2 is shown the P3HT-based emitter-collector diode and the SCLC is about 3V.
3.1.2 Pentacene-based space-charge-limited transistor
Pentacene-based SCLT was fabricated based on chapter 2. The characteristics of Pentacene-based SCLT with opening diameters of 2000 Å on the top insulator are shown in Figure 3.3(a). In Pentacene-based SCLT the carriers in the Al grid are blocked by the 30-nm silicon oxide and 60-nm Pentacene between Al (base) and Al (collector). The grid current is the reverse current of the Al/SIO/Pentacene/Al diode which is small but not zero. The current gain is therefore an important value to be maximized. As shown in Figure 3.3(a) and Figure 3.3(b), the grid current density JG
of Pentacene-based devices is in the order of 10-3mA/cm2 which is much smaller than JC. The current gain which is defined JC/JGis as large as 103. The collector current (JC) for fixed collector voltage is modulated by the grid voltage (VG) and the Au (emitter) is the common ground.The positive VG is used to introduce energy barrier for holes at the openings, and the off current can be reduced by increasing VGuntil a large leakage current between the grid and collector occurs. Figure 3.3(a) shows the on/off ratio of JC is 390 at VC = – 1 V for transistors with opening diameters of 2000 Å on the top insulator and the highestJC output is 0.2693mA/cm2for device dimension of 1 mm2. The device characteristics in double logarithmic scale with fixed VG are shown in Figure 3.4 for tracing the signature of SCLC. The dashed lines with slope equal to 1 and 2 are drawn in Figure 3.4 for indication. Indeed, the current follows the SCLC
once the barrier at the opening is suppressed by a sufficient negative VC. There is always a small ohmic current at low voltage. The polymer diode has a turn-on voltage where the current varies from a small leakage ohmic current to a quadratic SCLC current. The turn-on voltage is determined by both the level of the leakage and the difference between the work functions of the cathode and anode. Figure3.4 is shown the P3HT-based emitter-collector diode and the SCLC is about 4V.
3.1.3 C60-based space-charge-limited transistor
C60-based SCLT was fabricated based on chapter 2. The characteristics of C60-based SCLT with opening diameters of 2000 Å on the top insulator are shown in the Figure 3.5(a). In C60-based SCLT the carriers in the Al grid are blocked by the 30-nm silicon oxide and 80-nm C60 between Al (base) and Al (emitter). The grid current of Al/SIO/C60/Al diode is almost zero. Therefore, the current gain is an important value to be maximized. As shown in Figure 3.5(a) and Figure 3.5(b), the grid current density (JG) of C60-based devices is in the order of 10-3 which is smaller than JC. The current gainJC/JGis as 102. The collector current (JC) for fixed collector voltage is modulated by the grid voltage (VG) and the Al (emitter) is the common ground. The negative VG is used to introduce energy barrier for electrons at the openings, and the off current can be reduced by decreasing VG. Figure 3.5(a) shows the on/off ratio of JCis 589 at VC= 1 V for transistors with opening diameters of 2000 Å on the top insulator and the highestJC output is 0.411mA/cm2for device dimension area of 1 mm2. The total output current can be scaled up by using a larger area. The device characteristics in double logarithmic scale with fixed VG are shown in the Figure 3.6 for tracing the signature of SCLC. The dashed lines with slope equal to 1 and 2 are drawn in the Figure 3.6 for indication. Indeed, the current follows the SCLC once the barrier at the opening is suppressed by a positive enough VC. At low voltage there is always a small ohmic current. The polymer diode has a turn-on voltage where
the current switches from a small leakage ohmic current into a quadratic SCLC current. The turn-on voltage is determined by both the level of the leakage and the difference between the work functions of the cathode and anode. Figure3.6 is shown the P3HT-based emitter-collector diode and the SCLC is about 2V.