Depletion-Mode N-Channel Organic Field-Effect Transistors Based On

Solid-State Electronics 47 (2003) 1855–1858 www.elsevier.com/locate/sse

Depletion-mode n-channel organic ﬁeld-eﬀect transistors based on NTCDA Mo Zhu, Guirong Liang, Tianhong Cui *, Kody Varahramyan Institute for Micromanufacturing, Louisiana Tech University, P.O. Box 10137, 911 Hergot Avenue, Ruston, LA 71272, USA Received 15 March 2003; accepted 3 April 2003

Abstract The depletion-mode n-channel organic field-effect transistors (OFETs) based on naphthalene-tetracarboxylic-dianhydride (NTCDA) were fabricated and characterized. Electrical characteristics of the OFETs were demonstrated and analyzed under the depletion-mode operation. The mobility of NTCDA is about 0.016 cm2 V1 s1. The threshold voltage is )32 V, and cut off current is 1.76 nA. The on/off ratio extracted from transfer characteristics at saturation 2 region (Vds ¼ 60 V) is 2.25 10 . Ó 2003 Elsevier Ltd. All rights reserved.

Keywords: NTCDA; Organic ﬁeld-eﬀect transistor (OFET); Depletion-mode; Energy level

1. Introduction promising due to its high mobility of 0.06 cm2 V1 s1 [6] and investigated stabilities [7]. Conductive organic materials have been widely in- In this paper, the depletion-mode n-channel organic vestigated for the applications to the areas of micro- field-effect transistors (OFETs) based on NTCDA are electronics and optoelectronics, such as transistors and fabricated and characterized. From the discussions and light emitting diodes [1,2] due to their well-known ad- analyses of charge transport and energy bands, the vantages, such as low cost and flexibility. mechanism and function of this OFET are introduced Currently most of the studies on organic microelec- and demonstrated. tronic/optoelectronic devices are based on p-type materials like pentacene [3], a-sexithiophene (a-6T) [4], etc. Normally p-type organic materials are more stable than 2. Experiments n-type organic materials, and shows higher carrier mobility. The reported highest carrier mobility to our 2.1. Materials knowledge is 3 cm2 V1 s1 in pentacene [5]. Compared to p-type organic materials, n-type organic materials In the fabricated transistors, NTCDA acts as active have much lower mobility, and are more sensitive to the channel material due to its n-type conduction [7]. P-type environments due to the transport mechanism of or- conductive polymer polypyrrole (PPy) performs as the ganic materials. Among several n-type materials, naph- source and the drain in the transistors, showing good thalene-tetracarboxylic-dianhydride (NTCDA) is more characteristics of the p-type conducting polymer [8]. Aluminum is as the electrodes for the source and the drain. Dielectric material is poly(4-vinylphenol) (PVP) dissolved in isoproponal alcohol. All the organic chemicals, undoped NTCDA powder, 20 wt.% PPy * Corresponding author. Tel.: +1-318-257-5122; fax: +1-318- aqueous solution, and PVP solution, are purchased from 257-5104. Aldrich. Fig. 1 shows the molecular structures of E-mail address: [email protected] (T. Cui). NTCDA and PPy.

30.0n

25.0n

Vg=0

20.0n

15.0n Fig. 1. Molecular structures of NTCDA (left) and PPy (right). Vg=-20V 10.0n Vg=-40V

5.0n Vg=-60V

0.0 0 10203040506070

Fig. 3. Id versus Vgs curve of the OFET using NTCDA as the semiconductor and PPy as the gate.

0.00008 I 1/2 -V ,V =20V Fig. 2. Top contact (TOC) device structure. d gs ds

0.00007 2.2. Fabrication 0.00006 Normally, OFETs adopt two kinds of structures, bottom contact (BOC) and top contact (TOC) [9]. Here 0.00005 the TOC structure was employed in the devices, as il- lustrated in Fig. 2. The width of channel is 40 lm, and 0.00004 the length is 200 lm.

As shown in the schematic ﬁgure, the devices are built 0.00003 on the heavily doped silicon wafer, which also acts as the gate electrode. First, a layer of PVP 800 nm thick is spun -60 -50 -40 -30 -20 -10 0 on the wafer. A permittivity of 12.4 is determined. Next, (a) NTCDA 1.5 lm thick is thermally evaporated at the current of 20 amp and the working pressure of 1 107 0.00030 I 1/2 -V ,V = 60V d gs ds Torr. After the evaporation of NTCDA, PPy is depos- 0.00027 ited by spin-coating and Al 200 nm thick is thermally evaporated on top of PPy. Lithography process is used 0.00024 to pattern Al, and RIE (reactive ion etching) process to 0.00021 form the source region and the drain region. 0.00018

0.00015 3. Results 0.00012

NTCDA-based OFETs were characterized in the at- 0.00009 mosphere using KEITHLEY TEST SYSTEM (236 Source Measure Unit). Figs. 3 and 4 plot the electrical 0.00006 output and transfer characteristics, respectively. In Fig. -60 -40 -20 0 20 40 60 3, the drain current (Id) versus the drain voltage (Vds) (b) curve (output characteristics) shows the depletion-mode Fig. 4. Transfer characteristics at V 20 V (top) and V 60 operation. With the decrease of gate voltage, the drain ds ¼ ds ¼ V (bottom). current (Id) correspondently decreases step by step. The drain current (Id) versus the gate voltage (Vgs), transfer characteristics, at linear output region (Vds ¼ 20 V) are of )32 V and the cut oﬀ current of 1.76 nA. The on/oﬀ shown in Fig. 4(a), demonstrating the threshold voltage ratio extracted from the transfer characteristics at the M. Zhu et al. / Solid-State Electronics 47 (2003) 1855–1858 1857

2 saturation region (Vds ¼ 60 V) is about 2.25 10 ,as shown in Fig. 4(b). The electron mobility is calculated to be 0.016 cm2 V1 s1 from the models discussed in the next section, which is smaller than the results from the doped NTCDA [1] and larger than the former investigated undoped one [10].

4. Discussions

The depletion-mode FETs are normally on due to the electric field built by the existed electrons in the channel region between the source and the drain. For this type of device, it is functional, even though gate voltage (Vgs)is equal to zero. With the decrease of Vgs (Vgs is negative), the effective channel shrinks, and thus conduction ability of the active layer decreases simultaneously. Therefore the drain current (Id) decreases until the active channel disappears, and then the device is cut off. The gate voltage that turns off the device is known as the turn-off voltage, which is also the threshold voltage where the active channel forms. The traditional device models based on inorganic materials can still describe the devices made of organic Fig. 5. Interfacial alignment for zero gate voltage (a) and materials, the equation for the linear region is [11] negative gate voltage (b). W 1 Id ¼ lCox Vgs Vth Vds Vds ð1Þ L 2 completely open, and the electrons can be injected into the channel without barriers. The higher the drain cur- For the saturation region, the equation is rent (Vds), the more electrons injected, and the larger 1 W drain current until the channel becomes saturation. I ¼ lC ðV V Þ2 ð2Þ d 2 ox L gs th When the gate voltage is decreased, the conduction band of heavily doped silicon will rise above the lowest un- where W and L are the channel width and lengths re- occupied molecular orbital (LUMO) of NTCDA, thus spectively, l is the carrier mobility of the n-channel the positive charges are accumulated in the interface and device, Cox is the capacitance per unit area of the di- negative charges are partially depleted in the channel. electric layer, and Vth is the threshold voltage. The Moreover, the capability of electron-activated n-type electron mobility calculated from Eqs. (1) and (2) gives channel is weakened. If the gate voltage is below the the rough estimation compared to the traditional inor- threshold voltage, the channel will turns off, and the ganic devices. drain current will not be generated even collected with From the energy-band theory and the interfacial ef- the high drain voltage. fect, the insight of the OFET can be analyzed. Table 1 Another point is that the conducting channel is inside [12,13] gives the parameters (work function U, energy the bulk of the active layer of the depletion-mode op- gap Eg, and Fermi level or effective Fermi level EF) of the erated devices compared to the channel at the interface materials used in the fabricated OFETs. Fig. 5 illustrates between the dielectric layer and the active layer of the the interfacial energy structure with alignment and accumulation-mode operated devices, especially for the bending. At the gate voltage of zero, the channel is TOC structure. Fig. 5(b) clearly shows that the n- channel will be turned off from the gate side to the source/drain side when the negative gate voltage is ap- Table 1 plied. Parameters of used materials One of the most important applications of the deple- Materials U (eV) Eg (eV) EC (LUMO) tion-mode OFETs is as the load device of the inverter (eV) due to the symmetrical charging and discharging time NTCDA [12] 3.97 3.3 3.6 constants [14], much faster than the inverter with en- PPy [13] 5.19 3.16 2.5 hancement-mode transistors. Thus the depletion-mode P-type heavily doped Si 5.11 1.12 4.05 OFETs can be used in the organic integrated circuits. 1858 M. Zhu et al. / Solid-State Electronics 47 (2003) 1855–1858

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