Another type of all solid-state ISE that has had some commercial success is based on a field effect transistor (FET). This type of device has been available since the 1970s. A FET is a common semiconductor component that is used for electronic amplifiers. Transistors are made from doping pure silicon (or germanium) with small amounts of elements from the boron (group 13 or 3A) or nitrogen (group 15 or 5A) families. Figure 3.24 shows partial Lewis dot structures for different types of doped material. Pure silicon is a network solid in which all valence electrons are occupied in covalent bonds. Consequently, its electrons are not very mobile, and the material is a poor conductor of electricity. Arsenic is a com- mon dopant that has five valence electrons. Adding a small mole fraction of arsenic atoms to the silicon produces sites where an extra valence electron is available, because it is not associated with a bond. The conductivity of the resulting material is higher than that of
Si Si Si Si
Si
As Si Si Si
Si
Extra valence electron
B Si Si Si
Si
Electron
“hole”
Pure silicon network N-type semiconductor P-type semiconductor
FIGURE 3.24 Partial Lewis dot structures for silicon and doped silicon semiconductors. Elemental silicon is a network solid that conducts poorly in its pure form because all of its valence electrons are occupied in covalent bonds. If the silicon is doped with a small fraction of arsenic atoms, the “extra”
valence electrons from arsenic atoms are available to carry current. Doping with an element with only three valence electrons, such as boron, leaves one bond that is electron-deficient. Physicists refer to this as a “hole.” Electrons from nearby atoms can “hop” into these holes leaving behind positive holes.
k k pure silicon. This type of material is known as an N-type semiconductor because the major
charge carriers are negative (electrons). In a similar manner, doping with boron, which con- tributes only three valence electrons per boron atom, produces sites where one B—Si bond is electron-deficient. Physicists refer to the partially filled orbital as a “hole.” The energy barrier preventing electrons in neighboring atoms from hopping into a hole is relatively small so that at room temperature, a significant amount of movement is observed. In a sense, holes are moving around in this material. Because holes are positively charged, this type of material is referred to as a P-type semiconductor.
Transistors are made from combinations of N-type and P-type material that form three distinct regions. In the transistor shown in Figure 3.25, the substrate is made of lightly doped P-type material. Two other regions are formed from N-type material. These two regions are called the source and the drain and are coated with aluminum metal in order to make electrical contact with outside circuitry. The region between the source and drain
P-type N-type N-type
Source Gate Drain
SiO2 layer
P-type N-type N-type
Source Gate Drain
_
P-type N-type N-type
Source Gate Drain
_
_ __ _
_ _ _
+ + + + + +
_ _ _ _ _ _ _ - - - - e (a)
(b)
(c)
SiO2 layer
_ __ _
_ _ _
FIGURE 3.25 (a) Diagram of a conventional field effect transistor (FET) designed to operate in the enhancement mode. (b) No gate voltage; electrons are trapped in the source. (c) A positive voltage applied to the gate attracts electrons to carry current between the source and drain.
k k is called the gate. For conventional FETs, the gate is covered with a thin layer of silicon
dioxide or silicon nitride onto which an aluminum coating is vapor-deposited for making contact to other electronic components. It is through this contact that a signal voltage is applied to the gate.
In a conventional FET, a voltage signal applied to the gate either increases or decreases the number of charge carriers in a channel between the source and the drain. If the device is designed to increase the number of charge carriers when a signal is applied, then it is called an “enhancement mode” device. (Although it is built on a P-type substrate, the tran- sistor pictured here is an N-channel device. The name comes from the fact that negative charges [electrons] must be recruited to form a channel for current to move from the source to the drain.) Because the P-type substrate is lightly doped, the region between the source in the drain has few charge carriers and exhibits a high resistance normally. If the source is connected to the negative side of a battery or power supply (as in Figure 3.25b), then elec- trons move into the source but are trapped there. The high resistance between the source and the drain impedes any current between them. Now imagine that a positive voltage is applied to the gate as in Figure 3.25c. Positive charges build up on the metal contact, but go no further due to the nonconducting silicon dioxide layer. However, as in a capac- itor, the positive charges attract electrons (from the source) to the other side of the oxide layer. Once inside the P-type zone of the gate below the silicon dioxide, these electrons experience the field between the drain and the source and migrate toward the drain. The net effect is a channel of current between the source and the drain. In that manner, a small signal voltage controls a large quantity of current between the source and the drain, but uses a negligible amount of current from the device that introduces the signal to the gate.
An ion-sensing field effect transistor (ISFET) is designed to use the charge from the selective adsorption of ions from a sample solution onto the gate to create a signal voltage that subsequently controls the source to drain current. Instead of a metal contact over the gate, the silicon dioxide layer is left open to contact a sample solution (see Figure 3.26a).
Ion adsorption at this surface will control the charge at the gate and, therefore, control the current between the source and the drain. Proper design of the silicon dioxide layer pro- duces a pH-sensitive coating. Modern semiconductor manufacturing methods can make very tiny ISFET pH sensors. Figure 3.26b shows a picture of a pH ISFET sensor embedded
(a) P-type Si
H+ H+H+ N-type Si
SiO2 Al
~1 μm
(b) pH sensitive ISFET
Temperature sensor
FIGURE 3.26 Ion-sensing field effect transistor (ISFET). (a) Diagram of the transistor showing the exposed silicon dioxide layer acting as a pH sensitive glass that controls the voltage at the gate. (b) An ISFET embedded in epoxy inside a hypodermic needle. Source: Lisensky [40].
k k in the tip of a hypodermic needle [40]. As with other all-solid-state ISEs, the challenge to
making ISFETs for other analytes has been in creating a stable contact between the sensing membrane and the gate.