1. ISAT 436 Micro-/Nanofabrication and Applications Transistors David J. Lawrence Spring 2004

2. Transistor Applications H Transistors can be used for amplification and for switching. H They are used as discrete components and interconnected (often >> 1,000,000) in integrated circuits. H There are two primary types: u Bipolar Junction Transistors (BJTs or “bipolars”) u Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)

3. Bipolar Transistors H Simplified schematic diagram of an “npn” bipolar transistor amplifier: N N P V out V in collector base emitter

4. Bipolar Transistors H Bipolar transistors come in two “flavors”: u n p n u p n p H Bipolar transistors can also be constructed with heterojunctions for improved performance -- HBTs (higher frequency of operation and higher gain). H The structureof a real bipolar transistor is shown in Fig. 1.5 on page 8 of Jaeger.

5. MOS Transistors H An amplifier can also be constructed from a MOSFET. H A simplified cross section of a MOSFET is shown below. H There is no “easy” conduction path from the source to the drain. P-type substrate N+N+ N+N+ drainsource gate oxide

6. MOS Transistors H However, if we make the gate positive with respect to the source (“bias” the gate) we can attract electrons from the source and drain n + regions. H This forms a conductive channel that connects the source and the drain. P-type substrate N+N+ N+N+ drainsource + gate + channel

7. NMOS Transistor H Consequently, making the gate positive with respect to the source turns the transistor “on”. H Since electrons flow through the channel of this device when it is on, it is called an NMOS or n- channel MOS transistor. P-type substrate N+N+ N+N+ drainsource + gate + N-channel

8. PMOS Transistor H A “complementary” device can be made by replacing n-type regions with p-type, and conversely. H A simplified cross section of a PMOS MOSFET is shown below. H There is no “easy” conduction path from the source to the drain when there is no voltage applied to the gate. N-type substrate P+P+ P+P+ drainsource gate oxide

9. PMOS Transistor H However, if we make the gate negative with respect to the source (“bias” the gate) we can attract holes from the source and drain p + regions. H This forms a conductive channel that connects the source and the drain. N-type substrate P+P+ P+P+ drainsource  gate  + + + + + + P-channel

10. PMOS Transistor  gate  N-type substrate P+P+ P+P+ drainsource + + + + + + P-channel H Consequently, making the gate negative with respect to the source turns the transistor “on”. H Since holes flow through the channel of this device when it is on, it is called a PMOS or p- channel MOS transistor.

11. MOS Transistors H Several symbols are used to represent MOSFETS in electronic circuit diagrams. H Two common symbols are shown here: N-channel MOSFET NMOS P-channel MOSFET PMOS G D S G D S

12. MOS Transistors H MOSFETs can be used as switches, e.g., in memory and microprocessor chips. H MOSFETs can be used as amplifiers: G D S V in V out