The first transistor was demonstrated on Dec. 23, 1947, at Bell Labs by William Shockley. This new invention consisting of P type and N type semiconductive materials (in this case germanium) has completely revolutionized electronics. Transistors quickly replaced vacuum tubes in almost all applications (most notably those in discrete logic). Today when we think of transistors the first thing that comes to mind is computers. Advances in transistor technology and manufacturing processes as well as new materials being used for the semiconductor matrix and wiring have led to smaller, faster, cheaper, lower power transistors. Some of the basic principles behind semiconductor behavior and the restrictions currently faced by modern transistors will be discussed in the following pages.
Transistors are composed of a P type (positively doped) and N type (negatively doped) semiconductor material. These P-N junctions are the heart of both BJTs (Bipolar Junction Transistors) and FETs (Field Effect Transistors). BJTs have a physical connection between they current controlling input (base) and the input and output (collector and emitter). This results in a trickle current into the base. FETs have a physical separation between the control (gate) and the input and output (drain and source).
BJT and FET transistors are used in virtually every electronic device requiring current regulation or amplification. They make it very easy to precisely control power to a device reliably and with much greater efficiency than other methods. Another common use of transistors is their role in discrete logic. First used in DTL (Diode Transistor Logic) transistors compact nature and high switching speeds lend themselves well to use in logic ICs. In fact, almost every component of a modern computer is controlled by transistors. Video processing functions, system memory, system bus functions, and of course the main system processor. Many of these chips are composed of millions of transistors switching at very high frequency. The frequency that these logic transistors switch at is a primary factor in chip performance and is therefore of great significance. What factors limit switching speeds and how are chips being made to go faster and faster? Power consumption and gate temperature are two of the primary factors that affect switching speed.
Transistor gate switching speed is an area of primary concern in nearly all digital applications. Everyone wants faster stuff. The problem is being able to provide this without increasing power consumption or cost. There are many factors involved in determining the maximum frequency of a transistor which will be discussed here. A primary limiting factor is gate temperature.
Gate temperature is a byproduct of the resistance that is inherent to all electrical circuits. There is a small resistance in the wiring, there is resistance in the silicon substrate (which results in leakage current), and there is resistance within...