HMOS

HMOS, "high-performance n-channel MOS", also called depletion-load NMOS, is a digital circuit style which uses n-type MOSFETs to implement logic gates. It is based on NMOS logic technology, achieving greater speed at higher manufacturing cost. The performance gain, and price increase, derive from the inclusion of depletion-mode NMOS transistors in the manufacturing process. These "tuned" transistors are better ideal current sources. HMOS was the state of the art in the late 1970s, being used to make the Intel 8086 and Motorola 68000. It was superseded by CMOS in the early 1980s.

HMOS differs from NMOS in the way the "Vdd" voltage source, representing "1", connects to each gate. In both technologies, each gate contains one NMOS transistor which is permanently turned on and connected to Vdd. When the transistors connecting to "0" turn off, this pull-up transistor determines the output to be "1" by default. In standard NMOS, the pull-up is the same kind of transistor as is used for logic switches. As the output voltage approaches a value less than "Vdd", it gradually switches itself off. This slows the "0" to "1" transition, resulting in a slower circuit. HMOS replaces this transistor with a depletion-mode NMOS at a constant gate bias, with the gate tied directly to the source. This alternative type of transistor acts as a current source until the output approaches "1", then acts as a resistor. The result is a faster "0" to "1" transition.

In both NMOS and HMOS the connection to "1" is always active, even when the connection to "0" is also active. This results in a short circuit, wasting power. The amount of waste depends on the strength, or physical size, of the pull-up. Both normal (enhancement-mode) and depletion-mode pull-up transistors use greatest power when the output is stable at "0", so this loss is considerable. The sizes of the pull-ups are thus adjusted so that the "0" to "1" transition speed matches the "1" to "0" speed, but no more, as that would waste power. Because the strength of a depletion-mode transistor falls off less on the approach to "1", they may reach "1" faster despite starting slower, i.e. conducting less current at the beginning of the transition and at steady state.

The depletion-mode transistor is formed by increasing the amount of dopant in the channel region. Both NMOS and HMOS are obsolete, as CMOS technology eliminates nearly all short circuits completely.