It's a good idea to keep your stock portfolio diversified. For example, you might own some solid dividend-paying blue-chip companies and some faster-growing companies that don't offer dividend payouts. You would do well to spread your dollars across a variety of industries, as well, and perhaps include some investments in real estate (via real estate investment trusts (REITs) and foreign companies. Keeping some money in cash is a fine idea, too, so that you'll be able to jump on a great opportunity that arises.

Here are three companies to consider for the growth stock portion of your portfolio. Understand that growth stocks can often trade at lofty levels, so plan to hang on for a long time and to withstand some volatility as well.

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1. Universal Display

With a market value recently near $9 billion and a stock price recently down 27% from its 52-week high, Universal Display (OLED -0.92%) is looking rather attractive. The company specializes in energy-efficient "organic light emitting diode (OLED) technologies and materials for use in display and solid-state lighting applications."

There's a lot to like about Universal Display: It's growing briskly, with management expecting 30% year-over-year revenue growth this year while revenue grew 55% year over year in the first half of the year. The fact that it has plenty of cash and little to no debt is attractive, too, as the cash can help it grow via, say, acquisitions or additional hiring; little debt also means it's not having to divert valuable income toward debt repayments. Its OLED technology is likely to appear in more and more device and TV screens as well as lighting equipment over time, as it can consume less energy and can be better for our health, emitting less blue light.

The stock has slumped lately in part due to investors wanting more exciting growth projections, but rest assured: The company is growing briskly and has a rosy future, and management playing it conservative is better than the other way around.

2. DocuSign

If you've had to sign any important documents remotely lately, from your desktop computer or even your smartphone, you've very possibly used the technology of DocuSign (DOCU 2.45%). It's a leader in e-signatures, with an estimated market share between 60% and 70% , and plenty of opportunities for growth in other areas, such as notarization, identification, and contracts.

DocuSign already has more than a billion users worldwide, and more than a million paying customers. It's used by all 15 of the top 15 financial companies in the Fortune 500, and in its last quarter, revenue jumped 49% year over year -- with the lion's share of its revenue coming from subscriptions, which are often renewed. Between fiscal 2013 and now, it has been adding customers at a compounded average growth rate of 42%.

DocuSign's stock isn't looking cheap these days, with a recent price-to-sales ratio near 30, but buying shares now could pay off well over many years. Do your own research first, of course, and if you end up not quite sure about how attractively the stock is priced now, you might just add it to your watch list -- or plan to buy into the stock in chunks over time.

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3. Twilio

Now we come to Twilio (TWLO 1.91%), a $61 billion cloud computing business boasting the "the world's #1 Customer Data Platform (CDP)." It has been growing its core offerings at a rapid clip while introducing new ones, such as a toll-free messaging software service called Zipwhip -- which it acquired for $850 million.

In its second quarter, Twilio reported year-over-year revenue growth of 67%, with about a third of revenue generated abroad. Its top 10 customers recently accounted for only about 12% of revenue, which means it's not overly dependent on just one or a few customers for much of its income. All this makes Twilio a compelling investment, but it has some red flags, too, such as widening losses.

If, after further research, you are confident that Twilio will be turning its situation around soon, consider snapping up some shares. If you're on the fence, perhaps just add it to your watch list, or start with a small position.

These are just a few of many exciting growth stocks out there. The more time you spend reading up on others, the more solid candidates for your portfolio you'll uncover. Just one or two long-term big winners in your portfolio can make a big difference in your overall results.